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Nutrient Release Patterns of Coated Fertilizers Used for Citrus Production and Their Effect on Fruit Yield and Foliar Nu...

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NUTRIENT RELEASE PATTERNS OF CO ATED FERTILIZERS USED FOR CITRUS PRODUCTION AND THEIR EFFE CT ON FRUIT YIELD AND FOLIAR NUTRITION By CAROLINA MEDINA A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Carolina Medina

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To my family

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iv ACKNOWLEDGMENTS I would like to give my sincere gratitude and appreciation to Dr. Thomas Obreza and Dr. Jerry Sartain for their guidance, pr oblem solving and advice. Achieving this degree would not have been possible without th eir support. I would also like to thank the other members of my supervisory committee, Dr. Robert Rouse and Fritz Roka, whose knowledge and comments contributed significantly to this resear ch project. Further, my special thanks go to Ed Hopwood, Nahid Me nhaji and Zoe Shobert for their friendly assistance through laboratory anal ysis and field tasks for my research. Thanks also go to the Scotts Fertilizer Company for providing the funding for this research. Finally, I would like to thank all the members in my family and my boyfriend for their constant support and help.

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v TABLE OF CONTENTS Page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... ix CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................4 Fate of Nitrogen in a Citrus Environment....................................................................4 Plant Uptake..........................................................................................................4 Leaching and Runoff.............................................................................................6 Denitrification........................................................................................................9 Volatilization.......................................................................................................10 Citrus Management Practices.....................................................................................11 Fertilizer Management.........................................................................................12 Irrigation Management........................................................................................14 Leaf Analysis.......................................................................................................15 Controlled-Release Fertilizers....................................................................................16 Types of Controlled-Release Fertilizers..............................................................17 Predicting Nutrient Release from PCFs..............................................................20 Factors Influencing PC F Nutrient Release..........................................................20 3 NUTRIENT RELEASE CHARACTERIST ICS OF COATED FERTILIZERS UNDER GREENHOUSE AND FIELD CONDITIONS...........................................23 Introduction.................................................................................................................23 Materials and Methods...............................................................................................24 CRF Incubation and Nutrient Leaching Study....................................................25 Field Mesh Bag Study.........................................................................................27 Results and Discussion...............................................................................................29 CRF Incubation and Nutrient Leaching Study....................................................29 Field Mesh Bag Study.........................................................................................36 Conclusions.................................................................................................................46

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vi CRF Incubation and Nutrient Leaching Study....................................................46 Field Mesh Bag Study.........................................................................................46 4 EVALUATION OF CITRIBLEN ON FRUIT PRODUCTION AND FOLIAR NUTRIENT STATUS OF MATURE CITRUS TREES............................................49 Introduction.................................................................................................................49 Materials and Methods...............................................................................................50 Site A...................................................................................................................50 Site B...................................................................................................................51 Site C...................................................................................................................51 Site D...................................................................................................................52 Leaf Sampling of Commer cial Citrus Orchards..................................................52 Economics of CitriBlen Use on Co mmercial Mature Citrus Trees..................53 Results and Discussion...............................................................................................54 Leaf Sampling of Commer cial Citrus Orchards..................................................54 Economics of CitriBlen Use on Co mmercial Mature Citrus Trees..................57 Conclusions.................................................................................................................61 Leaf Sampling of Commer cial Citrus Orchards..................................................61 Economics of CitriBlen Use on Co mmercial Mature Citrus Trees..................62 5 CONCLUSIONS........................................................................................................63 CRF Incubation and Nutrient Leaching Study....................................................63 Field Mesh Bag Study.........................................................................................64 Leaf Sampling of Commer cial Citrus Orchards..................................................65 Economics of Citriblen Use on Co mmercial Mature Citrus Trees...................66 APPENDIX: COMMERCIAL YIELD DATA.................................................................67 LIST OF REFERENCES...................................................................................................68 BIOGRAPHICAL SKETCH.............................................................................................73

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vii LIST OF TABLES Table page 3-1 Nitrogen sources in each c ontrolled-release fertilizers............................................25 3-2 Controlled-release fert ilizer specifications...............................................................28 3-3 Effect of N source on total N re leased from four CRFs with time...........................33 3-4 Effect of controlled-relea se nitrogen fertilizer type and location on N release rates (% of applied)...........................................................................................................38 3-5 Percentage of controlled-release nitroge n (CRN) fertilizers re leased with time......39 3-6 Regression analysis of estimated N rel ease rate from different N sources against time using an exponential rise to a maximum model (Immokalee).........................43 3-7 Regression analysis of estimated N rel ease rate from different N sources against time using an exponential rise to a maximum model (Lake Alfred)........................43 4-1 Characteristics of site A...........................................................................................51 4-2 Characteristics of site B............................................................................................51 4-3 Characteristics of site D...........................................................................................52 4-4 Effect of soluble and controlled-release fe rtilizers on N, P, K, Ca, and Mg (site A).55 4-5 Leaf analysis standards for mature, bear ing citrus trees base d on 4 to 6-month old spring-cycle leaves from nonfruiting terminals........................................................55 4-6 Effect of soluble and controlled-release fe rtilizers on N, P, K, Ca, and Mg (site B).56 4-7 Effect of a dry, liquid an d controlled-release fertiliza tion program on N, P, K, Ca, and Mg (site D)........................................................................................................57 4-8 Partial budget for CitriBle n fertilization program.................................................58 4-9 Partial budget for CitriBle n fertilization program assu ming an increase in yield.60 A-1 Historic commercial fruit yield data for sites A, C and D........................................67

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viii LIST OF FIGURES Figure page 2-1 Citrus nitrogen cycle..................................................................................................5 2-2 Nutrient release mechanism for polymer-coated fertilizers.....................................19 3-1 Incubation lysimeters...............................................................................................26 3-2 Field study locations.................................................................................................27 3-3 Layout of mesh bags pla cement under citrus tree canopy.......................................28 3-4 Cumulative leaching of N forms..............................................................................31 3-5 Leaching of N forms from soil columns..................................................................32 3-6 Phosphorus leached from soil columns....................................................................34 3-7 Effect of K source on the quantit y of K leached from soil columns........................35 3-8 Effect of K source on K leaching.............................................................................36 3-9 Nitrogen released (% of applied) with time.............................................................37 3-10 Comparison of average daily te mperature (C) between locations...........................41 3-11.Comparison of rainfall distri bution (mm) between locations....................................42 3-12 Citrus orchard orientation at Imm okalee and Lake Alfred, respectively.................42 3-13 Nitrogen release curves for CitriBlen and Agrocote Type A at Immokalee and Lake Alfred, respectively.........................................................................................44 3-14 Nitrogen release curves for Agrocote Type C(D) and Agrocote Poly-S at Immokalee and Lake Alfred, respectively...............................................................45 3-15 CitriBlen N release curves for Immokalee and Lake Alfred.................................48 4-1 Response of Hamlin orange trees to cont rolled-release and water soluble fertilizers.59

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ix Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science NUTRIENT RELEASE PATTERNS OF CO ATED FERTILIZERS USED FOR CITRUS PRODUCTION AND THEIR EFFE CT ON FRUIT YIELD AND FOLIAR NUTRITION By Carolina Medina May 2006 Chair: Thomas Obreza Major Department: Soil and Water Science Citrus trees require nitrogen (N) fertiliz er to maintain optimum levels of fruit quality and productivity. Farmers have relied for many years on water-soluble fertilizers as the main method to provide N to Flor ida citrus trees. Howe ver, leaching of NO3-N from excessive use of N containing water-solubl e fertilizers can potentially contribute to contamination of groundwater, wh ich supplies more than half of the total fresh water used in Florida. Controlled-release fertiliz ers (CRFs) have the pot ential to gradually release nutrients to coinci de with the nutrient demand for crop growth, thereby maximizing N uptake efficiency while minimizing leaching losses. A laboratory study was conducte d to investigate the eff ect of various coated fertilizers (CitriBlen; Agrocote Type A; Agrocote Type C(D) and Agrocote PolyS) on nitrogen (N), phosphorus (P) and potas sium (K) leaching using a soil incubation and leaching technique. The quantity of N, P and K released depended on composition

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x and thickness of the coating. Release of N, P and K was de layed with CRF applications compared with water-soluble fertilizers. A 1-year field study in a mature citrus tree environment was used to estimate N release characteristics of the same CRFs and a water-soluble formulation. Similar studies were simultaneously conducted in central and southwest Florida. Mesh bags containing 3.5 g of elemental N from each source were plac ed on the soil surface within the irrigated zone under the tree canopy and were retr ieved from the field on a given. Despite differences in total amount of N released between locations, N release rates at both locations followed the same order: Wate r-soluble formulation>Agrocote Type A > CitriBlen > Agrocote Poly-S > Agrocote Type C (D). Quantity and frequency of irrigation and rainfall and orchard orienta tion were determined as potential factors affecting these differences. N release patterns coincided with the citrus fertilization strategy recommended as a Best Management Practice (BMP). Four commercial citrus orchards located in southwest and central Florida were used to compare the effects of CitriBlen and a conventional water-soluble fertilizer program on mature citrus production and nutrition. Leaf tissue was sampled at each orchard in August 2004 and 2005. Results suggested that Citr iBlen applied only once per year at half the water-soluble N rate has the potenti al to produce leaf nut rient concentrations within the optimum range according to guidelines. An economic analysis compared costs and benefits between the two fertilization programs. A reduc tion in net income indicated that using CitriBlen exclusively for citrus production is economically unfeasible due to its high cost. The implementation of a CRF pr ogram would not be attractive to citrus growers unless fertilizer prices change or a cost-share program was established.

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1 CHAPTER 1 INTRODUCTION Citrus plays an important ro le in Florida’s agricultural industry. According to the Florida Agricultural Statisti cal Service (2004), commercial citrus production occupies 302,940 ha in five major production regions, with an annual production of 11.4 million metric tons. Nitrogen (N) supply is more im portant in citrus nut rition than any other element. It has a large influence on tree fl owering, fruit set, appearance, and fruit production/quality (Zekri and Obreza, 2003). N is mainly provided to Florida citrus trees as dry soluble fertilizers. Because of Flor ida’s poor natural soil fertility and humid climate, citrus trees require frequent applications of sol uble N fertilizers at high annual rates to ensure sufficient vegetative growth, high yield and high fru it quality (Zekri and Koo, 1992). Efficient use of applied N is esse ntial to maintain hi gh quality trees while minimizing environmental hazards. N losses through leaching and/or volatilization are the main causes of low efficiency of applied N fertilizer. In sandy Florida soils, exce ssive use of N-containing fertilizer can potentially contribute to leaching of NO3-N and thus lead to cont amination of groundwater or surface water resources (Paramasivam and Alva, 1997). A circa 1990 groundwater quality study revealed that 63% of the dr inking water wells surveyed in the central Florida ridge counties of Lake, Polk, and Highlands contai ned detectable NO3-N and 15% contained NO3-N concentrations above the EPA Maximum Contaminant Level (MCL) of 10 mg L-1. Most of the contaminated wells were located close to commercial citrus orchards (Lamb et al., 1999). The pres ence of nitrate in dr inking water supplies

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2 above the EPA MCL represents a health hazar d for Florida citizens. Although the source of groundwater nitrate in central Florida has never been confirmed, proper nutrient management practices and j udicious use of existing fer tilizer technology may minimize or eliminate N leaching from citrus fertiliz ation and its potential to contribute to the nitrate problem. Controlled-release fertilizers (CRFs) have the potential to synchronize nutrient release patterns with crop demand and therefor e optimize nutrient uptake efficiency while reducing nutrient losses to the environment. Coated fertilizers occupy the largest share of controlled-release fertilizer t echnology due to their flexible nutrient release patterns and to their ability to control the release of other nutrients in addition to N. Despite continuing technological improvements and the co mmercial availability of several CRFs, their agricultural use remains limited. Many st udies conducted on citrus fertilization in past years have shown that CRFs have the potential to produce sim ilar or greater tree growth and fruit yield than water-soluble fertilizers. CRFs have also been shown to decrease N leaching potential. However, the higher cost of CRFs per unit of nutrient and the lack of experience about their performance in the field have caused Florida citrus growers to avoid them. Information is need ed regarding field performance and economic feasibility of coated N fertilizers applied in commercial citrus orchard environments. The objectives of this study were (1) to evaluate the cumulative N, P and K released from coated fertiliz ers with time using a soil incuba tion method; (2) to evaluate the N release patterns of coated fertilizers a pplied to a citrus orchard; (3) to develop N release curves for the coated fertilizers; (4 ) to evaluate the eff ects of a resin/polymersulfur coated mixture on fru it yield and foliar N, P, K, Ca and Mg concentrations of

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3 mature citrus trees; and (5) to evaluate the economic feasibility of using a controlledrelease fertilization program compared with a water-soluble fertilization program for commercial orange production.

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4 CHAPTER 2 LITERATURE REVIEW Fate of Nitrogen in a Citrus Environment Any nutrient added to the soil undergoes numerous comple x interactions between plant roots, soil microorganisms, chemical reactions and pathways for loss (Shaviv and Mikkelsen, 1993). For citrus, a maximum of 50 to 55% of the N fertilizer annually applied can be accounted for by plant uptake even considering exceptionally high fruit yield scenarios (He et al., 2000A). Applied nu trients not recovered by the trees and fruit may be lost to the environment by different mechanisms. Thus, there is a need to understand the fate of nitrogen in a citrus environment to maintain high quality trees while minimizing the effects of N fertiliza tion on the environment, particularly water resources. Fate of nitrogen in a citrus environm ent includes several mechanisms: 1) plant uptake; 2) runoff and leaching into groundwater and surface water; 3) denitrification; and 4) volatilization (He et al., 1999) Figure 2-1 illustrates N dynamics in a citrus ecosystem. Plant Uptake Nitrogen uptake efficiency (NUE), defined as the percentage of applied N taken up by crops, is often low in Florida soils because of the high mobility of N fertilizer (Obreza and Rouse, 1992). In sandy soils receiving 100to 125-cm of rainfall, the efficiency of N uptake by plants may not exceed 20 to 30% (Oertli and Lunt, 1962). Mattos (2000) estimated NUE for 6-year old ‘Valencia’ tr ees grown in a sandy soil to be 40% and 26% for ammonium nitrate and urea, respectivel y. Paramasivam et al. (2001) showed that

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5 NUE for 25-yr-old ‘Hamlin’ orange trees gr own in an Entisol ranged between 40 and 53%. Figure 2-1. Citrus nitrogen cycle NUE of young citrus trees can vary from 57 to 68% depending on tree growth rate, supply of other nutrients, and irrigation wate r quality (He et al., 1999). However, NUE is more complex to measure for bearing-citrus tr ees since they can stor e nutrients to sustain

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6 fruit production and growth. N removal in the harvested fruit can be a measure of NUE for mature citrus, since this is the only portion that is rem oved from the tree-soil system on an annual basis. Therefore, the annual fertilization program should aim to replenish nutrients removed by the harvested fruits along with adequate c onsideration of the nutrient requirement for the annual regrowth of leaves and roots, storage for flowering and fruit setting, application efficiency, and the contribution of nut rients from recycling of organic residues in the soil (Alva and Paramasivam, 1998B). Alva (1997) reported that the amount of N removed by harvested fruit was significantly correlated (quadratic relationshi p) with N rates. When N fertilizer was applied at 112 kg ha-1 year-1, total N in the fruit was equiva lent to 70 to 80% of the annual N applied depending on fertilizer source. This N recovery was higher than average, and is often difficult to reach under normal producti on conditions. Consequently, it is apparent that mineralization of N from crop residues plays an important role in supplying a fraction of the N requirement. Leaching and Runoff Currently, there is an increasing conc ern regarding large accumulations of NO3-N in ecosystems, mainly from N leaching from agricultural fields. High nitrate concentrations are related to (i) methemoglobanemia in infants and in ruminants; (ii) stomach cancer, for which a possible link with nitrates or nitrosoamines has been suggested; (iii) other diseases such as goiter, birth defects, and heart disease; and (iv) eutrophication of surface water (Shaviv and Mikkelsen, 1993). Transport of NO3-N through the soil profile is a function of many variables, including soil, climate factor s, biological N, and cultural characteristics. Edaphic characteristics include texture, porosity, stru cture, consistency, depth of profile, and

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7 percolation rates. Climatic characteris tics include amount, fr equency, duration, and timing of precipitation. Presence or absence of plant cover, depth of root zone, N use characteristics of the vegetation, and periods of plant growth also influence N dynamics in agricultural soils. Amount of organic matter and microbial popul ation affect leaching of NO3-N to groundwater (Alva, 1997). Under Florida’s warm, humid conditions, NO3-N easily moves through the soil profile due to rapid transformation of NH4-N into NO3-N, inherently low soil fertility, low cation exchange capacity, and unique hydr ologic features (e.g. a thin surface soil layer, high water table and porous limest one in many areas) (Tucker et al., 1995; Paramasivam et al., 2001). Consequently, a substantial portion of applied N fertilizer may leach from the root zone into su rface and groundwater. Paramasivam and Alva (1997) reported that about one -third of N applied to citr us on Florida’s extremely sandy soils is lost to leaching or volatilization. Similarly, leaching losses in a large southern California watershed planted with citrus were equivalent to 45% of the annual N applied (Paramavisam et al., 2001). Most Florida citrus orchards have been planted on Entisols, Spodosols or Alfisols, depending on geographical region (Obreza and Collins, 2002). Nearly 40% of citrus orchards are found on the deep sandy Entis ols along the Central Florida ridge, while more than 21.5% of total stat e citrus plantings are on the flatwoods and marsh soils of southwest Florida. A mixture of Alfisols a nd Spodosols is found in the Indian River citrus-growing area near the east coast. In a study conducted by Lamb et al. (1999), 15 months of baseli ne data indicated that groundwater NO3-N concentrations were above the EPA Maximum Contaminant

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8 Level (MCL) of 10 mg L-1 beneath mature citrus groves on the central Florida ridge. This area contains primarily Entisols (ridge soils ) with no confining subsurface soil horizon, very low organic matter content and sand cont ent > 96%. Most of th e well-drained soils of this region are classified as vulnerable to leaching of N (Tucker et al., 1995). Alva and Tucker (1993), in a leaching study on an Entis ol using coated and so luble fertilizers for young citrus, reported that NO3-N concentration detected 1.5 m below ground were above the MCL in the treatments that receiv ed soluble fertilizer at high rates. Results from another study on an Entisol showed that NO3-N concentrations in soil solution below the rooting depth (240 cm) peaked occasionally at 17 to 33 mg L-1, but under careful irrigation and N mana gement conditions concentrations were normally below 10 mg L-1. It was also demonstrated in the same study that NO3-N leaching losses below the rooting zone increased with increasing rate of N application and the amount of water drained (Paramavisam et al., 2001). In contrast to the well-drained Entiso ls, the acid, sandy Spodos ols (flatwoods soils) are typically poorly drained with a spodic a nd clay strata that impede water flow vertically from the profile. Consequently, wate r draining from these soils flows laterally along the top of the subsurface hardpan to a surface water body. Hence, potential leaching of NO3-N to the groundwater is more important in ridge soils than in flatwoods soils. However, in some cases, the hardpa n is broken during the bedding process, and thus, there could be a potential for downward migration of pollutants below the hardpan (Alva et al., 1997). Calvert ( 1975) demonstrated that NO3-N concentrations in the tile drainage water of a Spodosol varied from <1 to 8 mg L-1, depending on rainfall, irrigation and fertilization practices. In a study by He et al. ( 2000) on a Spodosol, solution NO3-N

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9 concentrations at 120and 180-cm depths incr eased with increasing fertilizer rates, but never exceed the MCL even at the hi ghest rate of fertilizer (168 kg N ha-1 yr-1). However, results from a study in west central Fl orida (He et al., 1999) revealed that NO3-N concentrations in groundwater exceed the MCL even at 3-m depth below surface on flatwoods soils. Mansell et al (1977) in a flatwood soils management study found that surface runoff typically occurs from deep-til led flatwoods during intense rainfall or irrigation of long duration after the soil profile has become water-saturated. Denitrification Denitrification is the gaseous loss of nitr ogen to the atmosphere via a microbial respiration process. This process occu rs under anaerobic conditions where microbes obtain their O2 from NO2 and NO3 with the accompanying release of N2 and N2O (Havlin et al., 1999). Environmental concerns about emission of nitrous oxides are mainly related to the effect on global warm ing and the role of nitrous oxides in ozone destruction. The destruction of O3 is catalyzed by NO, halogens, hydroxyl, and hydrogen. A possible source of NO is from N2O, the product of denitrification, which can diffuse into the upper atmosphere and lead to atmo spheric “holes”, hence causing problems for plants and animal life from excessive e xposure to ultraviolet radiation. However, depletion of the ozone layer is also greatly associated with the in tensive industrialization that has taken place during the pa st 5 decades (Shaviv, 2001). The presence of NO3-N in the soil profile, lack of O2, denitrifier population, and availability of soluble carbon sources are th e main factors determining the magnitude of denitrification activity. These ch aracteristics are often found in flatwoods soils, especially associated with a shallow water table. Re sults of a study conducted by Mansell et al. (1977), indicated that a significant portion of th e N fertilizer applied to citrus grown in a

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10 deep-tilled flatwoods soil was denitrifie d due to the relatively slow drainage characteristics and the capacity of the soil located in the lo wer portion of the profile to denitrify NO3-N. Another study on Spodosols (He et al., 2000A) showed that the concentrations of NO3-N were greater in the soil soluti on at the 120-cm depth than at the 180-cm depth, which might be due to great er denitrification at the 180-cm depth. Although some localized anaer obic microsites can exist in a well-drained soil, gaseous loss of N by denitrification is often insignificant in central Florida ridge soils (Alva and Paramasivam, 1998B). However, Para mavisam et al. (1999) demonstrated that denitrification occurred in well-drain ed sandy Entisols particularly at the soil/groundwater interface and was dependent on the amount of available carbon and denitrifier population. Volatilization Volatilization, the gaseous loss of amm onia from surface applied ammonium and urea fertilizers, is controlled by various so il properties and environmental factors and is directly proportional to a mmonium concentrations in the soil solution. Ammonia volatilizing from fertilized fields can accumu late in neighboring natural ecosystems, possibly causing damage to the vegetation. So me of the ammonia may be converted into nitric acid, and this produc t coupled with sulfuric acid (from industrial sources) forms acid rain that can affect plants directly a nd can acidify lakes, re sulting in aluminum toxicity in fish and plants (Newbould, 1989). Ammonia volatilization increase s with soil temperature and soil pH (Ernst et al., 1960). Nitrification, the transformation of NH4 to NO3, is inhibited at high temperature, resulting in increased availability of N as NH4, which contributes to increased volatilization losses. Ammonia vol atilization is favored in sandy soils with low buffering

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11 capacity, since the ability of NH4 to form electrostatic bonds with clay minerals and organic colloids to impair losses of soil and fe rtilizer N is low. In well-drained ridge soils with high pH, volatilization losses can account for 10 to 15% of NH4-N applied to the soil surface on an annual basis (A lva and Paramasivam, 1998B). He et al. (1999) measured ammonia volat ilization from four N fertilizer sources surface-applied to an Alfisol (Riviera fine sand, pH 7.9) using a sponge-trapping technique in the laborator y. Ammonia volatilization incr eased significantly with an increase in NH4-N application rate, and by 2and 3fold, respectively, with an increase in incubation temperature from 5 to 25 C, and from 25 to 45 C, respectively. Ammonia volatilization was minimal at pH of 3.5 and increased rapidly with increasing pH up to 8.5. In a field study by Mattos et al., (2003), ammonia volatilized from dry-granular ammonium nitrate and urea fe rtilizers surface-applied to a sandy Entisol was evaluated using a semi-open static system of ammonia sorbers. Ammonia volatilization losses from both N sources were greater when air was ci rculated inside the collection chamber to simulate ambient air movement compared with volatilization m easured with no air circulation. This result showed the remarkable effect of environmental conditions such as aeration, temperature, and soil mo isture on ammonia volatilization. Citrus Management Practices Most Florida citrus is grown on extremely sandy soils with inherently low fertility, low cation exchange capacity a nd low retention of applied pl ant nutrients. Due to these soil properties and climatic conditions in Flor ida, nitrate ions can fr eely leach from root zones into groundwater, potentially leading to pollution of drinking water supplies. Proper fertilization and irrigation management practices are required to ensure sufficient

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12 vegetative growth, high fruit yield, and good fr uit quality while minimizing detrimental effects on the environment. Fertilizer Management Citrus fertilization practices can be tracked back to th e late 1800s. In the early 1900s low analysis fertilizers and organi c N sources were used. Since the 1930s, inorganic fertilizers have played a major ro le in increasing citrus production per unit land area (He et al., 1999). Fertilizer Form Traditionally, broadcast applic ation of dry soluble fertiliz er material has been the main method to provide N to citrus trees in Florida. The majority of N applied has been ammonium nitrate either in gr anular or solution form. Also, the use of ammonium sulfate in sulfur-deficient or high soil pH conditi ons has increased in Fl orida during the past decade (Sartain, 2003). However, the soluble na ture of these materials in combination with Florida’s soil and climatic conditi ons may potentially cause leaching of NO3-N below the root zone. Fertigation, the delivery of liquid fert ilizers through the i rrigation system, has become a popular way to apply nutrients sinc e the introduction of microirrigation systems for citrus irrigation. Fertiga tion facilitates (i) placement of fertilizer under the canopy for efficient root uptake, and, (i i) increased frequency of a pplication without substantial increase in application cost (Alva et al., 1998). By increasing frequency of application, small amounts of fertilizer can be applied many times through the course of the growing season, improving nutrient uptake efficiency and reducing leachi ng losses. A study by Lamb et al. (1999) showed that when 142 kg N ha-1 was applied as a combination of fertigation and foliar spray, groundwater NO3-N concentration decr eased from 30 mg L-1

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13 to less than 10 mg L-1 while maintaining optimal fruit production and nutritional status of leaves. Some studies (Alva et al., 2002; Alva and Paramasivam, 1998A) showed that fruit yield was significantly greater for fertigation th an for a soluble, gra nular source applied at similar rates. These results suggest that nutri ent uptake efficiency may be greater with fertigation compared with the application of dry soluble fertilizers. However, fertigation does not always provide be tter efficiency. For example, Koo (1980) reported no significant differences in fruit yield and l eaf nutritional status between water-soluble granular fertilizers and fertigation. Controlled-release fertilizers were devel oped to improve nutrient use efficiency while reducing environmental hazards. Many studies (Alva and Tucker, 1993; Dou and Alva, 1998; Wang and Alva, 1996) have shown that controlled-release fertilizers applied in part or throughout a citrus fertilization program have potential to reduce N leaching on Florida sandy soils. It was also reported th at greater fruit yiel d was obtained using controlled-release fertilizers compared with water soluble fertilizers (Obreza et al., 1999; Zekri and Koo, 1992). However, due to its gr eater cost, the use of controlled-release fertilizers for citrus has been limited to young-tree situations (rese t or solid-set new plantings) where high frequency application of conventional fertilizer s is not feasible (Obreza and Rouse, 1992). Fertilizer Rate In general, increasing N rate tends to 1) increase juice volume, total soluble solids (TSS), acid content and juice color; 2) increas e number of green fruit at harvest, and incidence of creasing and scab; and 3) decreas e fruit size, weight and peel thickness (He et al., 1999).

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14 Applying less than the recommended N rate may substantially reduce yield and/or fruit quality, while over-application may increase the risk of nitrate contamination of the groundwater. Optimal N-fertilizer rates are di ctated by overall tree N-requirements and N-fertilizer use efficiency. Currently, fertilizer rates fo r non-bearing citrus trees are recommended in weight of a complete N-P-K fertilizer per plant (e.g. lbs tree-1), while for bearing citrus trees (4 years and older), fe rtilization is based on the expected production and N is recommended on a weight per unit area basis (e.g. lbs acre-1). The current recommended N rate for bearing citrus trees ranges from 134 to 269 kg N ha-1 yr-1 (120 to 240 lb N acre-1 yr-1) depending on variety and expected production volume per unit area (Tucker et al., 1995). A worldwide review of long-term citrus fertilization experiments indicated that appl ication of 202 kg N ha-1 yr-1 is sufficient to sustain optimal tree growth and maintain high fruit quality and producti on (Alva and Paramasivam, 1998B). Another study by Lamb et al. (1999) demonstrated th at when applying N at rates of 180 kg ha-1yr-1 to mature citrus located on the central Fl orida ridge, the groun dwater may on average comply with the MCL. Similarly, in a 4-yr study on an Entisol, Al va et al., (1998) showed that leaf N concentrations and fru it production did not significantly change when lowering N rates to 180 kg ha-1yr-1. Irrigation Management Since transport of water th rough the soil profile plays a major role in leaching of NO3-N, optimal irrigation management pr actices are important to minimize NO3-N leaching losses and to improve N uptake efficiency, principally in sandy soils. Traditionally, citrus was grown under overhea d irrigation, where the entire grove area was irrigated. More recently, due to the increas ing need to conserve water, microsprinkler irrigation has been introduced where the i rrigated area is greatly reduced to under the

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15 canopy, which is also the area of maximu m root activity. Young trees usually use microsprinklers that wet only a small area and provide efficient application of water and fertilizer, and cold protection when neede d. For mature trees with an expanded root system, the wetted area should cover at l east 50% of the gro und surface under the canopy in order to supply adequate ir rigation and fertigation, and av oid leaching (Tucker et al., 1995). The depth of wetting for each irrigation event should be restricted to the root zone, so that soluble N is maintained within the rooting depth and NO3-N uptake is facilitated. Therefore, irrigation duration s hould be limited to replenish the water storage capacity of the root zone (45 to 90cm) under the wetted ar ea in order to avoid leaching. The use of tools such as tensiometers, other soil moisture probes, and rainfall data is a recommended N-BMP for irrigation scheduling (Schumann, 2003) Timing of fertilizer application also plays a critical role in preventing groundw ater pollution. It is recommended to avoid fertilizer application during intense rainfa ll months (June through August) to minimize the risk of NO3-N leaching below the root zone. Severa l studies (Alva, 1997; Alva et al., 1998; Alva and Paramasivam, 1998A; He et al., 2000B; Paramasivam et al., 2001; Paramasivam et al., 2002) have shown that under appropriate irri gation scheduling and timing of fertilizer applicati on, optimal fruit production can be economically attained at lower N rates than recommended, leaching of NO3-N below the rooting depth can be minimized and N uptake efficiency can be increased. Leaf Analysis Leaf analysis has been extensively used in the past two decades as a research tool to gain valuable nutritional info rmation about citrus trees. Leaf analysis can be helpful in the following ways: 1) It can reflect the citrus tree nutritional status with respect to most

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16 nutrients, but is particularly e ffective for nutrients that read ily move with soil water like N and K; 2) It can help confirm visual nutri ent deficiency symptoms; 3) It can reveal nutritional problems where none are suspected to exist because of absence of marked deficiency symptoms (Smith, 1966). Leaf analys is plays an important role in formulating an efficient fertilization program, since tre nds in leaf nutrient content may indicate whether the supply of a partic ular element is inadequate, satisfactory or unnecessarily high. Leaf tissue sampling has been used in many studies (Alva and Tucker, 1993; Dou and Alva, 1998; Obreza, 1993; Obreza et al., 1999) as a technique to determine the effects of particular fert ilizer sources and rates on gr owth and production of both young and mature citrus trees. In Florida, 4to 6-month-old spring flush leaves are sampled following the procedure described by Obreza et al. (1992). Five ranges (deficient, low, optimum, high, and excess) for each element ha ve been established to classify the nutritional status of mature, bearing trees Maintenance of leaf sample elemental concentrations in the opti mum range is desirable. Soil sampling can also be important in fe rtilization decisions but for long-term crops such as citrus, leaf sampling is a better indicator of the effectiveness of soil-applied fertilizers. Soil sampling should be used for onl y those elements that have low mobility in most soils (such as P, Ca, and Mg) as support information to help make future fertilization decisions (Obreza, et al., 1992). Controlled-Release Fertilizers CRFs are designed and manufactured to gra dually deliver nutrients to plants at a rate that fits plant physiol ogical requirements during gr owth, while simultaneously reducing nutrient loss potential since only a small fraction of the total application is

PAGE 27

17 present in a readily available form at a ny one moment (Oertli, 1980). This type of fertilizer can provide many benefits to agri culture, such as (i) higher fertilizer use efficiency; (ii) reduced nutrient losse s via leaching, ammonia volatilization and denitrification; (iii) savings in labor and equipment costs for transportation, preparation, and application of the fertilizer since larg e single fertilizer app lications are possible without causing stress or toxicity to plants ; (iv) less soil compaction or mechanical damage to crops since fewer field operati ons are necessary; and (v) reduction of soil chemical processes that decrease the availabi lity of nutrients, such as the fixation of P (Lunt, 1971; Oertli, 1980; Sharma, 1979). There are, however, some concerns rela ted to the use of CRF. With regard to fertilizer longevity, nutrient release patte rns from some CRFs unde r laboratory testing (data provided by the manufacturer) do not co rrelate with the actual nutrient release pattern under field conditions A study done by Meadows and Fuller (1983) revealed that nutrient release periods of several polymer-c oated CRFs were shorter than those claimed by the manufacturers. The initial nutrient rele ase with sulfur coated controlled-release fertilizers may be too rapid, causing damage to the crop a nd a higher fertilization cost compared with non-coated water soluble fertilizers (Trenkel, 1997). Lastly, with regard to residual effects, there is a possibility that nutrient release from CRF may continue during the non-cropped season and result in seri ous leaching losses (Shaviv and Mikkelsen, 1993). All the possible disadvantages from CRF mentioned above may result in environmental or crop damage and economic losses. Types of Controlled-Release Fertilizers Controlled-release fertilizers can be classi fied into four types: (i) materials of limited water solubility containing plant av ailable nutrients (e .g. metal ammonium

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18 phosphates); (ii) materials of limited water so lubility which, during their chemical and/or microbial decomposition, release plant availa ble nutrients (e.g. ureaforms, oxamides); (iii) water-soluble or relatively water soluble materials that graduall y decompose, thereby releasing plant available nutrien ts (e.g. guanylurea salts), and (iv) water soluble materials where dissolution is controlled by a physical barrier, e.g. by an impermeable or semiimpermeable coating (Hauck, 1985). Coated fe rtilizers represent the fastest growing segment in controlled release fertilizer techno logy because of their improved flexibility in nutrient release patterns compared with other CRF products, and the flexibility in controlling the release of othe r nutrients in addition to N (Sartain, 1999). Currently, CRF coating materials are composed of either sulf ur or polymeric materi als or hybrid products that utilize a multilayer coating of sulfur and polymer (Sartain and Kruse, 2001). As shown in Figure 2-2, for nutrient release of polymer-coated fertilizers (PCF), water (mainly vapor) passes in thr ough the coating. The vapor cond enses on the solid core and dissolves part of it, thus i nducing a build-up of internal pr essure. At this stage, two pathways are possible. If the internal pressure exceeds the membrane resistance, the coating ruptures and the entire content of the granule is re leased instantaneously. If the membrane resists the internal pressure, the fe rtilizer is released by diffusion driven by a concentration gradient across the coating, by ma ss flow driven by a pressure gradient or by combination of the two (Shaviv, 2001). For polymer/sulfur coated fertilizers, the nutrient release mechanism is through a comb ination of diffusion and capillary action. Water vapor must first diffuse through the continuous polymeric membrane layer. Once at the sulfur/polymer interface, the water subsequently penetrat es the defects in the sulfur coat through capillary action and solubilizes th e fertilizer core. The solubilized fertilizer

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19 then exits the particle in re verse sequence (Sartain and Krus e, 2001). PCFs are the most sophisticated and advanced means of controllin g fertilizer durability and nutrient release. The use of most polymer-coated products has been generally limited to high value applications due to the high cost of the coatings (Sartain, 1999). The potential of PCF to produce comparab le or improved plant growth compared with water-soluble forms has been demons trated. For example, 5-year-old bearing Hamlin orange trees responded better when a resin/Poly-S mixture was applied once per year at 101 kg N ha-1 than water-soluble fertilizer appl ied three times per year at 202 kg N ha-1 (Obreza and Rouse, 2004). However, there are still concerns about whether or not nutrient release patterns from PCFs matc h plant nutrient demands. A study by Cabrera (1997) indicated that despite similar longev ity ratings, the intensity and pattern of nutrient release can be significantly differe nt among polymer-coated CRFs. Therefore, there is a need to better understand the ba sis for differences in PCF nutrient release characteristics and the effects of environmen tal factors on PCF nutrient release patterns. An increased understanding of these factors co uld potentially lead to more efficient use of PCFs. Figure 2-2. Nutrient release mechan ism for polymer-coated fertilizers. P water va p o r dissolution rupture (“failure”) immediate release diffusion / mass flow swelling

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20 Predicting Nutrient Release from PCFs Efforts have been made during the last decade to develop empirical, semiempirical, and mechanistic models describing nu trient release from coated fertilizers. Most of these models were based on the assu mption that the release of nutrients from coated CRFs is either controlled by the rate of solute diffusion from the fertilizers or by the rate of water vapor pe netration into the CRF thr ough the coating (Shaviv, 2001). The nutrient release patterns of PCFs have been studied by several investigators. Shaviv (2001) described a diffusion release pr ocess from PCFs where the driving force is the vapor pressure gradient across the coati ng. This release course consists of three stages: (1) the initial stage during which almo st no release is observed (lag period), (2) the constant-release stage, and (3) the stage wh ere a gradual decay in release rate occurs. Kochba et al. (1990) considered nutrient rel ease to be a first-orde r kinetic process where water vapor movement into the fertilizer is the rate-limiting process. The authors suggested a release sequence consisting of two stages: wa ter vapor diffusion into the granule, and solution flow out of the coa ting. Ahmed et al. (1963) demonstrated that water condensation on salts increases with the lowering of va por pressure by the saturated salt solution and thus the rate of release is affected. Other investigators, such as Oertli and Lunt (1962) and L unt and Oertli (1962), used several elution and leaching experiments to conclude that the mechanis m controlling the nutrient release is the diffusion of salts out of the fertilizer granules. Factors Influencing PCF Nutrient Release Temperature Temperature is the most important envir onmental factor influencing PCF nutrient release (Oertli and Lunt, 1962) The nutrient release rate was found to significantly

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21 increase with an increase in temperature (e.g., an increase in temperature from 10 to 20 C almost doubled the initial releas e rate). Because the release rate increased much greater than would have been expected from a si mple diffusion mechanism, Oertli and Lunt (1962) speculated that properties of the co ating materials could possibly change with temperature. Ahmed et al. (1963) showed in a pot study th at nutrient release rate was directly related to temperature. The investigators s uggested that the direct relation between temperature and the rate of release could possibl y have been due to either an increase in viscosity of water at the lower temperature if it had entered as a liquid or to a reduction in water vapor pressure if it entered as a vapor Kochba et al. (1990) determined in a soil incubation study that the change of the nutrient release rate with temp erature is expected to be exponential since vapor pressure is an exponential function of temperature. Cabrera (1997) studied the N leaching patterns of diffe rent PCFs in containers under greenhouse conditions during the growing season. It was found that some PCFs exhibited N leaching patterns that closely followed changes in average daily ambient temperature over the season. This relationship was curvilinear, with N leaching rates being highly responsive to temperature changes between 20 and 25 C. Lamont et al. (1987) investigated the nutrient release rate of PCFs in beakers of distilled water at different temperatures. It was found that the nutrient release rate was affected by both incubation temperature and time. Generally, as temperature increased, nutrien t release increased. S ubsequently, nutrient release decreased with time after high initial release rates. Other Factors Oertli and Lunt (1962) found that the releas e rate was independent of pH as well as microbial activity. Coating thickness also had an effect on release rate. The release rates

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22 from heavily coated materials were relatively low and from lightly coated material were high. Furthermore, they determined that there was an effect of ionic species; nitrate and ammonia were released more rapidly than potassium and phosphate under comparable conditions. Lunt and Oertli (1962) found that moisture level exceeding the range of permanent wilting percentage to field capacity in a loam soil did not significantly affect the rate of nutrient transfer through the memb rane of coated fertilizers mi xed in the soil. This result supports the hypothesis of Kochba et al. (1990) that substrate vapor pressure is the ratelimiting step in nutrient release, since loweri ng the substrate moisture level within range of field capacity does not have a marked eff ect on the substrate vapor pressure. Likewise, they found that the time for nutrient releas e through a membrane was substantially extended if the fertilizer wa s top-dressed compared with incorporated. This finding was apparently due to intermittent drying of t op-dressed material between watering. Cabrera (1997) also found that top-dres sing decreased release rates re lative to incorporation. The release rate from PCF can also be altered by composition of the coating and the fertilizer N source being coated (Sartain and Kruse, 2001).

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23 CHAPTER 3 NUTRIENT RELEASE CHARACTERISTICS OF COATED FERTILIZERS UNDER GREENHOUSE AND FIELD CONDITIONS Introduction The efficient use of applied N fertiliz ers is influenced by soil, plant, and environmental conditions. High mobility of N fertilizers in deep sandy Florida soils and poorly distributed annual rainfall of around 1250 mm combine to make applied N highly leachable, therefore large N doses are require d to maintain high yield and quality of citrus. High N fertilizer rates may cause envi ronmental damage while at the same time increasing production costs. Cont rolled-release ferti lizers (CRF) are a possible alternative to minimize N losses to the environment and increase N uptake efficiency while meeting production goals of citrus growers. CRFs contain one or more plant nutrients in a form that extends their availability to the plant considerably longer than rapidly-available water-s oluble fertilizers. This gradual release of nutrients brings a potential to match plant nut rient demand. The use of CRFs has currently increased due to pressure from environmental groups and regulatory agencies to overcome environmental impacts. Hence, the use of controlled-release materials in fertilization programs is now being considered as a Best Management Practice (BMP). A BMP is defined as a “reco mmended technique that is technically and economically feasible, which will minimize wate r quality impact with no adverse effects on the agricultural production and/or quality, as well as net returns” (Alva et al., 2002). In 1994, the Florida legislature passed a ‘N itrogen Best Manageme nt Practice (N-BMP)’

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24 law that mandated the state to develop crop sp ecific nitrogen BMPs designed to meet groundwater standards. An interim BMP for c itrus was established at that time based on previous N rate studies a nd current IFAS recommendations. In 2002, a revised citrus BMP was established as an incentive-based program that contains fertilization and irrigation guidelines designed to minimize the ri sk of leaching nitrates from fertilizers to groundwater. Despite recommendation of CRFs in BMPs, th e lack of experience about their field performance is one reason why citrus grow ers avoid them. Information regarding the release periods and patterns of individual CRFs is needed to increase acceptance of CRFs for citrus production. Different techniques have been used to estimate release characteristics of controlled-r elease N-fertilizers. Accordi ng to Sharma (1979), the most direct and widely used technique is the so il incubation methodology that determines some or all of the mineral N released during CRF incubation in soil. Methods used for laboratory evaluation of N fertilizers include: determination of fertilizer fractions soluble in cold water, hot water, buffer solutions, or permanganate solution; direct incubation in soil; indirect incubation in soil; Neubauer tests; short-term nutrient uptake and microbiological assays. The objectives of this study were: 1. Determine N, P and K release patterns of four coated fertilizers and water-soluble fertilizer in a short-te rm laboratory incubation. 2. Measure the N release characteristics of the fertilizers in a long-term field evaluation. Materials and Methods Nutrient release patterns were simulta neously evaluated in greenhouse and field studies from spring 2004 to spring 2005.

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25 CRF Incubation and Nutrient Leaching Study Nitrogen (N), phosphorus (P) and potassium (K) release patterns were evaluated using a soil incubation-column leaching study in the greenhouse. Four controlled-release fertilizers (CRF) and a water-soluble produc t were compared for 270 days. CRFs were CitriBlen; Agrocote Type A; Agrocote Type C(D) and Agrocote Poly-S (Table 3-1). The soluble formulation was a Hydro 21-7-14 product. CitriBlen is a mixture composed of coated (Agrocote Type A; Agrocote Type C(D) and Agrocote PolyS) and water-soluble (Hydro, Potassium-magne sium sulfate, Potassium chloride and Iron) nutrients. Table 3-1. Nitrogen sources in each controlled-release fertilizers Source Formulation (N-P2O5-K2O) Ammoniacal % Nitrate % WSON1 % CitriBlen 15-3-19 5.3 4.5 5.2 Agrocote Type A 19-6-12 10 9 0 Agrocote Type C(D) 18-7-12 10 8 0 Agrocote Poly-S 37-0-0 0 0 37 1Water-soluble organic N (primarily urea) The leaching column technique as describe d by Sartain et al. (2004) was used in this study. A surface layer (0 to 5 cm dept h) of Arredondo fine sand (90 g) (Loamy siliceous, hyperthermic, Grossarenic Paleudul t) from central Florida was mixed with noncoated white sand (1710 g) and the equivale nt of 450 mg N from each source. These mixtures were placed in 30-cm long, 7.5cm di ameter PVC incubation lysimeters (Figure 3-1). The sand/soil/N source mixture was br ought to 10% moisture by adding 180 mL of 0.01% citric acid solution. A 50 mL beaker containing 20 mL of 0.2 M H2SO4 was placed in the head space of the incubation lysimeter as an ammonia tr ap. This solution was replaced and analyzed for NH4-N by titration every 7 days to dete rmine volatile-N. The soil columns were

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26 incubated at about 24 C in a greenhouse. Each lysimeter was leached after 7, 14, 28, 42, 56, 84, 112, 140, 180, 210, 240 and 270 days with one pore volume of 0.01% citric acid (500 mL) using a vacuum manifold for 2 min. Leachate volume was recorded and an aliquot was frozen for later analysis of N, P and K. All samples were analyzed for NO3-N and NH4-N using an air segmented Rapid Flow Analyzer (RFA). The concentration of urea-N was measured using a colorimetric method (Bremner, 1982). An estimation of the total N released with time was calculated by adding the three forms of N present in the leachate and the volatile-N. The concentrations of P and K in the leachates were analyzed at the University of Florida Analytical Research Laboratory following USEPA method 200.7 (USEPA, 1994) using an Inductively C oupled Plasma Spectrophotometer (ICP). The electrical conductivity (EC) and pH of each leachate fraction were also measured. Non-amended controls were included, and tr eatments were replicated four times in a randomized complete block design. Statisti cal analysis of data was performed using Statistical Analysis System (SAS) software (SAS Institute, 1999), and means were compared with Duncan’s Multiple Range Test ( =0.05). Figure 3-1. Incubation lysimeters.

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27 Field Mesh Bag Study A 1-year field study in a mature citrus tree environment was used to measure N release patterns of four contro lled-release fertilizers and a water-soluble material. Similar studies were conducted in central (Citrus Re search and Education Center, Lake Alfred) and southwest (Southwest Florida Research and Education Center, Immokalee) Florida simultaneously since most Florida citrus is grown under these rainfall and temperature conditions (Figure 3-2). Mesh bags (13 x 13 cm ) were constructed from typical fiberglass window screen, using heat to seal the edges. Each bag was filled w ith 3.5 g of elemental N from each source and then placed on the gr ound surface within the irrigated zone under bearing orange trees (Figure 3-3). Six trees were used as replicates. Each line of five bags contained the five fertilizer sources and was retrieved from the field on a given date. A control treatment (15-2-18) fe rtilizer consisting of water-soluble N, P and K (ammonium nitrate, concentrated superphosphate, and potassium chloride) was included as a conventional standard. It was applied thr ee times during the year (February, May and September), while controlled-release materials (Table 3-2) were applied only once at the beginning of the experiment. Figure 3-2. Field study locations. Lake Alfred Immokalee

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28 Figure 3-3. Layout of mesh bags placement under citrus tree canopy. Table 3-2. Controlled-release fertilizer specifications. Formulation (N-P2O5-K2O) Release duration (months)1 Principle source2 Source N P2O5 K2O CitriBlen 15-3-19 12 AN, AP, PSCU AP,CP KMgS, KS Agrocote Type A 19-6-12 3-4 AN,AP AP,CP KS Agrocote Type C(D) 18-7-12 12-14 AN,AP AP,CP KS Agrocote Poly-S 37-0-0 6 PSCU ------1Approximate at 21C soil temperature 2AN= ammonium nitrate; AP= ammonium phosphates; CP=calcium phosphate; PSCU= polymer sulfur coated urea; KMgS= pota ssium-magnesium sulfate; KS= potassium sulfate. Six replicates of each fertilizer material were removed from the field after 14, 28, 42, 60, 90, 120, 150, 180, 240, 300 and 360 days. They were air-dried in the greenhouse Tree trunk Emitter 30.5 cm 91.4 cm

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29 and then stored in plastic ba gs at room temperature for later analysis of urea-N, NO3-N, and NH4-N in residual fert ilizer granules. NO3-N, and NH4-N were analyzed using an air segmented Rapid Flow Analyzer (RFA) unit. The concentration of urea-N was measured using a colorimetric method (Bremner, 1982). To tal nitrogen (TN) in residual fertilizer granules was calculated by a dding the three N forms detected in the granules. An estimation of the TN released with time was calculated by subtracting the TN in the residual fertilizer granules from the 3.5g N applied. The average daily ambient temperature 60 cm above the ground and daily rainfall (mm) for both locations were collected fr om the Florida Automated Weather Network (FAWN). FAWN’s weather stations at bot h locations were located close to the experimental sites. Treatments were arrang ed in a randomized complete block design. Separation of means was accomplished with the general linear model procedure (PROC GLM) and single degree of freedom contrasts at P 0.05 (SAS Institute, 1999). Nonlinear regression curves were fitted to the N release data separately for each material at each location to develop N release curves. Results and Discussion CRF Incubation and Nutrient Leaching Study The pH of the leachate varied from 6.4 to 7.1 in the non-amended soil. The soil amended with CRFs maintained a pH between 5.0 and 6.9 until the fifth leaching event (56 days) and then gradually decreased to 4.0 until the termination of the experiment. This low pH was probably a result of the citr ic acid solution used in the study. However, pH of the leachate from soil columns ame nded with a urea-based controlled release fertilizer (Agrocote Poly-S) increased fr om 6.0 to 7.0 during the initial three leaching events and then decreased to pH 5.5. The initia l increase in leachate pH was likely due to

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30 hydrolysis of urea into ammonium carbonate through the action of the urease enzyme. The pH of leachate then decrea sed as a result of production of nitrate through nitrification (Paramavisan and Alva, 1997). N was recovere d from the soil columns amended with CRFs despite the low pH of 4.0 observed durin g the last leaching events, since their nutrient release mechanism is not pH depende nt. However, the lower N recovery from Agrocote Poly-S was probably due to low pH since the rate of rel ease from this type of fertilizer is affected by pH and microbial activity. Among the coated fertilizers, CitriBlen had the highest initial N release. This result was expected since water-soluble N co mponents are present in this blend. After the completion of the twelve leaching events, th e cumulative recoveries of total N in the leachate were 90, 86, 85, 82 and 69% of the to tal N applied as CitriBlen, Hydro, Agrocote Type C(D), Agrocote Type A and Agrocote Poly-S respectively. Almost all N applied as Hydro was leached after the 1st week. The low recovery of total N from the soil amended with Agro cote Poly-S could also be explained by losses through denitrification, sin ce minimum losses of N due to NH3 volatilization were obtained. Furthermore, it was likely that there was still some N left inside the prills after the 270 day incubation. Table 3-3 shows the e ffect of N source on N release rate. The total NH4-N recovered in twelve leachates was 46.6, 35.8, 33.8, 31.4 and 27.9% of the total N applied as Hydro, C itriBlen, Agrocote Type C(D), Agrocote Type A, and Agrocote Poly-S respectively (Figure 3-4, for each fertilizer, bars with the same letter were not significantly di fferent). The peak concentration of NH4-N for all fertilizers but Agrocote Type C(D) was pr esent after 7 days of incubation and then decreased gradually (Figure 3-5). The decrease in leaching of NH4-N was likely due to

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31 the transformation of NH4-N to NO3-N by nitrification. A gradua l increase in leaching of NH4-N observed from the first to the eighth leaching event from Agrocote Type C(D) was likely due to its slower release characte ristics compared with the other materials. CitriBlenAgr.Type (A)Agr.Type C(D)Agr. Poly-SHydro N forms in leachate (% of applied) 0 10 20 30 40 50 60 Urea NH 4 -N NO 3 -N A B C C A A A B B A B A Figure 3-4. Cumulative leaching of N forms. In the case of soils amended with urea-b ased CRFs, peak concentrations of urea (1.7 and 3.0% of total N app lied as CitriBlen and Agro cote Poly-S respectively) occurred in the first leachate and then d ecreased significantly by th e third leachate. As shown in Figure 3-5, leaching of urea stopped after the third event as a result of the hydrolysis of urea. Similar leaching behavior s were expected from both materials since Agrocote Poly-S is the only urea-based component of CitriBlen. The total NO3-N recovered during the expe riment accounted for 51.2, 50.5, 50.4, 39.4 and 35.5% of the total N applied as Citr iBlen, Agrocote Type C(D), Agrocote Type A, Hydro, and Agrocote Poly-S respectively (Figure 3-4). For urea-based CRFs, a gradual decrease in urea-N after the first leachate and a stable increase of NO3-N

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32 in the subsequent leachate fractions suggested that the urea-N releas ed was being rapidly hydrolyzed and nitrif ied (Figure 3-5). 0 50 100 150 200 Cumulative N Leached (mg) 0 50 100 150 200 CitriBlen Agrocote Type A Agrocote Type C(D) Agrocote Poly-S Hydro Days Incubation 050100150200250300 0 50 100 150 200 Urea-N NH 4 -N NO 3 -N Figure 3-5. Leaching of N forms from soil columns.

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33Table 3-3. Effect of N source on total N released from four CRFs with time. Total Nitrogen Released (mg) Time (days) N Source 7 14 28 42 56 84 112 140 180 210 240 270 Cumul. (mg) CitriBlen 113.6a 13.8b 28.7b 52.1a 24.9b 49.5a 38.9a 40.4b 23.5b 9.3b 6.9a 1.6ab 403.1a Agrocote Type A 62.2b 25.7a 38.9ab 53.9a 37.3a 53.3a 45.8a 22.4b 13.6b 8.5b 5.4a 2.8ab 369.6b Agrocote Type C(D) 20.9c 14.7b 26.7b 43.0a 24.6b 45.2a 42.3a 68.5a 54.3a 23.8a 12.6a 5.2a 381.6ab Agrocote Poly-S 25.4c 18.8b 44.7a 44.4a 25.1b 43.5a 39.9a 26.3b 22.4b 7.3b 9.2a 3.1ab 309.9c Statistical Significance1 *** * NS ** NS NS ** ** *** NS ** 1NS = not significant, *= significant P<0.05, **= significant P<0.01 and ***= significant P<0.001. 2Means with the same letter within columns are not significantly different.

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34 Inorganic P added to soil that is not absorbed by plant roots or immobilized by microorganisms can be adsorbed to minera l surfaces or precipitated as secondary P compounds. Surface adsorption and precipitation r eactions collectively are called fixation or retention (Havlin et., al 1999) Relatively little P leached from any fertilizer treatment suggested P fixation in the soil columns. P leaching was appreciably retarded with all CRFs except CitriBlen (Figure 3-6). A high initial P release from CitriBlen was expected since 40% of its P2O5 is water-soluble, then a similar lag period was observed between the third and fifth leaching event. P leached from all CRFs generally increased with time after the fifth leachate (56 days of incubation). Total P leached was higher for the Hydro formulation than for the CRFs since it is a readily-soluble material. Days Incubation 050100150200250300 Cumulative P Released (%applied) 0 5 10 15 20 25 30 35 40 CitriBlen Agrocote Type A Agrocote Type C(D) Hydro Figure 3-6. Phosphorus leached from soil columns. The ionic composition of th e fertilizer source and the charged component of the soil have a significant influence on leaching loss es of K source fertilizers. The soil used in this experiment (Arredondo fine sand) is composed of 960 g kg-1 sand and has a cation

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35 exchange capacity (CEC) of 7.7 cmol(+) kg-1 (Sartain, 2002), thus the potential for K leaching is great. Sartain (2002) reported sign ificant K leaching from this soil. As shown in Figure. 3-7, small differences in the tota l quantity of K leached relative to K sources were obtained. Of all the materials studied, the Hydro formulation leached the largest quantity of K. This result was expected since it is a water-soluble formulation. However, a similar trend was obtained from one of th e CRFs (CitriBlen), likely due to the large amount (80%) of water-soluble K components present in this blend. K recovered from these materials (CitriBlen and Hydro) was gr eater than the amount applied. It is likely that the actual amount of K2O in these fertilizer granules was greater than the claimed analysis. Agrocote Type A and Agrocote Ty pe C(D) did not diffe r in quantity of K leached. Similar K leaching might have been due to the same ionic composition of the K source (K2SO4) and the same amount (83%) of K coated for slow-release. Potassium Sources CitriBlenAgrocote Type (A)Agrocote Type C(D)Hydro Total K Leached (% applied) 0 20 40 60 80 100 120 A B B A Figure 3-7. Effect of K source on the quant ity of K leached from soil columns. Potassium release patterns are presented in Figure 3-8. Timing and quantity of K leached was influenced by K source. Most of the K from Hydro leached during the 1st

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36 week and then decreased for the next two ev ents, at which point all the applied K had been leached. Similarly, CitriBlen leached 85% of the applied K after 1 week and then declined during the rest of the experiment. So me retardation in quantity of K leached was observed with Agrocote Type A and Type C(D). For Agrocote Type C(D), the initial peak of K was delayed for longer (81 days). A slower release of K from this material was expected since it has a thicke r polymer coating. This resu lt showed the influence of coating technology on K release. Days Incubation 050100150200250300 Cumulative K Released (% of applied) 0 20 40 60 80 100 CitriBlen Agrocote Type A Agrocote Type C(D) Hydro Figure 3-8. Effect of K source on K leaching. Field Mesh Bag Study After 365 days in the field, the percentages of N released were 99, 95, 93 and 88% of the total N applied as Agrocote Type A, CitriBlen, Agrocote Poly-S, and Agrocote Type C(D), respectively at Imm okalee, and 97, 90, 81, and 79% of the total N applied as Agrocote Type A, CitriBlen Agrocote Poly-S, and Agrocote Type C(D), respectively at Lake Alfred (Figure 3-9). The entire N from the water-soluble

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37 formulation was released afte r the first rainfall it was e xposed to, which was a 5-cm event. N Released (%applied) 0 20 40 60 80 100N Released (%applied) 050100150200250300350400 0 20 40 60 80 100 Da y s in field 050100150200250300350400 CitriBlen Agrocote Type A Agrocote Type C (D) Agrocote Poly-S Immokalee Lake Alfred Figure 3-9. Nitrogen released (% of applied) with time. The effect of N source, location and the interaction of these factors (N source x location) on N release rates are shown in Tabl e 3-4. The interaction of the two factors had generally no effect on N rele ase during the study. N sources had a significant influence on the quantity of to tal N released.

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38 Table 3-4. Effect of controlled-release nitr ogen fertilizer type and location on N release rates (% of applied). Main Effect1 Days in field Fertilizer Source Location Source x Location 14 *** NS NS 28 ** NS NS 42 *** ** 60 *** ** NS 90 *** ** NS 120 *** *** NS 150 *** *** 180 *** *** NS 240 *** *** *** 300 *** *** *** 360 *** *** 1Indicates whether main effects of fertilizer type, location, or fert ilizer type x location affected the data. NS, *, **, *** represent no t significant, and significant where P< 0.05, 0.01 and 0.001, respectively. Single degree of freedom contrasts were us ed to compare means of total N released among N sources (Table 3-5). Generally, N re leased from CitriBlen was significantly different from the other N sources. Simila r release patterns among N sources were observed after 180 days in the field at Imm okalee. When comparing Agrocote Type A with Agrocote Type C(D), a highly signifi cant difference in N release rates was found during the entire study. Although both are resin coated materials, a slower N release rate was expected from Agrocote Type C(D) sin ce it has a thicker coating. However, when Agrocote Type C(D) was compared with Ag rocote Poly-S (polymer/sulfur coating), no differences in N release patterns were observed periodically during the experiment. This result suggested that polymer/sulfur coat ed fertilizers (PSCF) have the potential to release N approaching polymer-coated fertiliz ers performance. This is an interesting finding since PSCFs are produced at a much reduced cost and therefore are more affordable to farmers

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39 Table 3-5. Percentage of controlled-release nitr ogen (CRN) fertilizers released with time. DIF1 CRN Type2 Contrast3 1 2 3 4 1 vs. rest 2 vs. 3 3 vs. 4 14 IMM4 26.4 15.9 2.8 7.6 *** *** LAL 25.7 22.3 2.2 15.2 ** 28 IMM 35.5 33.6 17.9 22.4 NS NS NS LAL 34.9 26.8 8.2 26.2 * 42 IMM 51.8 50.6 36.0 27.0 *** ** LAL 35.3 30.6 14.7 29.2 NS * 60 IMM 59.4 56.2 33.9 49.4 ** *** ** LAL 45.4 37.5 22.5 32.8 *** ** 90 IMM 64.3 65.3 46.3 56.6 ** *** ** LAL 59.9 58.0 42.1 45.0 ** ** NS 120 IMM 76.5 82.7 54.6 61.8 *** *** LAL 58.2 67.9 42.8 53.6 NS *** ** 150 IMM 81.8 89.2 62.8 66.4 *** NS LAL 68.7 75.4 42.6 62.3 ** *** *** 180 IMM 85.7 91.7 67.4 79.1 ** *** *** LAL 77.6 85.1 51.5 66.2 ** *** ** 240 IMM 92.2 97.1 83.9 85.4 NS *** NS LAL 83.9 92.8 61.2 72.9 ** *** *** 300 IMM 94.9 98.6 90.7 86.3 NS ** NS LAL 87.3 94.6 71.5 75.2 ** *** NS 360 IMM 94.7 99.0 88.8 93.0 NS *** NS LAL 89.7 96.5 78.8 80.6 NS *** NS 1Days in field 2CRN type: 1, 2, 3 and 4 represent CitriBlen, Agrocote Type A, Agrocote Type C(D) and Agrocote Poly-S, respectively. 3Single degree of freedom contrasts were generated using SAS GLM Proc. NS, *, **, *** represent not significant, and significa nt where P< 0.05, 0.01 and 0.001, respectively. 4IMM and LAL represent Immokalee and Lake Alfred, respectively.

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40 Location had a significant influence on N rel ease rate. In general, slower release rates and less N released dur ing the 365-day experimental period were observed at Lake Alfred. For this study temperature and rain fall were considered as the potential environmental factors that caused differentia l N release rates between locations. The 12month average temperatures were 22.3 a nd 22.0 C for Immokalee and Lake Alfred, respectively. Figure 3-10 compares daily av erage temperatures during the study for both locations. Temperature trends were very similar between locations. Sporadic lower temperatures were observed after 250 days at Lake Alfred. However, at this point most N had been released from all sources. Since little difference was found in annual average temperature and temperature trends between locations, it was concluded that temperature was not the main reason for differential N release rates between locations. Total amount of rainfall and irrigation were used as an estimation of the amount of water received by the fertilizers during the e xperiment. The trees were irrigated using under-canopy microsprinklers (one emitter per tree). Irrigation was scheduled based on rainfall and season (spring/fa ll or winter) and it was normally applied when there was no rain and delayed when rainfall occurred. At Immokalee, Maxi-jet green jets with a delivery rate of 6.05 x 10-2 m3 h-1 were used to irrigate th ree times per week for 4 hours per application. At Lake Alfred, Maxi-jet vi olet jets with a delivery rate of 5.79 x 10-2 m3 h-1 were used to irrigate twice per week fo r 4 hours per application. An estimation of the total amount of water received by the fertil izers through irrigation was calculated based on these parameters. It was found that approx imately twice as much irrigation water was delivered at Immokalee (23.22 m3) compared with Lake Alfred (11.06 m3), thus the fertilizer bags at Immokalee were exposed to more irrigation.

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41 Days 050100150200250300350 Average daily temperature (C) 5 10 15 20 25 30 Immokalee Lake Alfred Figure 3-10. Comparison of average daily temperature (C) between locations. Rainfall distribution at each location is compared in Figure 3-11. The total volumes of rainfall during the study were 829 a nd 854 mm for Immokalee and Lake Alfred, respectively. Although total rainfall was very sim ilar between locations, rainfall distribution was not the same. There were mo re rainfall events at Immokalee (92 rainy days) than at Lake Alfred (77 rainy days). Quantity and frequency of irrigation and rainfall did probably influence the N release rates between locations. Less and slower N release rates observed at Lake Alfred were presumably due to more frequent intermittent drying of fertilizer materials between wetti ng by irrigation or rainfall. Similar results were found by Kochba et al. (1990). Furthermore, differences in orchard orie ntation may have al so contributed to different N release patterns between locatio ns. At Immokalee, ro ws were north-south oriented while at Lake Alfred they were orie nted in an east-west direction (Figure 3-12). In citrus orchards, more sunlight is intercep ted by trees planted in rows oriented northsouth than east-west (Tucker et al., 1994) Thus, the amount of sunlight that was intercepted by the fertilizer bags on the ground was greater in rows planted in an east-

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42 west direction than north-south. This occurrence resulted in more fr equent drying periods of the fertilizer granules which extended the ti me for N release through the coating. Rainfall (mm) 0 20 40 60 80 100 Days 050100150200250300350 Rainfall (mm) 0 20 40 60 80 100 Immokalee Lake Alfred Figure 3-11.Comparison of rainfall di stribution (mm) between locations. North – South East West Figure 3-12. Citrus orchard orientation at Immokalee and Lake Alfred, respectively.

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43 Nitrogen Release Curves Regression coefficients, R2 and P values of the non-lin ear regression equations are provided in Table 3-6 and 37 for Immokalee and Lake Alfred, respectively. The R2 values for all the equations at both locations were close to unity, and all relationships were statistically significant at the P < 0.0001. This result indicated that the equations provided a good approximation of the N release ra te (% of applied) for a given time in the field. Figures 3-13 and 3-14 show N release curves for all materials at both locations. Table 3-6. Regression analysis of estimated N release rate from different N sources against time using an exponential rise to a maximum model (Immokalee). N Source Y0 a b R2 P-value CitriBlen1 15.16 80.90 0.011 0.99 <0.0001 Agrocote Type A2 99.85 0.014 0.99 <0.0001 Agrocote Type C(D)2 98.79 0.007 0.98 <0.0001 Agrocote Poly-S)2 93.70 0.009 0.98 <0.0001 1Y = Y0 + a (1exp-bx) where X = time, Y0 = mean value of %NR when t equals zero and a, and b are regression coefficients. 2Y = a (1-exp-bx) where X = time and a, and b are regression coefficients. Table 3-7. Regression analysis of estimated N release rate from different N sources against time using an exponential rise to a maximum model (Lake Alfred). N Source Y0 a b R2 P-value CitriBlen1 18.38 78.40 0.007 0.98 <0.0001 Agrocote Type A1 7.11 97.98 0.008 0.99 <0.0001 Agrocote Type C(D)2 95.61 0.005 0.97 <0.0001 Agrocote Poly-S)1 8.52 76.87 0.007 0.99 <0.0001 1Y = Y0 + a (1exp-bx) where X = time, Y0 = mean value of %NR when t equals zero and a, and b are regression coefficients. 2Y = a (1-exp-bx) where X = time and a, and b are regression coefficients.

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44 050100150200250300350400 N Released (%applied) 0 20 40 60 80 100N Released (%applied) 050100150200250300350400 050100150200250300350400 0 20 40 60 80 100 Days in field 050100150200250300350400 Release Data Fitted curve release data Agrocote Type A CitriBlen Immokalee Lake Alfred Immokalee Lake Alfred Figure 3-13. Nitrogen release curves for CitriBlen and Agrocote Type A at Immokalee and Lake Alfred, respectively.

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45 050100150200250300350400 N Released (%applied) 0 20 40 60 80 100N Released (%applied) 050100150200250300350400 050100150200250300350400 0 20 40 60 80 100 Days in field 050100150200250300350400 Release Data Fitted curve release data Agrocote Poly-S Agrocote Type C(D)Immokalee Lake Alfred ImmokaleeLake Alfred Figure 3-14. Nitrogen release cu rves for Agrocote Type C(D) and Agrocote Poly-S at Immokalee and Lake Alfred, respectively.

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46 Conclusions CRF Incubation and Nutrient Leaching Study N source had a significant effect on N rel eased to the soil solution. Among the controlled-release formulations, N release followed the order: CitriBlen > Agrocote Type C(D) > Agrocote Type A > Agro cote Poly-S, and the proportion of NO3-N in twelve leachate fractions was gr eater than either urea-N or NH4-N. This result indicated that nitrification was taking place and thus suggested that the system was microbiologically active, mimicking the condi tions of a natural so il system. Little P recovered (~30% of applied) from any fertiliz er treatment suggested P fixation in the soil due to chemical reactions. Incorporation of P in the soil columns also made P more vulnerable to fixation. Fertilizer source had a significant effect on quantity and timing of K released to the soil. The low CEC of the soil also accelerat ed K leaching. Rapid release of N, P and K from the Hydro formulation was due to its large water solubility. This study demonstrated that the time to transfer a gi ven fraction of N, P and K through a membrane was considerably longer with CRFs applications than water-soluble fer tilizers. Also, N, P and K release patterns varied depending on composition and thickness of the coating. Field Mesh Bag Study CRFs showed different intensities and patte rns of N release due to differences in coating material and technology. The N release patterns measured were similar to those claimed by the manufacturer. Environmenta l conditions were more favorable for N release at Immokalee than at Lake Alfre d. Despite differences in total amount of N released between locations, N release patterns at both locations followed the same order: Agrocote Type A > CitriBlen > Agroco te Poly-S > Agrocote Type C (D).

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47 Potential factors affecting these differences were quantity and freque ncy of irrigation and rainfall, and orchard orientation. It is susp ected that intermittent drying of fertilizer granules between wetting by irrigation or rainfall extended the time for N release. CitriBlen, a complete N-P-K controlledrelease fertilizer composed mostly of coated nutrients is made and marketed exclus ively for mature Florida citrus as a single annual application material. CitriBlen was de veloped to gradually release nutrients in such a way that matches tree nutritional requirements and thus increases nutrient uptake efficiency while reducing nutrient losses to the environment. In addition, CitriBlen nutrient release mechanism is temperature depe ndent and therefore it potentially provides nutrients to the tree anytime growth is indu ced as a result of warm growing conditions. Citrus trees require the highe st amount of nutrients for each year from late winter through early summer when flowering and fr uit development compete with the spring flush of growth. After the flower-fruitlet sh edding process is completed in May-June, the tree is left with only the fru it it can satisfactorily support to maturity. Fewer nutrients are required for fruit development after this peri od, with the best fruit quality being obtained with moderately low nutritional levels, main ly N, during fall and early winter. Based on these nutrient requirements, current UF-IFAS citrus fertilizer guidelines recommend that 2/3 of the tree nutritional requirements s hould be made available between March and June 15th (105 day period) and the remaining 1/3 can be applied after September 15th. This study demonstrated that N release pa tterns from CitriBlen matched tree nutritional requirements recommended by BMPs The dashed line in Figure 3-15 shows that after 105 days in the field approx imately 70 and 60% of total N applied as

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48 CitriBlen was released at Immokalee and Lake Alfred, respectiv ely. Then a gradual release of the remaining N was observed until termination of the 1-yr field experiment. Days in field 050100150200250300350400 N Released (%applied) 0 20 40 60 80 100 Immokalee Lake Alfred Figure 3-15. CitriBlen N release curves for Immokalee and Lake Alfred.

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49 CHAPTER 4 EVALUATION OF CITRIBLEN ON FR UIT PRODUCTION AND FOLIAR NUTRIENT STATUS OF MATURE CITRUS TREES Introduction A structured fertilization ma nagement program is needed to ensure high yields and optimal fruit quality while minimizing environm ental impacts and costs. Leaf analysis is the best indicator of proper fertilization in l ong-term crops such as ci trus. It is a useful management tool for making fertilization de cisions since the composition of the plant tissue reflects prior fertiliza tion and production practices (Tucke r et al., 1995). It can also assist with diagnostic problems within the orchard or reveal symptomless nutritional problems. Leaves have to be properly samp led, handled, processed and analyzed to ensure that analytical results are meaningful and can be used as guidelines for managing citrus nutritional programs. In Florida, 4to 6-month-old spring flush leaves are sampled following the procedure describe d by Obreza et al. (1992). CRFs have the potential to produce comparable or impr oved fruit yield relative to water-soluble fertilizer. Many studies have evaluated the effects of CRFs on production of both young and mature citrus trees. Increa sed fruit yield has been reported using controlled-release sources of N compared with water solu ble sources (Koo, 1986; Alva and Paramasivam, 1998; Obreza et al., 1999). The economic feasibility however of using CRFs exclusively to produce citrus needs to be further evaluated. The objectives of this study were:

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50 1. Compare leaf nutrient status of commercial orange trees subjected to a controlledrelease nutrient management program with trees fertilized using a conventional water-soluble fertilizer program. 2. Evaluate the economic feasibility of using a controlled-release fertilizer program relative to a conventional water-solu ble nutrient management program for commercial orange production. Materials and Methods Three commercial citrus orchards in southwest Florida (C ollier and Hendry counties) and one in central Florida (Polk county) were used to assess the potential use of CitriBlen on mature citrus production and nutrition. All management practices were done on a commercial basis. The orchards us ed in this study repr esent the two major sections of the citrus industry. Three of the four sites were located on poorly drained soils on the flatwoods (site A, B, and C). The four th site (D) located in central Florida was planted on well drained sandy Entisols. Characteri stics of the studied citrus orchards are described below. Site A Characteristics of site A are shown in Table 4-1. Be ginning in 2000, CRF (CitriBlen) was applied once per year (late March) at a rate of 101 kg N ha-1 yr-1 while the soluble conventional fertilizer was a pplied four times (March, June, August and October) at a rate of 202 kg N ha-1 year-1. Blocks 1 and 3 received the conventional water-soluble treatment during the first 2 year s, and then CitriBlen was applied for the rest of the experiment. Blocks 2 and 4 rece ived CitriBlen and the conventional watersoluble fertilizer, respectively, during the entire experiment.

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51 Table 4-1. Characteristics of site A. Variables Block number/(total area of the block in the study) Block 1 (25 ha) Block 2 (38 ha) Block 3 (34 ha) Block 4 (18 ha) Scion Hamlin Hamlin Hamlin Hamlin Rootstock Swingle Cleopatra Cleopatra Carrizo Year planted 1987 1987 1987 1987 Tree density (no. ha-1) 373 299 299 299 Spacing (m.) 3.7 x 7.3 4.6 x 7.3 4.6 x 7.3 4.6 x 7.3 Percentage of resets 15 20 11 5 Site B Scion/rootstock combinations at site B are shown in Table 4-2. Two treatments were applied to each block. The water-solu ble N standard was applied three times per year (late March, June and September) at an N rate depending on fruit production. Trees received 202 kg N ha-1 year-1 in 2002 and 224 kg N ha-1 year-1 from 2003 through 2005. CitriBlen was applied once per year in late March at a rate of 50% of the total N applied to the standard plots. Table 4-2. Characteristics of site B. Block number/(total area of the block in the study) Variables Block 1 (113 ha) Block 2 (30 ha) Scion Valencia Hamlin Rootstock Carrizo Carrizo Site C An 8-ha fertilizer source comparison wa s conducted on mature Valencia orange trees budded on Swingle citrumelo (Citrus paradisi Macf. x Poncirus trifoliata (L.) Raf.) rootstock planted in 1990 in a commercial citr us orchard. Tree rows were spaced 7.3 m apart, with 3.0 m between trees within each row (448 trees ha-1). Percentage of non-

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52 bearing resets was negligible Conventional fertilization prac tices consisted of a watersoluble fertilizer compound a pplied three times per year (late March, June and September) at a rate of 224 kg N ha-1 year-1 from 2000 to 2002. Then CitriBlen was applied once per year at a rate of 101 kg N ha-1 year-1 from 2003 to 2005. Site D The orchard characteristics are shown in Table 4-3. Percentage of non-bearing resets was negligible. Three fertilizer treatments were applied in this orchard. A fertigation program that consisted of 202 kg N ha-1 year-1 split in 4 applications per year (February, March, August, and October) was applied to blocks 1 and 4. A conventional dry water-soluble fertilizer compound was applied to blocks 2 and 3 under the same specifications as the liquid program. CitriBle n was applied to block 5 once per year (late March) at a rate of 101 kg N ha-1 year-1. Table 4-3. Characteristics of site D. Variables Block number/(total area of the block in the study) #1 (11 ha) #2 (11 ha) #3 (11 ha) #4 (12 ha) #5 (8 ha) Scion Valencia Valencia Valencia Valencia Valencia Rootstock Cleopatra / Carrizo Carrizo Cleopatra / Carrizo Carrizo Cleopatra Year Planted 1997 1998 1997 1998 1997 Trees (ha-1) 287 287 287 287 287 Spacing (m.) 4.6 x 7.6 4.6 x 7.6 4.6 x 7.6 4.6 x 7.6 4.6 x 7.6 Leaf Sampling of Commercial Citrus Orchards Leaf tissue was sampled at sites A, B a nd D. Treatment blocks were partitioned into management units of approximately 8 ha and about 20 trees were sampled within each management unit. About 100 4-mont h-old spring flush leaves from each management unit were collected from non-fr uiting twigs in late August 2004 and 2005.

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53 Leaves at the edge of orchard blocks were avoided when sampling because of possible surface contamination that could lead to meas urement errors. The leaves were dried at 70 C for 3 days and then finely-ground. Sample s were sent to a co mmercial agricultural laboratory for analysis of total N, P, K, Ca and Mg concentrations. Statistical analysis was performed on the leaf tissue data independe ntly for each site and sampling date using the Statistical Analysis System (S AS) software (SAS Institute, 1999). Economics of CitriBlen Use on Commercial Mature Citrus Trees A partial budget analysis compared the costs and benefits of using a CRF (CitriBlen) program with a conventional wa ter-soluble fertilizer program for mature orange trees. Partial budgeting is a planning and decision-ma king tool used to compare the costs and benefits of a lternatives faced by a farm bus iness. It focuses only on the changes in income and expenses that w ould result from implementing a specific alternative while all aspect s of farm profits that ar e unchanged by the decision are ignored (Roth and Hyde, 2002). It is based on the principle that a small change in a farm business eliminates or reduces some costs and returns, adds costs, and/or adds revenues. Costs of the fertilization programs were estimated by adding the fertilizer product and application cost for each or chard. Fertilizer cost ($ ha-1) was calculated by multiplying the price of fertilizer ($ kg-1 product) by its applica tion rate (kg product ha-1). Thus, fertilization costs varied based on fertilizer rate, sour ce, and application frequency. Fruit yield data for each block were obtained from the growers for sites A, C and D in terms of quantity (box ha-1)1 and quality (pound-solids box-1)2. 1 One box is equal to 41kg for oranges. 2 Pound-solids per box is an expression of total solubl e solids per unit weight of fruit and is the basis on which a grower gets paid for his fruit.

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54 The standard water-soluble fertilizat ion program was taken as 202 kg N ha-1 year-1 split in four applications per year, while the CitriBlen program consisted of 101 kg N ha-1 year-1 applied once per year. The CitriBlen (15-3-19-2.5Mg) price was obtained from the distributor. The price of the st andard water-soluble formulation (15-2-152.4Mg) and the average cost of a single dry fertilizer applicati on including labor and equipment were taken from Muraro et al. ( 2004). Even though the two fertilizer products did not have exactly the same P and K analysis, they were considered to be equal for this economic analysis. Results and Discussion Leaf Sampling of Commercial Citrus Orchards Results are described independently for each study site due to differences in scion/rootstock combinations and treatment applications. Site A No statistical analysis was performed on th ese data due to diffe rences in rootstock types between blocks. Mean le af nutrient concentrations ar e summarized in Table 4-4. Similar leaf N concentration trends were observed for both sampli ng dates. Trees that received only the conventional water-soluble fertilizer (Block 4) had numerically the highest leaf N content. Since water-soluble N was applied to those trees only a few weeks before sampling, leaf N concentration was e xpected to be the hi ghest among blocks. A variation in leaf nut rient concentrations was obser ved from 2004 to 2005. Yearly variations in macronutrient c oncentrations have been c onfirmed in other long-term studies.

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55 Table 4-4. Effect of soluble and controlled-rel ease fertilizers on N, P, K, Ca, and Mg (site A). Mean leaf element concentration (%) Block N P K Mg Ca 08/0408/05 08/0408/0508/04 08/0508/0408/05 08/04 08/05 1 2.84 2.53 0.13 0.16 1.09 1.45 0.31 0.32 4.50 4.21 2 2.69 2.20 0.14 0.16 1.39 1.74 0.38 0.40 3.97 4.76 3 2.70 2.22 0.13 0.16 1.45 1.78 0.36 0.42 4.10 4.75 4 2.92 2.54 0.13 0.15 1.38 1.60 0.36 0.36 4.41 4.80 In fact, a consistent pattern from year to year appears to be the exception rather than the rule. Various phenologi cal factors such as light intensity, temperature, relative humidity and water availability interact with edaphic and physiological factors in such a way as to produce profound changes in leaf com position from one year to the next (Smith 1966). Lower leaf N occurred when the rainfall was about 35% greate r than the previous year. Perhaps because of increased vegetative growth of the trees, the concentration of N was lower in the leaves during this year. Generally all treatments resulted in leaf P, K, Ca and Mg status in the optimum range according to current guidelines (Table 45). Overall, trees that were planted on the same rootstock type (blocks 2 and 3) showed similar leaf c oncentrations for all nutrients. This result suggested that r ootstock type may have influe nced leaf nutrient status. Table 4-5. Leaf analysis standards for mature bearing citrus trees based on 4 to 6-month old spring-cycle leaves from nonfruiting terminals. Element Deficient Low Optimum High Excessive Nitrogen (N) (%) <2.2 2.2-2.4 2.5-2.8 2.9-3.2 >3.3 Phosphorus (P) (%) <0.09 0.09-0.11 0.12-0.17 0.18-0.29 >0.30 Potassium (K) (%) <0.7 0.7-1.1 1.2-1.7 1.8-2.3 >2.4 Calcium (Ca) (%) <1.5 1.5-2.9 3.0-5.0 5.1-6.9 >7.0 Magnesium (Mg) (%) <0.20 0.20-0.29 0.30-0.50 0.51-0.70 >0.80 Adapted from Obreza et al. (1992).

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56 Site B The effect of fertilizer source on leaf nutri ent concentration is shown in Table 4-6. For Block 1, leaf nutrient concentrations did not differ between treatments on any sampling date, except for K and Mg in the first year. Trees treated with the standard water-soluble fertilizer had the highest leaf N. However, leaf N concentrations of CitriBlen-treated trees were within the high range (Table 45) and then decreased to the optimum range. Leaf P, K, Mg and Ca were within optimum ranges through the entire study regardless of treatment. For Block 2, leaf nutrient concentrations did not differ between treatments either year. CitriBlen-treated tr ees had the highest leaf N in August 2005, while the conventional standard had the highest leaf N in August 2004 and the lowest in August 2005. These results suggested that the N in the water-soluble fe rtilizer had short residual effects on leaf N compared with that in CitriBlen. A study by Zekri and Koo (1992) showed similar results. All treatments resulted in leaf P, K, Mg and Ca status in the optimum or high range according to guidelines (Table 4-5). Table 4-6. Effect of soluble and controlled-rel ease fertilizers on N, P, K, Ca, and Mg (site B). Mean leaf element concentration (%) Source N P K Mg Ca ----------------------------------Block 1------------------------------------08/04 08/05 08/0408/0508/04 08/0508/0408/05 08/0408/05 CitriBlen 2.90 2.54 0.15 0.18 1.32 1.45 0.47 0.48 3.62 3.70 Std. Sol. 2.96 2.60 0.15 0.17 1.45* 1.48 0.43* 0.45 3.52 3.68 ----------------------------------Block 2------------------------------------08/04 08/05 08/0408/0508/04 08/0508/0408/05 08/0408/05 CitriBlen 2.52 2.61 0.15 0.19 1.35 1.26 0.44 0.41 3.97 4.13 Std. Sol. 2.64 2.49 0.14 0.17 1.37 1.32 0.42 0.41 3.70 4.40 *Significant at P<0.05.

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57 Site D These data were not statistically analyzed since treatments were replicated on trees that had different root stock types. Rootstock selection influences the c oncentration of major elements in leaves appreciably (Smith 1966). A summary of the leaf mineral status for each block is shown in Table 4-7. Generall y, leaf N content was within the low range of 2.2 to 2.4% regardless of the ferti lizer source in 2004, but was optimum in 2005. Compared with analysis standards (Table 4-5), leaf P, K, Mg and Ca were usually within the optimum or high range in both years regard less of fertilizer treatment. CitriBlentreated trees (Block 5) show ed numerically higher leaf P and K in both years than the conventional dry and liquid fertilization programs. Table 4-7. Effect of a dry, liquid and controlled-release fert ilization program on N, P, K, Ca, and Mg (site D). Mean leaf element concentration (%) Block N P K Mg Ca 08/0408/05 08/0408/0508/04 08/0508/0408/05 08/04 08/05 1 2.27 2.56 0.12 0.15 1.23 1.67 0.40 0.39 4.54 3.94 2 2.57 2.74 0.12 0.14 1.23 1.30 0.53 0.54 4.37 4.08 3 2.35 2.64 0.12 0.16 1.35 1.65 0.43 0.49 4.06 4.70 4 2.31 2.70 0.12 0.15 1.23 1.43 0.47 0.47 4.32 4.08 5 2.18 2.65 0.13 0.17 1.44 1.74 0.38 0.39 4.61 4.49 Economics of CitriBlen Use on Commercial Mature Citrus Trees Commercially-obtained fruit yield and ju ice quality data collected from each orchard are shown in the appendix (Table A-1) No statistical analysis was performed on these data due to the lack of replicate plots for each fertilization program. Any conclusions on yield effects from these data can be misleading since CitriBlen-treated

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58 trees were planted on a less pr oductive rootstock (Cleopatra) than trees subjected to a water-soluble fertilization program (Carrizo). Differences in rootstock type are likely to influence fruit yield. A partial budget analysis was used to evaluate the effects from changes on fertilization costs on change s in net income. The partia l budget, with a detailed description of the positive and negative imp acts and net change in income, is shown in Table 4-8. Reduced costs listed the fertiliza tion costs that were no longer incurred when the CitriBlen program was initiated. Adde d costs included additional expenses that occurred when the CitriBlen program took place. Table 4-8. Partial budget for Citr iBlen fertilization program. Proposed Change Replacing a water-soluble formulation (15-2-15-2.4Mg) with CitriBlen (15-3-19-2.5Mg) POSITIVE IMPACTS NEGATIVE IMPACTS $ per ha $ per ha Reduced Costs Added Costs $0.20 kg-1 (15-2-15) $0.77 kg-1 (15-3-19) @ 202 kg N ha-1yr-1 Std. Sol. 269.3@ 101 kg N ha-1yr-1 CitriBlen 518.5 $18 ha-1@ 4 applications yr-1 72.0$18 ha-1 @ 1 application yr-1 18.0 Total reduced costs 341.3Total additional costs 536.5 Total positive impacts 341.3Total negative impacts 536.5 Change in net income (Total positive impacts) minus (Total negative impacts) (195.2)

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59 Assuming no change in yield, economic be nefits of using CitriBlen as one application per year at half the N rate applie d as opposed to applying fertilizer at the full rate over four applications were not suffici ent to offset its higher cost. A study by Obreza and Rouse (2004), however, on bearing Hamlin orange trees in a commercial citrus orchard demonstrated that a resin/polymer-s ulfur coated urea (Poly-S) mixture applied once per year at 101 kg N ha-1 year-1 yielded about 4 pound solids per tree more than the standard water-soluble N in 5 years at the 202 kg N ha-1 year-1 rate split in three applications (Figure 4-1, adapted from Obreza and Rouse (2004)). The resin/Poly-S mixture evaluated in this trial served as the forerunner to the suite of CitriBlen. N rate ( k g /ha ) 50100150200 5-year cumulative lbs solids per tree 60 65 70 75 80 85 90 Water-soluble N Resin coated Poly-S coated Resin/Poly-S mixture Figure 4-1. Response of Hamlin orange trees to controlle d-release and water soluble fertilizers. A partial budget analysis constructed assu ming that CitriBlen had a similar yield effect to that mentioned above when it was ap plied in large scale to the commercial citrus orchards is shown in Table 4-9. Since fruit production does not increase sufficiently to offset the higher cost of CitriBlen, econom ic incentives will be needed to encourage

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60 growers to utilize CRF sources on mature citr us trees. One option is for state regulatory agencies to designate CRFs a Best Manageme nt Practice (BMP) and provide cost-share funds. If the cost difference ($195.20 per ha) was totally supp orted by regulatory agencies funds, the cost-share program would require about $13.1 million annually, assuming that the CitriBlen fertilization pr ogram was adopted for all orange-producing orchards located in the vulnerable soils of the central Florida ridge (Polk, Highlands and Lake county). The cost-share program cost w ould be a relatively small cost to maintain the Florida orange production with a pot ential value of $508.5 million annually. Table 4-9. Partial budget for CitriBlen fer tilization program assuming an increase in yield. Proposed Change Replacing a water-soluble formulation (15-2-15-2.4Mg) with CitriBlen (15-3-19-2.5Mg) POSITIVE IMPACTS NEGATIVE IMPACTS $ per ha $ per ha Reduced Costs Added Costs $0.20 kg-1 (15-2-15) $0.77 kg-1 (15-3-19) @ 202 kg N ha-1yr-1 Std. Sol. 269.3@ 101 kg N ha-1yr-1CitriBlen 518.5 $18 ha-1 @4 applications yr-1 72.0$18 ha-1 @ 1 application yr-1 18.0 Total reduced costs 341.3Total additional costs 536.5 Added Returns Increase in yield 136.0 0.8 p.s. tr-1 @ 6 p.s. bx-1 0.13 bx tr-1 @ $2.89 bx-1 $0.38 tr-1 @ 358 tr ha-1 Total Added Returns 136.0 Total positive impacts 477.3Total negative impacts 536.5 Change in net income (Total positive impacts) minus (Total negative impacts) (59.2)

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61 Furthermore, there are environmetnal bene fits that should be considered when comparing the value of using CitriBlen with the standard soluble fertilization practices. Those benefits include: 1) CitriBlen gr adually releases nutr ients matching plant demands and consequently maximizes nutrient up take efficiency. Therefore, there is less opportunity for nutrient losses to the environment, and 2) The CitriBlen fertilization program requires fewer field operations and a lower N rate. One trip per year through the field may result in less soil compaction. Like wise, heavy fertilizer loads may significantly affect soil physical, chemical and biological reactions. With a reduced fertilizer load, the potential for soil degradation or st ructural damage is minimized. Conclusions Leaf Sampling of Commercial Citrus Orchards For site B, no differences in leaf nutri ent concentration were found due to N source except for K and Mg in Block 1. Variability in leaf nutrient st atus was found among different rootstock types rega rdless of N source, while the opposite was observed when the same rootstock was used. This result suggest ed that leaf mineral patterns might have been modified by rootstock selection. Genera lly, leaf N concentration was numerically higher when water-soluble fertilizers were applied compared with CitriBlen applications. However, leaf N concentrations with the CitriBlen treatment were usually within the optimum range according to accepted standards (Table 4-5). Leaf P, K, Mg and Ca concentrations were always in the optimum or high range for any treatment at any given time. However, generally CitriBlen-treated trees had numerically higher leaf P, K, and Mg than the water-soluble treated trees. This study demonstrated that CitriBlen had the potential to maintain leaf nutrient status within the

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62 optimum range with only one application per year at a N rate reduced by one half compared with that of water-soluble fertilizer. Economics of CitriBlen Use on Commercial Mature Citrus Trees Conclusions drawn from the provided yiel d data regarding the CitriBlen impact on fruit production are misleadi ng since fertilizer treatments were applied on trees that had rootstocks with different productivity potential. A partial budget analysis that compared costs between the two fertilization programs showed a negative change in net income. This finding indicated that using CitriBlen exclusively to produce mature citrus is economically not feasible because of too high fertilizer costs. However, if the state regulatory agencies designated CRFs a Best Management Practice (BMP) and provided cost-share funds the implementation of a CRF program for orange production would become economically at tractive for citrus growers. A cost-share program for CRF use would be nefit growers and regulatory agencies by helping them meet their production and environmental goals while providing bette r water quality to Florida citizens and reducing environmental hazards.

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63 CHAPTER 5 CONCLUSIONS Providing sufficient N fertiliza tion to citrus is critical to achieve high fruit quality and yields. However, with Florida citrus grown mainly under conditions of extremely sandy soils and high-volume rainfall, a structur ed fertilization program is needed to maximize N uptake efficiency and minimize environmental hazards. Excessive use of water-soluble N fertilizer can potentially lead to groundwater contamination. Controlledrelease fertilizers (CRF) can be utilized as a management tool to supply nutrients during an extended period of time while reducing pot ential nutrient losses to the environment. Four studies were conducted to evaluate the effectiveness of polymer coated fertilizers in matching citrus nutrient requi rements and achieving optimal fruit production and foliar nutrition. CRF Incubation and Nutrient Leaching Study The goal of this study was to determine th e cumulative N, P and K released from coated fertilizers with time in a short-term laboratory incuba tion. Fertilizer material had an effect on the quantity of N, P and K releas ed to the soil solution. This differential release of nutrients was likely influenced by the composition and thic kness of the coating material. Rapid release of N, P and K from the Hydro formulation was due to its high water solubility. Among the controlled-releas e formulations, N release followed the order: Citriblen > Agrocote Type C(D) > Agrocote Type A > Agrocote Poly-S. Low recovery of P (~30% of applied) fr om any fertilizer tr eatment was probably due to P fixation in the soil columns. Some retardation of P release was observed from

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64 the CRFs. Relative release of P from the fertilizers followed the same order as N release. P release patterns from Citriblen were si milar to those of its components (Agrocote Type A, Agrocote Type C(D) and Hydro), with a high initial P release due to its water-soluble component and then a gradual re lease until termination of the experiment. The soil used in this experiment had a gr eat potential for K leaching due to its low CEC. Citriblen released 85% of the applie d K after 1 week of incubation and then a gradual release of the remaining portion was observed. This release pattern was likely due to the large amount (80%) of water-sol uble K components present in this blend. When comparing Agrocote Type A with Agrocote Type C(D), similar release patterns were found, with a slower release of K from Agrocote Type C(D) likely due to its thicker coating. Field Mesh Bag Study The objective of this study was to measur e the N release characteristics of polymer coated fertilizers and a standard water-solub le fertilizer applied to a bearing citrus orchard. Differential N release among CRFs was likely due to differences in coating material and technology. The N release patterns measured were similar to those claimed by the manufacturer. The entire N from the wa ter-soluble formulation was released after the first rainfall. Despite diffe rences in total amount of N released between locations, N release patterns at both locations followed the same order: Agrocote Type A > CitriBlen > Agrocote Poly-S > Agrocote Type C (D). Environmental conditions were more favorable for N release at Immokalee than at Lake Alfred. Quantity and frequency of irri gation and rainfall and orchard orientation probably influenced the differential N release between locations. Slow er release rates and

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65 less N released during the 1-yr field experiment at Lake Al fred were probably due to a more frequent drying of fertilizer granul es between wettings by rain or irrigation. It was found that Citriblen, a complete N-P-K controlledrelease coated blend that is made and marketed exclusively for ma ture Florida citrus as a one-application per year fertilizer, matched tree nutritional re quirements recommended by current UF-IFAS citrus fertilizer guidelines. These recommendations indicate that 2/3 of the tree nutritional requirements should be made available between March and June 15th (105 day period), and the remaining 1/3 can be applied after September 15th. About 70 and 60% of total N applied as Citriblen was re leased after 105 days in the field at Immokalee and Lake Alfred, respectively, and then a gradual release of the residual N was observed. This finding indicated that Citriblen can potentially increase N uptake efficiency while reducing leaching losses since only the portion of the N needed by the tree is available at a given time. Furthermore, the nutrient release mechanism of Citriblen is temperature dependent and therefore it potenti ally provides nutrient s to the tree anytime growth is induced as a result of warm growing conditions. Leaf Sampling of Commercial Citrus Orchards Three commercial citrus orchards were used to compare the effects of a CRF program with a standard water-soluble program on leaf nutrient status of mature orange trees. Leaf N, P, K, Ca and Mg concentrations were usually within the optimum or high range according to guidelines regardless th e fertilization program. However, trees receiving the Citriblen program had numerically higher leaf P, K, and Mg concentrations than the water-s oluble treated trees. Furtherm ore, results suggested that Citriblen can potentially pr oduce leaf mineral concentrations within the optimum range

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66 with only one application per ye ar at a N rate reduced by one half compared with that of conventional water-soluble fertilizer programs. Economics of Citriblen Use on Commercial Mature Citrus Trees The objective of this study was to comp are the costs and benefits of using Citriblen and a conventional water-solubl e fertilizer program on commercial orange orchards. A partial budget analysis used to ev aluate the positive and negative impacts of using Citriblen compared with a standard water-soluble fertilizer program indicated a negative net change in income. Break-eve n prices required to cover Citriblen fertilization costs were higher than th e current on-tree per box market prices. These results suggested that using Citrible n exclusively to produce mature citrus is economically not feasible be cause of excessive fertilizer costs. The use of CRFs would be unattractive to citrus growers unless th ey were designated as a BMP and regulatory agencies provided cost-share funds. A fully funded cost-share program would require annually about $13.1 million. This amount would be a relatively small cost to maintain the Florida orange production with a pot ential value of $508.5 millions annually. Environmental benefits should also be consid ered when evaluating the use of Citriblen in a fertilization program. By using Citriblen, N uptake efficiency is maximized and leaching losses to the groundwater are potentially reduced.

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67 APPENDIX COMMERCIAL YIELD DATA Table A-1. Historic commercial fruit yield data for sites A, C and D. Fertilizer Source CitriBlen Std. water-soluble Crop Year Yield Juice quality Yield Juice quality (box/ha) (p.s./box) (box/ha) (p.s./box) ---------------------------------------Site A-------------------------------------Hamlin / Cleopatra Hamlin / Carrizo 2000-01 1,236 6.59 1,312 6.21 2001-02 1,260 5.71 1,092 5.08 2002-03 912 5.62 954 4.98 2003-04 1,371 5.55 1,846 5.24 2004-05 578 6.30 1,344 6.56 ---------------------------------------Site C-------------------------------------Valencia / Swingle 1999-00 ----891 6.85 2000-01 ----1,202 7.28 2001-02 ----912 6.61 2002-03 1,096 6.99 ----2003-04 831 6.86 ----2004-05 709 7.67 -------------------------------------------Site D-------------------------------------Valencia / Cleopatra Val./Carr.Val./CleoCarr. Val./Carr. Val./CleoCarr. 2001-02 68 5.13 78 53 5.36 5.20 2002-03 81 5.71 103 102 5.79 5.61 2004-05 96 5.48 146 128 5.43 5.39

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68 LIST OF REFERENCES Ahmed, I.U., O.J. Attoe, L.E. Engelbert, a nd R.E. Corey. 1963. Factors affecting the rate of release from fertilizers from capsules. Agron. J. 55:495-499. Alva, A.K. 1997. Best management practice for fe rtilization of Florid a citrus on the ridge area to minimize nitrate contamination of groundwater. Univ. of Fla. IFAS Citrus Res. and Ed. Center. Lake Alfred, FL. Alva, A.K., S Paramasivam, W.D. Gr aham, and T.A. Wheaton. 2002. Best N and irrigation management practices for citrus production in sandy soils. Water, Air and Soil Pollution. 143(1-4):139-154. Alva, A.K., and S. Paramasivam. 1998 A. Nitrogen management for high yield and quality of citrus in sandy soils. Soil Sci. Soc. Am. J. 62:1335-1342. Alva, A.K., and S. Paramasivam. 1998 B. An evaluation of nutrient removal by citrus fruits. Proc. Fla. State Hort. Soc. 111:126-128. Alva, A.K., S. Paramasiva m, and W.D. Graham. 1998. Im pact of N management practices on nutritional status and yield of Valencia or ange trees and groundwater nitrate. J. Environ. Qual. 27:904-910. Alva, A.K., Y.C. Li, D.V. Ca lvert, and D.J. Banks. 1997. Best management practice for irrigation ad fertilization of Florida citr us in flatwoods soils. Completion report for project 93W280. Univ. of Fla. IFAS Citrus Res. and Ed. Center. Lake Alfred, FL. Alva, A.K., and D.P.H. Tucker. 1993. Evaluation of a resin coat ed nitrogen fertilizers for young citrus trees on a deep sand. Proc Fla. State Hort. Soc. 106:4-8. Bremner, J.M. 1982. Nitrogen-Urea. P. 699705. In Methods of Soil Analysis, Part2. Chemical and Microbiological Prope rties. Agronomy Monograph no. 9 (2nd edition). ASA-SSSA, Madison, WI. Cabrera, R.I. 1997. Comparative evaluation of nitrogen release patterns from controlledrelease fertilizers by nitrogen leachin g analysis. HortScience 32(4):669-673. Calvert, D.V. 1975. Nitrate, phosphate and po tassium movement into drainage lines under three soil management system s. J. Environ. Qual. 4:183-186.

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69 Dou, H., and A.K. Alva. 1998. Nitrogen uptak e and growth of two citrus rootstock seedlings in a sandy soil receiving differ ent controlled-release fertilizer sources. Biol. Fertil. Soils 26:169-172. Ernst, J.W., and H.F. Massey. 1960. The effect s of several factors on volatilization of ammonia formed from urea in the soil. Soil Sci. Soc. Amer. Proc. 24(2):87-90. Florida Agricultural Statisti cs Service. 2004. Commercial citrus inventory 2004. Fla. Dept. Agric. and USDA, Orlando, FL. Hauck, R.D. 1985. Slow release a nd bio-inhibitor-amended nitr ogen fertilizers. Engelstad OP (ed) Fertilizer technology and use, pp 293-322. Third ed. SSSA, Madison, WI. Havlin, J.L., J.D. Beaton, S.L. Tisdale and W.L. Nelson. 1999. Soil fertility and fertilizers. Sixth Edition. Prenti ce Hall. Upper Saddle River, N.J. He, Z.L., D.V. Calvert, A.K. Alva, D.J. Banks, and Y.C. Li. 2000A. Nutrient leaching potential of mature grapefruit trees in a sandy soil. Soil Sci. 165(9):748-758. He, Z.L., D.V. Calvert, A.K. Alva, and Y.C. Li. 2000B. Ma nagement of nutrients in citrus production systems in Florida: An overview. Proc. Soil Crop Sci. Soc. Fla. 59:2-10. He, Z. L., A.K. Alva, D.V. Calvert, a nd D.J. Banks. 1999. Ammonia volatilization from different fertilizer sources and eff ects of temperature and soil pH. Soil Sci. 164(10):750-758. Kochba, M., S. Gambash, and Y. Avnimelec h. 1990. Studies on slow release fertilizers:1. Effects of temperature, soil moisture and water vapor pressure. Soil Sci. 149(6):339-343. Koo, R.C.J. 1980. Results of citrus fertigation studies. Proc. Fla. State Hort. Soc. 93:3336. Lamb, S.T., W. D. Graham, C.B. Harrison, and A.K. Alva. 1999. Impact of alternative citrus management practices on groundwater nitrate in the Ce ntral Florida Rdge: Part 1. Field Investigation. Transa ctions of the ASAE 42(6):1653-1668. Lamont, G.P., R.J. Worrall, and M.A. O Connell. 1987. The effects of temperature and time on the solubility of resin-coated c ontrolled-release fertil izers under laboratory and field conditions. Scientia Hort. 32:265-273. Lunt, O.R. 1971. Controlled-release fertilizer : Achievements and potential. J. Agr. Food Chem. 19(5):797-800.

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70 Lunt, O.R., and J.J. Oertli. 1962. Controll ed release of fertilizer minerals by encapsulating membranes: II. Efficiency of recovery, influence of soil moisture, mode of application, and other considerati ons related to use. Soil Sci Soc. Amer. Proc. 26:584-587. Lunt, O.R., A.M. Kofranek, and J.J. Oertli. 1961. Coated fertilizers: General description and applications. Calif. Agr. 15(12):2-3. Mansell, R.S., D.V.Calvert, E.H. Stewart, W. B. Wheeler, J.S. Rogers, D.A. Graetz, L.H. Allen, A.R. Overman, and E.B. Knipling. 1977. Fetilizer and Pesticide movement from citrus groves in Florida flawood soils. Completion report for Project R800517.USEPA Environ. Res. Lab., Athens, GA. Mattos, D. 2000. Citrus response functions to N, P, and K fertilization. Ph.D. Dissertation, University of Florida, Gainesville, Fl. Mattos, D. Jr., A.K. Alva, S. Paramasivam, and D.A. Graetz. 2003. Nitrogen volatilization and mineraliz ation in a sandy Entisol of florida under citrus. Commun. Soil Sci. Plant Anal. 34(13&14):1803. Meadows, W.A., and D.L. Fuller. 1983. Nitroge n and potassium release patterns of five formulations of Osmocote fertilizers a nd two micronutrients mixes for containergrown woody ornamentals. Proc. Southern Nurserymens Assn. Res. Conf. 9:28-34 Muraro, R.P., F.M. Roka, and R.E. Rouse. 2004. Budgeting costs and returns for southwest Florida citrus production, 2003-2004. Univ. of Fla. IFAS Food and Resource Econ. Dep. EDIS document FE528. Newbould, P. 1989. The use of fertilizer in agriculture. Where do we go practically and ecologically? Plan t Soil 115:297-311. Obreza, T.A. 1993. Program fertilization for esta blishment of orange trees. J. Prod. Agric. 6:546-552. Obreza, T. A. and R.E. Rouse. 2004. Controll ed-release fertilizers for Florida citrus production. Univ. of Fla. IFAS Soil and Water Sci. Dep. Fact Sheet SL-214. Obreza, T. A. and M.E. Collins. 2002. Comm on soils used for citrus production in Florida. Univ. of Fla. IFAS Soil a nd Water Sci. Dep. Fact Sheet SL-193. Obreza, T.A., R.E. Rouse, and J.B. Sherrod. 1999. Economics of controlled-release fertilizer use on young citrus trees. J. Prod. Agric. 12:69-73. Obreza, T.A., and R.E. Rouse. 1992. Cont rolled-release fertilizers use on young Hamlin orange trees. Proc. Soil Crop Sci. Soc. Fla. 51:64-68.

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71 Obreza, T.A., A.K. Alva, E.A. Hanlon, and R.E. Rouse.1992. Citrus leaf tissue and soil testing: Sampling, analysis, and interpreta tion. Univ. of Fla. IFAS Soil and Water Sci. Dep. Fact Sheet SL-115. Oertli, J.J. 1980. Controlled-release fertilizers. Fert. Res. 1:103-123. Oertli, J.J., and O.R. Lunt. 1962. Controlled re lease of fertilizer mi nerals by incapsulating membranes: I. Factors influencing the rate of release. Soil Sci. Soc. Amer. Proc. 26:579-583. Paramasivam, S., A.K. Alva, A. Fares, and K.S. Sajwan. 2002. Fate of nitrate and bromide in an unsaturated zone of a sa ndy soil under citrus production. J. Environ. Qual. 31:671-681. Paramasivam, S., A.K. Alva, A. Fares, and K.S. Sajwan. 2001. Estimation of nitrate leaching in an Entisol under optimum citrus production. Soil Sci. Soc. Amer. J. 65:914-921 Paramasivam, S., A.K. Alva, O. Prakash, and S.L. Cui. 1999. Denitrification in the vadose zone and in surficial groundwater of a sandy entisol with citrus production. Plant and Soil. 208:307-319. Paramasivam, S., and A.K. Alva. 1997. Leaching of nitrogen forms from controlledrelease nitrogen fertilizers. Commun. Soil Sci. Plant Anal. 28(17&18):1663. Roth, S. and J. Hyde. 2002. Partial budge ting for agricultural businesses. The Pennsylvania State Univ. College of Agricultural Sci. January 2006 from: http://pubs.cas.psu.edu/freepubs/pdfs/ua366.pdf. Sartain, J.B. 2003. Selecting nutrients sour ces for citrus fertilization. Nutrient management for optimum citrus tree growth and yield short course. Univ. of Fla. IFAS Citrus Res. and Ed. Center. Lake Alfred, FL. Sartain, J. B. 2002. Tifway bermudgrass respon se to potassium fertilization. Crop Sci. 42:507-512. Sartain, J.B. 1999. Latest advances in rese arch on new fertiliz ers with emphasis on controlled-release fertilizers: An economic perspective. Proc. International Symposium on Integrated Nu trient Mgmt. Systems in Banana Prod. Costa Rica. Sartain, J.B., Hall, W.L., Littell, R.C. and Hopwood, E.W. 2004. New tools for the analysis and characterization of slow-relea sed fertilizers. Environmental Impact of fertilizer on Soil and Water. Am. Chem Soc. Symposium Series 872. Washington, DC. 13:180-195. Sartain, J. B., and J.K. Kruse. 2001. Selected fertilizers used in turfgrass fertilization. Univ. of Fla. IFAS Soil and Water Sci. Dep. CIR 1262.

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72 Schumman, A.W. 2003. Nutrient BMPs for the ridge citrus production region. Nutrient management for optimum citrus tree growth and yield short course. Univ. of Fla. IFAS Citrus Res. and Ed. Center. Lake Alfred, FL. Sharma, G.C. 1979. Controlled-rel ease fertilizers and horticultural applications. Scientia Hort. 11:107-129. Shaviv, A. 2001. Advances in controlled-rel ease fertilizers. Advances in Agronomy. 71:1-49. Shaviv, A., and R.L. Mikkelsen. 1993. Controlled -release fertilizers to increase efficiency of nutrient use and minimize environmental degradationA revi ew. Fert. Res. 35:112. Smith, P.F. 1966. Leaf analysis of citrus. Te mperate to Tropical Fruit Nutrition. Childers, N.F., Ed., Horticultural P ublications, Rutgers The State University, USA. 208228. Statistical Analysis System Institute. SAS Release 8.02; SAS Institute, Inc. Cary, NC, 1999. Trenkel, M.E. 1997. Controlled-release and stabilized fertiliz ers in agriculture. International Fertilizer Industry Assn., Paris. Tucker, D.P.H., A.K. Alva, L.K. Jackson, and T.A. Wheaton. 1995. Nutrition of Florida citrus trees. Univ. of Fla. Coop. Ext. Serv. SP 169. Tucker, D.P.H., T.A. Wheaton, and R.P. Mura ro. 1992. Citrus tree spaci ng. Univ. of Fla. IFAS Horticultural Sci. Dep. and Coop. Ext. Serv. Fact Sheet HS 143. USDA, NASS, Florida field office. 2005. C itrus Summary 2004-05. February 2006 from: http://www.nass.usda.gov/fl/rtoc0e.htm US EPA (United States Environmen tal Protection Agency) Method 200.7. 1994. Determination of Metals and Trace Elemen ts in Water and Wastes by Inductively Coupled Plasma-atomic Emission Spectro metry. Revision 4.4. Methods for the Determination of Metals in the Envi ronmental Samples Supplement I, EPA600/R-94-111. Wang, F.L. and A.K. Alva. 1996. Leaching of ni trogen from slow-release urea sources in sandy soils. Soil Sci. Soc. Am. J. 60:1454-1458. Zekri, M. and T.A. Obreza. 2003. Plant nutrients for citrus trees. Univ. of Fla. IFAS Soil and Water Sci. Dep. Fact Sheet SL-200. Zekri, M., and R.C.J. Koo. 1992. Use of cont rolled release ferti lizers for young citrus trees. Scientia Hort iculturae. 49:233-241.

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73 BIOGRAPHICAL SKETCH Carolina Medina was born in Guayaquil, Ecuador, on April 30, 1981. She attended the Polytechnic School of the Littoral in Gu ayaquil, Ecuador, for two years and then moved to Gainesville as a transfer student where she received her bachelor’s degree in agricultural operations management from th e University of Fl orida in 2003. She continued further studies in th e Soil and Water Science Depart ment at the University of Florida, obtaining the Master of Science degree in May 2006. After earning her degree, Carolina would like to continue he r work towards a Ph.D. degree.


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Title: Nutrient Release Patterns of Coated Fertilizers Used for Citrus Production and Their Effect on Fruit Yield and Foliar Nutrition
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Copyright Date: 2008

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NUTRIENT RELEASE PATTERNS OF COATED FERTILIZERS USED FOR
CITRUS PRODUCTION AND THEIR EFFECT ON FRUIT YIELD AND FOLIAR
NUTRITION














By

CAROLINA MEDINA


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Carolina Medina

































To my family
















ACKNOWLEDGMENTS

I would like to give my sincere gratitude and appreciation to Dr. Thomas Obreza

and Dr. Jerry Sartain for their guidance, problem solving and advice. Achieving this

degree would not have been possible without their support. I would also like to thank the

other members of my supervisory committee, Dr. Robert Rouse and Fritz Roka, whose

knowledge and comments contributed significantly to this research proj ect. Further, my

special thanks go to Ed Hopwood, Nahid Menhaji and Zoe Shobert for their friendly

assistance through laboratory analysis and field tasks for my research. Thanks also go to

the Scotts Fertilizer Company for providing the funding for this research. Finally, I would

like to thank all the members in my family and my boyfriend for their constant support

and help.




















TABLE OF CONTENTS

Page


ACKNOWLEDGMENT S .............. .................... iv


LI ST OF T ABLE S ........._..__ .... .___ .............._ vii...


LIST OF FIGURES ........._.._ ..... ._._ ..............viii...


AB STRAC T ................ .............. ix


CHAPTER


1 INTRODUCTION ................. ...............1.......... ......


2 LITERATURE REVIEW .............. ...............4.....


Fate of Nitrogen in a Citrus Environment .............. ...............4.....
Plant Uptake .............. ...............4.....
Leaching and Runoff ............... ...............6.
D enitrification............... .............
Volatilization ................ ...............10......._.. .....
Citrus Management Practices ............_. ...._.. ...._... ............1
Fertilizer Management. ............_. ...._.. ...._... ............1
Irrigation Management ............_. ...._.. ...._... ............1
Leaf Analy si s ............._. ...._... ............... 15...
Controlled-Release Fertilizers ............... ...............16....

Types of Controlled-Release Fertilizers ............._. ...._... .. ...._.._........17
Predicting Nutrient Release from PCFs .............. ...............20....
Factors Influencing PCF Nutrient Release ................. ................ ......... .20


3 NUTRIENT RELEASE CHARACTERISTICS OF COATED FERTILIZERS
UNDER GREENHOUSE AND FIELD CONDITIONS .............. ....................2


Introducti on ................. ...............23.................
M materials and M ethods .............. ... .... ... ......... .......2
CRF Incubation and Nutrient Leaching Study .............. ...............25....
Field Mesh Bag Study .............. ...............27....
Results and Discussion .............. ... .... ... ......... .......2
CRF Incubation and Nutrient Leaching Study .............. ...............29....
Field M esh Bag Study .............. ...............36....
Conclusions............... ..............4












CRF Incubation and Nutrient Leaching Study .............. ...............46....
Field Mesh Bag Study .............. ...............46....


4 EVALUATION OF CITRIBLEN@ ON FRUIT PRODUCTION AND FOLIAR
NUTRIENT STATUS OF MATURE CITRUS TREES ................. .....................49


Introducti on ................. ...............49.................
Materials and Methods .............. ...............50....
Site A ............... ...............50...
Site B .............. ...............51....
Site C .............. ...............51....
Site D .............. ... .. .... ..... ..... ... ........ .. ...........5
Leaf Sampling of Commercial Citrus Orchards.................... .. ............... ...._.52
Economics of CitriBlen@ Use on Commercial Mature Citrus Trees..................53
Results and Discussion .................. ...... .......... ..........5
Leaf Sampling of Commercial Citrus Orchards.................... .. ............... ...._.54
Economics of CitriBlen@ Use on Commercial Mature Citrus Trees..................57
Conclusions................... ... ... .. .... .......6
Leaf Sampling of Commercial Citrus Orchards.................... .. .......................61
Economics of CitriBlen@ Use on Commercial Mature Citrus Trees..................62


5 CONCLUSIONS .............. ...............63....


CRF Incubation and Nutrient Leaching Study .............. ...............63....
Field M esh Bag Study .................. .......... .. ..........6
Leaf Sampling of Commercial Citrus Orchards ................. ... ......_.__........._65
Economics of Citriblen@ Use on Commercial Mature Citrus Trees...................66


APPENDIX: COMMERCIAL YIELD DATA .............. ...............67....


LIST OF REFERENCES ................. ...............68.._. ......


BIOGRAPHICAL SKETCH .............. ...............73....


















LIST OF TABLES


Table pg

3-1 Nitrogen sources in each controlled-release fertilizers .............. ....................2

3-2 Controlled-release fertilizer specifications............... .............2

3-3 Effect of N source on total N released from four CRFs with time. ................... .......3 3

3-4 Effect of controlled-release nitrogen fertilizer type and location on N release rates
(% of applied) ................. ...............38........... ....

3-5 Percentage of controlled-release nitrogen (CRN) fertilizers released with time......39

3-6 Regression analysis of estimated N release rate from different N sources against
time using an exponential rise to a maximum model (Immokalee). ........................43

3-7 Regression analysis of estimated N release rate from different N sources against
time using an exponential rise to a maximum model (Lake Alfred)........................43

4-1 Characteristics of site A. ............. ...............51.....

4-2 Characteristics of site B............... ...............51...

4-3 Characteristics of site D. ............. ...............52.....

4-4 Effect of soluble and controlled-release fertilizers on N, P, K, Ca, and Mg (site A).55

4-5 Leaf analysis standards for mature, bearing citrus trees based on 4 to 6-month old
spring-cycle leaves from nonfruiting terminals............... ...............5

4-6 Effect of soluble and controlled-release fertilizers on N, P, K, Ca, and Mg (site B).56

4-7 Effect of a dry, liquid and controlled-release fertilization program on N, P, K, Ca,
and M g (site D). ............. ...............57.....

4-8 Partial budget for CitriBlen@ fertilization program..........._.._.. ........._...........58

4-9 Partial budget for CitriBlen@ fertilization program assuming an increase in yield.60

A-1 Historic commercial fruit yield data for sites A, C and D............... ...................67

















LIST OF FIGURES


Figure pg

2-1 Citrus nitrogen cycle .............. ...............5.....

2-2 Nutrient release mechanism for polymer-coated fertilizers. ............. ...................19

3-1 Incubation lysimeters. ............. ...............26.....

3-2 Field study locations............... ...............2

3-3 Layout of mesh bags placement under citrus tree canopy. ............. ...............28

3-4 Cumulative leaching of N forms. ............. ...............31.....

3-5 Leaching of N forms from soil columns. ............. ...............32.....

3-6 Phosphorus leached from soil columns. .............. ...............34....

3-7 Effect of K source on the quantity of K leached from soil columns. .......................35

3-8 Effect of K source on K leaching. .............. ...............36....

3-9 Nitrogen released (% of applied) with time. ............. ...............37.....

3-10 Comparison of average daily temperature (C) between locations. ..........................41

3-1 1.Comparison of rainfall distribution (mm) between locations. ........._.... ..............42

3-12 Citrus orchard orientation at Immokalee and Lake Alfred, respectively. ........._.....42

3-13 Nitrogen release curves for CitriBlen@ and Agrocote@ Type A at Immokalee and
Lake Alfred, respectively. ............. ...............44.....

3-14 Nitrogen release curves for Agrocote@ Type C(D) and Agrocote@ Poly-S@ at
Immokalee and Lake Alfred, respectively. ............. ...............45.....

3-15 CitriBlen@ N release curves for Immokalee and Lake Alfred. .............. .... .........._.48

4-1 Response of Hamlin orange trees to controlled-release and water soluble fertilizers.59
















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

NUTRIENT RELEASE PATTERNS OF COATED FERTILIZERS USED FOR
CITRUS PRODUCTION AND THEIR EFFECT ON FRUIT YIELD AND FOLIAR
NUTRITION

By

Carolina Medina

May 2006

Chair: Thomas Obreza
Major Department: Soil and Water Science

Citrus trees require nitrogen (N) fertilizer to maintain optimum levels of fruit

quality and productivity. Farmers have relied for many years on water-soluble fertilizers

as the main method to provide N to Florida citrus trees. However, leaching of NO3-N

from excessive use of N containing water-soluble fertilizers can potentially contribute to

contamination of groundwater, which supplies more than half of the total fresh water

used in Florida. Controlled-release fertilizers (CRFs) have the potential to gradually

release nutrients to coincide with the nutrient demand for crop growth, thereby

maximizing N uptake efficiency while minimizing leaching losses.

A laboratory study was conducted to investigate the effect of various coated

fertilizers (CitriBlen@; Agrocote@ Type A; Agrocote@ Type C(D) and Agrocote@ Poly-

S@) on nitrogen (N), phosphorus (P) and potassium (K) leaching using a soil incubation

and leaching technique. The quantity of N, P and K released depended on composition









and thickness of the coating. Release of N, P and K was delayed with CRF applications

compared with water-soluble fertilizers.

A 1-year field study in a mature citrus tree environment was used to estimate N

release characteristics of the same CRFs and a water-soluble formulation. Similar studies

were simultaneously conducted in central and southwest Florida. Mesh bags containing

3.5 g of elemental N from each source were placed on the soil surface within the irrigated

zone under the tree canopy and were retrieved from the field on a given. Despite

differences in total amount of N released between locations, N release rates at both

locations followed the same order: Water-soluble formulation>Agrocote@~ Type A >

CitriBlen@ > Agrocote@ Poly-S@ > Agrocote@ Type C (D). Quantity and frequency of

irrigation and rainfall and orchard orientation were determined as potential factors

affecting these differences. N release patterns coincided with the citrus fertilization

strategy recommended as a Best Management Practice (BMP).

Four commercial citrus orchards located in southwest and central Florida were used

to compare the effects of CitriBlen@ and a conventional water-soluble fertilizer program

on mature citrus production and nutrition. Leaf tissue was sampled at each orchard in

August 2004 and 2005. Results suggested that CitriBlen@ applied only once per year at

half the water-soluble N rate has the potential to produce leaf nutrient concentrations

within the optimum range according to guidelines. An economic analysis compared costs

and benefits between the two fertilization programs. A reduction in net income indicated

that using CitriBlen@ exclusively for citrus production is economically unfeasible due to

its high cost. The implementation of a CRF program would not be attractive to citrus

growers unless fertilizer prices change or a cost-share program was established.















CHAPTER 1
INTTRODUCTION

Citrus plays an important role in Florida' s agricultural industry. According to the

Florida Agricultural Statistical Service (2004), commercial citrus production occupies

302,940 ha in five maj or production regions, with an annual production of 1 1.4 million

metric tons. Nitrogen (N) supply is more important in citrus nutrition than any other

element. It has a large influence on tree flowering, fruit set, appearance, and fruit

production/quality (Zekri and Obreza, 2003). N is mainly provided to Florida citrus trees

as dry soluble fertilizers. Because of Florida' s poor natural soil fertility and humid

climate, citrus trees require frequent applications of soluble N fertilizers at high annual

rates to ensure sufficient vegetative growth, high yield and high fruit quality (Zekri and

Koo, 1992). Efficient use of applied N is essential to maintain high quality trees while

minimizing environmental hazards. N losses through leaching and/or volatilization are

the main causes of low efficiency of applied N fertilizer.

In sandy Florida soils, excessive use of N-containing fertilizer can potentially

contribute to leaching of NO3-N and thus lead to contamination of groundwater or

surface water resources (Paramasivam and Alva, 1997). A circa 1990 groundwater

quality study revealed that 63% of the drinking water wells surveyed in the central

Florida ridge counties of Lake, Polk, and Highlands contained detectable NO3-N and

15% contained NO3-N concentrations above the EPA Maximum Contaminant Level

(MCL) of 10 mg L 1. Most of the contaminated wells were located close to commercial

citrus orchards (Lamb et al., 1999). The presence of nitrate in drinking water supplies









above the EPA MCL represents a health hazard for Florida citizens. Although the source

of groundwater nitrate in central Florida has never been confirmed, proper nutrient

management practices and judicious use of existing fertilizer technology may minimize

or eliminate N leaching from citrus fertilization and its potential to contribute to the

nitrate problem.

Controlled-release fertilizers (CRFs) have the potential to synchronize nutrient

release patterns with crop demand and therefore optimize nutrient uptake efficiency while

reducing nutrient losses to the environment. Coated fertilizers occupy the largest share of

controlled-release fertilizer technology due to their flexible nutrient release patterns and

to their ability to control the release of other nutrients in addition to N. Despite

continuing technological improvements and the commercial availability of several CRFs,

their agricultural use remains limited. Many studies conducted on citrus fertilization in

past years have shown that CRFs have the potential to produce similar or greater tree

growth and fruit yield than water-soluble fertilizers. CRFs have also been shown to

decrease N leaching potential. However, the higher cost of CRFs per unit of nutrient and

the lack of experience about their performance in the Hield have caused Florida citrus

growers to avoid them. Information is needed regarding Hield performance and economic

feasibility of coated N fertilizers applied in commercial citrus orchard environments.

The obj ectives of this study were (1) to evaluate the cumulative N, P and K

released from coated fertilizers with time using a soil incubation method; (2) to evaluate

the N release patterns of coated fertilizers applied to a citrus orchard; (3) to develop N

release curves for the coated fertilizers; (4) to evaluate the effects of a resin/polymer-

sulfur coated mixture on fruit yield and foliar N, P, K, Ca and Mg concentrations of









mature citrus trees; and (5) to evaluate the economic feasibility of using a controlled-

release fertilization program compared with a water-soluble fertilization program for

commercial orange production.















CHAPTER 2
LITERATURE REVIEW

Fate of Nitrogen in a Citrus Environment

Any nutrient added to the soil undergoes numerous complex interactions between

plant roots, soil microorganisms, chemical reactions and pathways for loss (Shaviv and

Mikkelsen, 1993). For citrus, a maximum of 50 to 55% of the N fertilizer annually

applied can be accounted for by plant uptake even considering exceptionally high fruit

yield scenarios (He et al., 2000A). Applied nutrients not recovered by the trees and fruit

may be lost to the environment by different mechanisms. Thus, there is a need to

understand the fate of nitrogen in a citrus environment to maintain high quality trees

while minimizing the effects of N fertilization on the environment, particularly water

resources.

Fate of nitrogen in a citrus environment includes several mechanisms: 1) plant

uptake; 2) runoff and leaching into groundwater and surface water; 3) denitrification; and

4) volatilization (He et al., 1999). Figure 2-1 illustrates N dynamics in a citrus ecosystem.

Plant Uptake

Nitrogen uptake efficiency (NUE), defined as the percentage of applied N taken up

by crops, is often low in Florida soils because of the high mobility of N fertilizer (Obreza

and Rouse, 1992). In sandy soils receiving 100- to 125-cm of rainfall, the efficiency of N

uptake by plants may not exceed 20 to 30% (Oertli and Lunt, 1962). Mattos (2000)

estimated NUE for 6-year old 'Valencia' trees grown in a sandy soil to be 40% and 26%

for ammonium nitrate and urea, respectively. Paramasivam et al. (2001) showed that
























































Figure 2-1. Citrus nitrogen cycle

NUE of young citrus trees can vary from 57 to 68% depending on tree growth rate,

supply of other nutrients, and irrigation water quality (He et al., 1999). However, NUE is

more complex to measure for bearing-citrus trees since they can store nutrients to sustain


NUE for 25-yr-old 'Hamlin' orange trees grown in an Entisol ranged between 40 and

53%.









fruit production and growth. N removal in the harvested fruit can be a measure of NUE

for mature citrus, since this is the only portion that is removed from the tree-soil system

on an annual basis. Therefore, the annual fertilization program should aim to replenish

nutrients removed by the harvested fruits along with adequate consideration of the

nutrient requirement for the annual regrowth of leaves and roots, storage for flowering

and fruit setting, application efficiency, and the contribution of nutrients from recycling

of organic residues in the soil (Alva and Paramasivam, 1998B).

Alva (1997) reported that the amount of N removed by harvested fruit was

significantly correlated (quadratic relationship) with N rates. When N fertilizer was

applied at 1 12 kg ha-l year total N in the fruit was equivalent to 70 to 80% of the annual

N applied depending on fertilizer source. This N recovery was higher than average, and is

often difficult to reach under normal production conditions. Consequently, it is apparent

that mineralization of N from crop residues plays an important role in supplying a

fraction of the N requirement.

Leaching and Runoff

Currently, there is an increasing concern regarding large accumulations of NO3-N

in ecosystems, mainly from N leaching from agricultural Hields. High nitrate

concentrations are related to (i) methemoglobanemia in infants and in ruminants; (ii)

stomach cancer, for which a possible link with nitrates or nitrosoamines has been

suggested; (iii) other diseases such as goiter, birth defects, and heart disease; and (iv)

eutrophication of surface water (Shaviv and Mikkelsen, 1993).

Transport of NO3-N through the soil profie is a function of many variables,

including soil, climate factors, biological N, and cultural characteristics. Edaphic

characteristics include texture, porosity, structure, consistency, depth of profile, and










percolation rates. Climatic characteristics include amount, frequency, duration, and

timing of precipitation. Presence or absence of plant cover, depth of root zone, N use

characteristics of the vegetation, and periods of plant growth also influence N dynamics

in agricultural soils. Amount of organic matter and microbial population affect leaching

of NO3-N to groundwater (Alva, 1997).

Under Florida' s warm, humid conditions, NO3-N easily moves through the soil

profile due to rapid transformation of NH4-N into NO3-N, inherently low soil fertility,

low cation exchange capacity, and unique hydrologic features (e.g. a thin surface soil

layer, high water table and porous limestone in many areas) (Tucker et al., 1995;

Paramasivam et al., 2001). Consequently, a substantial portion of applied N fertilizer

may leach from the root zone into surface and groundwater. Paramasivam and Alva

(1997) reported that about one-third of N applied to citrus on Florida' s extremely sandy

soils is lost to leaching or volatilization. Similarly, leaching losses in a large southern

California watershed planted with citrus were equivalent to 45% of the annual N applied

(Paramavisam et al., 2001).

Most Florida citrus orchards have been planted on Entisols, Spodosols or Alfisols,

depending on geographical region (Obreza and Collins, 2002). Nearly 40% of citrus

orchards are found on the deep sandy Entisols along the Central Florida ridge, while

more than 21.5% of total state citrus plantings are on the flatwoods and marsh soils of

southwest Florida. A mixture of Alfisols and Spodosols is found in the Indian River

citrus-growing area near the east coast.

In a study conducted by Lamb et al. (1999), 15 months of baseline data indicated

that groundwater NO3-N concentrations were above the EPA Maximum Contaminant









Level (MCL) of 10 mg L-1 beneath mature citrus groves on the central Florida ridge. This

area contains primarily Entisols (ridge soils) with no confining subsurface soil horizon,

very low organic matter content and sand content > 96%. Most of the well-drained soils

of this region are classified as vulnerable to leaching of N (Tucker et al., 1995). Alva and

Tucker (1993), in a leaching study on an Entisol using coated and soluble fertilizers for

young citrus, reported that NO3-N concentration detected 1.5 m below ground were

above the MCL in the treatments that received soluble fertilizer at high rates. Results

from another study on an Entisol showed that NO3-N concentrations in soil solution

below the rooting depth (240 cm) peaked occasionally at 17 to 33 mg L- but under

careful irrigation and N management conditions concentrations were normally below 10

mg L^1. It was also demonstrated in the same study that NO3-N leaching losses below the

rooting zone increased with increasing rate of N application and the amount of water

drained (Paramavisam et al., 2001).

In contrast to the well-drained Entisols, the acid, sandy Spodosols (flatwoods soils)

are typically poorly drained with a spodic and clay strata that impede water flow

vertically from the profile. Consequently, water draining from these soils flows laterally

along the top of the subsurface hardpan to a surface water body. Hence, potential

leaching of NO3-N to the groundwater is more important in ridge soils than in flatwoods

soils. However, in some cases, the hardpan is broken during the bedding process, and

thus, there could be a potential for downward migration of pollutants below the hardpan

(Alva et al., 1997). Calvert (1975) demonstrated that NO3-N concentrations in the tile

drainage water of a Spodosol varied from <1 to 8 mg L- depending on rainfall, irrigation

and fertilization practices. In a study by He et al. (2000) on a Spodosol, solution NO3-N









concentrations at 120- and 180-cm depths increased with increasing fertilizer rates, but

never exceed the MCL even at the highest rate of fertilizer (168 kg N ha-l yr- ). However,

results from a study in west central Florida (He et al., 1999) revealed that NO3-N

concentrations in groundwater exceed the MCL even at 3-m depth below surface on

flatwoods soils. Mansell et al. (1977) in a flatwood soils management study found that

surface runoff typically occurs from deep-tilled flatwoods during intense rainfall or

irrigation of long duration after the soil profile has become water-saturated.

Denitrification

Denitrification is the gaseous loss of nitrogen to the atmosphere via a microbial

respiration process. This process occurs under anaerobic conditions where microbes

obtain their Ol frOm NO2- and NO3- with the accompanying release of N2 and N20

(Havlin et al., 1999). Environmental concerns about emission of nitrous oxides are

mainly related to the effect on global warming and the role of nitrous oxides in ozone

destruction. The destruction of 03 is catalyzed by NO, halogens, hydroxyl, and hydrogen.

A possible source of NO is from N20, the product of denitrification, which can diffuse

into the upper atmosphere and lead to atmospheric "holes", hence causing problems for

plants and animal life from excessive exposure to ultraviolet radiation. However,

depletion of the ozone layer is also greatly associated with the intensive industrialization

that has taken place during the past 5 decades (Shaviv, 2001).

The presence of NO3-N in the soil profile, lack of Oz, denitrifier population, and

availability of soluble carbon sources are the main factors determining the magnitude of

denitrification activity. These characteristics are often found in flatwoods soils, especially

associated with a shallow water table. Results of a study conducted by Mansell et al.

(1977), indicated that a significant portion of the N fertilizer applied to citrus grown in a










deep-tilled flatwoods soil was denitrified due to the relatively slow drainage

characteristics and the capacity of the soil located in the lower portion of the profie to

denitrify NO3-N. Another study on Spodosols (He et al., 2000A) showed that the

concentrations of NO3-N were greater in the soil solution at the 120-cm depth than at the

180-cm depth, which might be due to greater denitrifieation at the 180-cm depth.

Although some localized anaerobic microsites can exist in a well-drained soil,

gaseous loss of N by denitrification is often insignificant in central Florida ridge soils

(Alva and Paramasivam, 1998B). However, Paramavisam et al. (1999) demonstrated that

denitrification occurred in well-drained sandy Entisols particularly at the

soil/groundwater interface and was dependent on the amount of available carbon and

denitrifier population.

Volatilization

Volatilization, the gaseous loss of ammonia from surface applied ammonium and

urea fertilizers, is controlled by various soil properties and environmental factors and is

directly proportional to ammonium concentrations in the soil solution. Ammonia

volatilizing from fertilized fields can accumulate in neighboring natural ecosystems,

possibly causing damage to the vegetation. Some of the ammonia may be converted into

nitric acid, and this product coupled with sulfuric acid (from industrial sources) forms

acid rain that can affect plants directly and can acidify lakes, resulting in aluminum

toxicity in fish and plants (Newbould, 1989).

Ammonia volatilization increases with soil temperature and soil pH (Ernst et al.,

1960). Nitrification, the transformation ofNH4 to NO3, iS inhibited at high temperature,

resulting in increased availability of N as NH4, which contributes to increased

volatilization losses. Ammonia volatilization is favored in sandy soils with low buffering










capacity, since the ability of NH4 to form electrostatic bonds with clay minerals and

organic colloids to impair losses of soil and fertilizer N is low. In well-drained ridge soils

with high pH, volatilization losses can account for 10 to 15% of NH4-N applied to the

soil surface on an annual basis (Alva and Paramasivam, 1998B).

He et al. (1999) measured ammonia volatilization from four N fertilizer sources

surface-applied to an Alfisol (Riviera fine sand, pH 7.9) using a sponge-trapping

technique in the laboratory. Ammonia volatilization increased significantly with an

increase in NH4-N application rate, and by 2- and 3- fold, respectively, with an increase

in incubation temperature from 5 to 25 C, and from 25 to 45 C, respectively. Ammonia

volatilization was minimal at pH of 3.5 and increased rapidly with increasing pH up to

8.5. In a field study by Mattos et al., (2003), ammonia volatilized from dry-granular

ammonium nitrate and urea fertilizers surface-applied to a sandy Entisol was evaluated

using a semi-open static system of ammonia sorbers. Ammonia volatilization losses from

both N sources were greater when air was circulated inside the collection chamber to

simulate ambient air movement compared with volatilization measured with no air

circulation. This result showed the remarkable effect of environmental conditions such as

aeration, temperature, and soil moisture on ammonia volatilization.

Citrus Management Practices

Most Florida citrus is grown on extremely sandy soils with inherently low fertility,

low cation exchange capacity and low retention of applied plant nutrients. Due to these

soil properties and climatic conditions in Florida, nitrate ions can freely leach from root

zones into groundwater, potentially leading to pollution of drinking water supplies.

Proper fertilization and irrigation management practices are required to ensure sufficient










vegetative growth, high fruit yield, and good fruit quality while minimizing detrimental

effects on the environment.

Fertilizer Management

Citrus fertilization practices can be tracked back to the late 1800s. In the early

1900s low analysis fertilizers and organic N sources were used. Since the 1930s,

inorganic fertilizers have played a major role in increasing citrus production per unit land

area (He et al., 1999).

Fertilizer Form

Traditionally, broadcast application of dry soluble fertilizer material has been the

main method to provide N to citrus trees in Florida. The maj ority of N applied has been

ammonium nitrate either in granular or solution form. Also, the use of ammonium sulfate

in sulfur-deficient or high soil pH conditions has increased in Florida during the past

decade (Sartain, 2003). However, the soluble nature of these materials in combination

with Florida' s soil and climatic conditions may potentially cause leaching of NO3-N

below the root zone.

Fertigation, the delivery of liquid fertilizers through the irrigation system, has

become a popular way to apply nutrients since the introduction of microirrigation systems

for citrus irrigation. Fertigation facilitates (i) placement of fertilizer under the canopy for

efficient root uptake, and, (ii) increased frequency of application without substantial

increase in application cost (Alva et al., 1998). By increasing frequency of application,

small amounts of fertilizer can be applied many times through the course of the growing

season, improving nutrient uptake efficiency and reducing leaching losses. A study by

Lamb et al. (1999) showed that when 142 kg N ha-l was applied as a combination of

fertigation and foliar spray, groundwater NO3-N concentration decreased from 30 mg L^1









to less than 10 mg L1 while maintaining optimal fruit production and nutritional status of

leaves. Some studies (Alva et al., 2002; Alva and Paramasivam, 1998A) showed that fruit

yield was significantly greater for fertigation than for a soluble, granular source applied at

similar rates. These results suggest that nutrient uptake efficiency may be greater with

fertigation compared with the application of dry soluble fertilizers. However, fertigation

does not always provide better efficiency. For example, Koo (1980) reported no

significant differences in fruit yield and leaf nutritional status between water-soluble

granular fertilizers and fertigation.

Controlled-release fertilizers were developed to improve nutrient use efficiency

while reducing environmental hazards. Many studies (Alva and Tucker, 1993; Dou and

Alva, 1998; Wang and Alva, 1996) have shown that controlled-release fertilizers applied

in part or throughout a citrus fertilization program have potential to reduce N leaching on

Florida sandy soils. It was also reported that greater fruit yield was obtained using

controlled-release fertilizers compared with water soluble fertilizers (Obreza et al., 1999;

Zekri and Koo, 1992). However, due to its greater cost, the use of controlled-release

fertilizers for citrus has been limited to young-tree situations (reset or solid-set new

plantings) where high frequency application of conventional fertilizers is not feasible

(Obreza and Rouse, 1992).

Fertilizer Rate

In general, increasing N rate tends to 1) increase juice volume, total soluble solids

(TSS), acid content and juice color; 2) increase number of green fruit at harvest, and

incidence of creasing and scab; and 3) decrease fruit size, weight and peel thickness (He

et al., 1999).









Applying less than the recommended N rate may substantially reduce yield and/or

fruit quality, while over-application may increase the risk of nitrate contamination of the

groundwater. Optimal N-fertilizer rates are dictated by overall tree N-requirements and

N-fertilizer use efficiency. Currently, fertilizer rates for non-bearing citrus trees are

recommended in weight of a complete N-P-K fertilizer per plant (e.g. lbs tree- ), while for

bearing citrus trees (4 years and older), fertilization is based on the expected production

and N is recommended on a weight per unit area basis (e.g. lbs acre- ). The current

recommended N rate for bearing citrus trees ranges from 134 to 269 kg N ha-l yr- (120 to

240 lb N acre-l yr- ) depending on variety and expected production volume per unit area

(Tucker et al., 1995). A worldwide review of long-term citrus fertilization experiments

indicated that application of 202 kg N ha-l yr- is sufficient to sustain optimal tree growth

and maintain high fruit quality and production (Alva and Paramasivam, 1998B). Another

study by Lamb et al. (1999) demonstrated that when applying N at rates of 180 kg ha- yr-

to mature citrus located on the central Florida ridge, the groundwater may on average

comply with the MCL. Similarly, in a 4-yr study on an Entisol, Alva et al., (1998)

showed that leaf N concentrations and fruit production did not significantly change when

lowering N rates to 180 kg ha- yr-

Irrigation Management

Since transport of water through the soil profile plays a maj or role in leaching of

NO3-N, optimal irrigation management practices are important to minimize NO3-N

leaching losses and to improve N uptake efficiency, principally in sandy soils.

Traditionally, citrus was grown under overhead irrigation, where the entire grove area

was irrigated. More recently, due to the increasing need to conserve water, microsprinkler

irrigation has been introduced where the irrigated area is greatly reduced to under the









canopy, which is also the area of maximum root activity. Young trees usually use

microsprinklers that wet only a small area and provide efficient application of water and

fertilizer, and cold protection when needed. For mature trees with an expanded root

system, the wetted area should cover at least 50% of the ground surface under the canopy

in order to supply adequate irrigation and fertigation, and avoid leaching (Tucker et al.,

1995).

The depth of wetting for each irrigation event should be restricted to the root zone,

so that soluble N is maintained within the rooting depth and NO3-N uptake is facilitated.

Therefore, irrigation duration should be limited to replenish the water storage capacity of

the root zone (45 to 90cm) under the wetted area in order to avoid leaching. The use of

tools such as tensiometers, other soil moisture probes, and rainfall data is a recommended

N-BMP for irrigation scheduling (Schumann, 2003). Timing of fertilizer application also

plays a critical role in preventing groundwater pollution. It is recommended to avoid

fertilizer application during intense rainfall months (June through August) to minimize

the risk of NO3-N leaching below the root zone. Several studies (Alva, 1997; Alva et al.,

1998; Alva and Paramasivam, 1998A; He et al., 2000B; Paramasivam et al., 2001;

Paramasivam et al., 2002) have shown that under appropriate irrigation scheduling and

timing of fertilizer application, optimal fruit production can be economically attained at

lower N rates than recommended, leaching of NO3-N below the rooting depth can be

minimized and N uptake efficiency can be increased.

Leaf Analysis

Leaf analysis has been extensively used in the past two decades as a research tool

to gain valuable nutritional information about citrus trees. Leaf analysis can be helpful in

the following ways: 1) It can reflect the citrus tree nutritional status with respect to most









nutrients, but is particularly effective for nutrients that readily move with soil water like

N and K; 2) It can help confirm visual nutrient deficiency symptoms; 3) It can reveal

nutritional problems where none are suspected to exist because of absence of marked

deficiency symptoms (Smith, 1966). Leaf analysis plays an important role in formulating

an efficient fertilization program, since trends in leaf nutrient content may indicate

whether the supply of a particular element is inadequate, satisfactory or unnecessarily

high.

Leaf tissue sampling has been used in many studies (Alva and Tucker, 1993; Dou

and Alva, 1998; Obreza, 1993; Obreza et al., 1999) as a technique to determine the

effects of particular fertilizer sources and rates on growth and production of both young

and mature citrus trees. In Florida, 4- to 6-month-old spring flush leaves are sampled

following the procedure described by Obreza et al. (1992). Five ranges (deficient, low,

optimum, high, and excess) for each element have been established to classify the

nutritional status of mature, bearing trees. Maintenance of leaf sample elemental

concentrations in the optimum range is desirable.

Soil sampling can also be important in fertilization decisions, but for long-term

crops such as citrus, leaf sampling is a better indicator of the effectiveness of soil-applied

fertilizers. Soil sampling should be used for only those elements that have low mobility in

most soils (such as P, Ca, and Mg) as support information to help make future

fertilization decisions (Obreza, et al., 1992).

Controlled-Release Fertilizers

CRFs are designed and manufactured to gradually deliver nutrients to plants at a

rate that fits plant physiological requirements during growth, while simultaneously

reducing nutrient loss potential since only a small fraction of the total application is









present in a readily available form at any one moment (Oertli, 1980). This type of

fertilizer can provide many benefits to agriculture, such as (i) higher fertilizer use

efficiency; (ii) reduced nutrient losses via leaching, ammonia volatilization and

denitrification; (iii) savings in labor and equipment costs for transportation, preparation,

and application of the fertilizer since large single fertilizer applications are possible

without causing stress or toxicity to plants; (iv) less soil compaction or mechanical

damage to crops since fewer field operations are necessary; and (v) reduction of soil

chemical processes that decrease the availability of nutrients, such as the fixation of P

(Lunt, 1971; Oertli, 1980; Sharma, 1979).

There are, however, some concerns related to the use of CRF. With regard to

fertilizer longevity, nutrient release patterns from some CRFs under laboratory testing

(data provided by the manufacturer) do not correlate with the actual nutrient release

pattern under field conditions. A study done by Meadows and Fuller (1983) revealed that

nutrient release periods of several polymer-coated CRFs were shorter than those claimed

by the manufacturers. The initial nutrient release with sulfur coated controlled-release

fertilizers may be too rapid, causing damage to the crop and a higher fertilization cost

compared with non-coated water soluble fertilizers (Trenkel, 1997). Lastly, with regard to

residual effects, there is a possibility that nutrient release from CRF may continue during

the non-cropped season and result in serious leaching losses (Shaviv and Mikkelsen,

1993). All the possible disadvantages from CRF mentioned above may result in

environmental or crop damage and economic losses.

Types of Controlled-Release Fertilizers

Controlled-release fertilizers can be classified into four types: (i) materials of

limited water solubility containing plant available nutrients (e.g. metal ammonium










phosphates); (ii) materials of limited water solubility which, during their chemical and/or

microbial decomposition, release plant available nutrients (e.g. ureaforms, oxamides);

(iii) water-soluble or relatively water soluble materials that gradually decompose, thereby

releasing plant available nutrients (e.g. guanylurea salts), and (iv) water soluble materials

where dissolution is controlled by a physical barrier, e.g. by an impermeable or semi-

impermeable coating (Hauck, 1985). Coated fertilizers represent the fastest growing

segment in controlled release fertilizer technology because of their improved flexibility in

nutrient release patterns compared with other CRF products, and the flexibility in

controlling the release of other nutrients in addition to N (Sartain, 1999). Currently, CRF

coating materials are composed of either sulfur or polymeric materials or hybrid products

that utilize a multilayer coating of sulfur and polymer (Sartain and Kruse, 2001). As

shown in Figure 2-2, for nutrient release of polymer-coated fertilizers (PCF), water

(mainly vapor) passes in through the coating. The vapor condenses on the solid core and

dissolves part of it, thus inducing a build-up of internal pressure. At this stage, two

pathways are possible. If the internal pressure exceeds the membrane resistance, the

coating ruptures and the entire content of the granule is released instantaneously. If the

membrane resists the internal pressure, the fertilizer is released by diffusion driven by a

concentration gradient across the coating, by mass flow driven by a pressure gradient, or

by combination of the two (Shaviv, 2001). For polymer/sulfur coated fertilizers, the

nutrient release mechanism is through a combination of diffusion and capillary action.

Water vapor must first diffuse through the continuous polymeric membrane layer. Once

at the sulfur/polymer interface, the water subsequently penetrates the defects in the sulfur

coat through capillary action and solubilizes the fertilizer core. The solubilized fertilizer





Figure 2-2. Nutrient release mechanism for polymer-coated fertilizers.


then exits the particle in reverse sequence (Sartain and Kruse, 2001). PCFs are the most

sophisticated and advanced means of controlling fertilizer durability and nutrient release.

The use of most polymer-coated products has been generally limited to high value

applications due to the high cost of the coatings (Sartain, 1999).

The potential of PCF to produce comparable or improved plant growth compared

with water-soluble forms has been demonstrated. For example, 5-year-old bearing

Hamlin orange trees responded better when a resin/Poly-S mixture was applied once per

year at 101 kg N ha-l than water-soluble fertilizer applied three times per year at 202 kg

N ha-l (Obreza and Rouse, 2004). However, there are still concerns about whether or not

nutrient release patterns from PCFs match plant nutrient demands. A study by Cabrera

(1997) indicated that despite similar longevity ratings, the intensity and pattern of

nutrient release can be significantly different among polymer-coated CRFs. Therefore,

there is a need to better understand the basis for differences in PCF nutrient release

characteristics and the effects of environmental factors on PCF nutrient release patterns.

An increased understanding of these factors could potentially lead to more efficient use

of PCFs.


rupur ("~"'l;"failure")


water vanor


diffusion / mass


swelling









Predicting Nutrient Release from PCFs

Efforts have been made during the last decade to develop empirical, semi-

empirical, and mechanistic models describing nutrient release from coated fertilizers.

Most of these models were based on the assumption that the release of nutrients from

coated CRFs is either controlled by the rate of solute diffusion from the fertilizers or by

the rate of water vapor penetration into the CRF through the coating (Shaviv, 2001).

The nutrient release patterns of PCFs have been studied by several investigators.

Shaviv (2001) described a diffusion release process from PCFs where the driving force is

the vapor pressure gradient across the coating. This release course consists of three

stages: (1) the initial stage during which almost no release is observed (lag period), (2)

the constant-release stage, and (3) the stage where a gradual decay in release rate occurs.

Kochba et al. (1990) considered nutrient release to be a first-order kinetic process where

water vapor movement into the fertilizer is the rate-limiting process. The authors

suggested a release sequence consisting of two stages: water vapor diffusion into the

granule, and solution flow out of the coating. Ahmed et al. (1963) demonstrated that

water condensation on salts increases with the lowering of vapor pressure by the

saturated salt solution and thus the rate of release is affected. Other investigators, such as

Oertli and Lunt (1962) and Lunt and Oertli (1962), used several elution and leaching

experiments to conclude that the mechanism controlling the nutrient release is the

diffusion of salts out of the fertilizer granules.

Factors Influencing PCF Nutrient Release

Temperature

Temperature is the most important environmental factor influencing PCF nutrient

release (Oertli and Lunt, 1962). The nutrient release rate was found to significantly










increase with an increase in temperature (e.g., an increase in temperature from 10 to 20 C

almost doubled the initial release rate). Because the release rate increased much greater

than would have been expected from a simple diffusion mechanism, Oertli and Lunt

(1962) speculated that properties of the coating materials could possibly change with

temperature.

Ahmed et al. (1963) showed in a pot study that nutrient release rate was directly

related to temperature. The investigators suggested that the direct relation between

temperature and the rate of release could possibly have been due to either an increase in

viscosity of water at the lower temperature if it had entered as a liquid or to a reduction in

water vapor pressure if it entered as a vapor. Kochba et al. (1990) determined in a soil

incubation study that the change of the nutrient release rate with temperature is expected

to be exponential since vapor pressure is an exponential function of temperature. Cabrera

(1997) studied the N leaching patterns of different PCFs in containers under greenhouse

conditions during the growing season. It was found that some PCFs exhibited N leaching

patterns that closely followed changes in average daily ambient temperature over the

season. This relationship was curvilinear, with N leaching rates being highly responsive

to temperature changes between 20 and 25 C. Lamont et al. (1987) investigated the

nutrient release rate of PCFs in beakers of distilled water at different temperatures. It was

found that the nutrient release rate was affected by both incubation temperature and time.

Generally, as temperature increased, nutrient release increased. Subsequently, nutrient

release decreased with time after high initial release rates.

Other Factors

Oertli and Lunt (1962) found that the release rate was independent of pH as well as

microbial activity. Coating thickness also had an effect on release rate. The release rates









from heavily coated materials were relatively low and from lightly coated material were

high. Furthermore, they determined that there was an effect of ionic species; nitrate and

ammonia were released more rapidly than potassium and phosphate under comparable

conditions.

Lunt and Oertli (1962) found that moisture level exceeding the range of permanent

wilting percentage to field capacity in a loam soil did not significantly affect the rate of

nutrient transfer through the membrane of coated fertilizers mixed in the soil. This result

supports the hypothesis of Kochba et al. (1990) that substrate vapor pressure is the rate-

limiting step in nutrient release, since lowering the substrate moisture level within range

of field capacity does not have a marked effect on the substrate vapor pressure. Likewise,

they found that the time for nutrient release through a membrane was substantially

extended if the fertilizer was top-dressed compared with incorporated. This finding was

apparently due to intermittent drying of top-dressed material between watering. Cabrera

(1997) also found that top-dressing decreased release rates relative to incorporation. The

release rate from PCF can also be altered by composition of the coating and the fertilizer

N source being coated (Sartain and Kruse, 2001).















CHAPTER 3
NUTRIENT RELEASE CHARACTERISTICS OF COATED FERTILIZERS UNDER
GREENHOUSE AND FIELD CONDITIONS

Introduction

The efficient use of applied N fertilizers is influenced by soil, plant, and

environmental conditions. High mobility of N fertilizers in deep sandy Florida soils and

poorly distributed annual rainfall of around 1250 mm combine to make applied N highly

leachable, therefore large N doses are required to maintain high yield and quality of

citrus. High N fertilizer rates may cause environmental damage while at the same time

increasing production costs. Controlled-release fertilizers (CRF) are a possible alternative

to minimize N losses to the environment and increase N uptake efficiency while meeting

production goals of citrus growers.

CRFs contain one or more plant nutrients in a form that extends their availability to

the plant considerably longer than rapidly-available water-soluble fertilizers. This gradual

release of nutrients brings a potential to match plant nutrient demand. The use of CRFs

has currently increased due to pressure from environmental groups and regulatory

agencies to overcome environmental impacts. Hence, the use of controlled-release

materials in fertilization programs is now being considered as a Best Management

Practice (BMP). A BMP is defined as a "recommended technique that is technically and

economically feasible, which will minimize water quality impact with no adverse effects

on the agricultural production and/or quality, as well as net returns" (Alva et al., 2002).

In 1994, the Florida legislature passed a 'Nitrogen Best Management Practice (N-BMP)'









law that mandated the state to develop crop specific nitrogen BMPs designed to meet

groundwater standards. An interim BMP for citrus was established at that time based on

previous N rate studies and current IFAS recommendations. In 2002, a revised citrus

BMP was established as an incentive-based program that contains fertilization and

irrigation guidelines designed to minimize the risk of leaching nitrates from fertilizers to

groundwater.

Despite recommendation of CRFs in BMPs, the lack of experience about their field

performance is one reason why citrus growers avoid them. Information regarding the

release periods and patterns of individual CRFs is needed to increase acceptance of CRFs

for citrus production. Different techniques have been used to estimate release

characteristics of controlled-release N-fertilizers. According to Sharma (1979), the most

direct and widely used technique is the soil incubation methodology that determines some

or all of the mineral N released during CRF incubation in soil. Methods used for

laboratory evaluation of N fertilizers include: determination of fertilizer fractions soluble

in cold water, hot water, buffer solutions, or permanganate solution; direct incubation in

soil; indirect incubation in soil; Neubauer tests; short-term nutrient uptake and

microbiological assays.

The obj ectives of this study were:

1. Determine N, P and K release patterns of four coated fertilizers and water-soluble
fertilizer in a short-term laboratory incubation.

2. Measure the N release characteristics of the fertilizers in a long-term field
evaluation.

Materials and Methods

Nutrient release patterns were simultaneously evaluated in greenhouse and field

studies from spring 2004 to spring 2005.









CRF Incubation and Nutrient Leaching Study

Nitrogen (N), phosphorus (P) and potassium (K) release patterns were evaluated

using a soil incubation-column leaching study in the greenhouse. Four controlled-release

fertilizers (CRF) and a water-soluble product were compared for 270 days. CRFs were

CitriBlen@; Agrocote@ Type A; Agrocote@ Type C(D) and Agrocote@ Poly-S@ (Table

3-1). The soluble formulation was a Hydro@ 21-7-14 product. CitriBlen@ is a mixture

composed of coated (Agrocote@ Type A; Agrocote@ Type C(D) and Agrocote@ Poly-

S@) and water-soluble (Hydro@, Potassium-magnesium sulfate, Potassium chloride and

Iron) nutrients.

Table 3-1. Nitrogen sources in each controlled-release fertilizers
Source Formulation Ammoniacal Nitrate WSON
(N-P205-K20) % % %
CitriBlen@ 15-3-19 5.3 4.5 5.2
Agrocote@ Type A 19-6-12 10 9 0
Agrocote@ Type C(D) 18-7-12 10 8 0
Agrocote@ Poly-S@ 37-0-0 0 0 37
Water-soluble organic N (primarily urea)

The leaching column technique as described by Sartain et al. (2004) was used in

this study. A surface layer (0 to 5 cm depth) of Arredondo fine sand (90 g) (Loamy

siliceous, hyperthermic, Grossarenic Paleudult) from central Florida was mixed with non-

coated white sand (1710 g) and the equivalent of 450 mg N from each source. These

mixtures were placed in 30-cm long, 7.5cm diameter PVC incubation lysimeters (Figure

3-1). The sand/soil/N source mixture was brought to 10% moisture by adding 180 mL of

0.01% citric acid solution.

A 50 mL beaker containing 20 mL of 0.2 M H2SO4 WAS placed in the head space

of the incubation lysimeter as an ammonia trap. This solution was replaced and analyzed

for NH4-N by titration every 7 days to determine volatile-N. The soil columns were









incubated at about 24 C in a greenhouse. Each lysimeter was leached after 7, 14, 28, 42,

56, 84, 112, 140, 180, 210, 240 and 270 days with one pore volume of 0.01% citric acid

(500 mL) using a vacuum manifold for 2 min. Leachate volume was recorded and an

aliquot was frozen for later analysis of N, P and K. All samples were analyzed for NO3-N

and NH4-N using an air segmented Rapid Flow Analyzer (RFA). The concentration of

urea-N was measured using a colorimetric method (Bremner, 1982). An estimation of the

total N released with time was calculated by adding the three forms of N present in the

leachate and the volatile-N. The concentrations of P and K in the leachates were analyzed

at the University of Florida Analytical Research Laboratory following USEPA method

200.7 (USEPA, 1994) using an Inductively Coupled Plasma Spectrophotometer (ICP).

The electrical conductivity (EC) and pH of each leachate fraction were also measured.

Non-amended controls were included, and treatments were replicated four times in

a randomized complete block design. Statistical analysis of data was performed using

Statistical Analysis System (SAS) software (SAS Institute, 1999), and means were

compared with Duncan's Multiple Range Test (a=0.05).


Figure 3-1. Incubation lysimeters.










Field Mesh Bag Study

A 1-year Hield study in a mature citrus tree environment was used to measure N

release patterns of four controlled-release fertilizers and a water-soluble material. Similar

studies were conducted in central (Citrus Research and Education Center, Lake Alfred)

and southwest (Southwest Florida Research and Education Center, Immokalee) Florida

simultaneously since most Florida citrus is grown under these rainfall and temperature

conditions (Figure 3-2). Mesh bags (13 x 13 cm) were constructed from typical fiberglass

window screen, using heat to seal the edges. Each bag was filled with 3.5 g of elemental

N from each source and then placed on the ground surface within the irrigated zone under

bearing orange trees (Figure 3-3). Six trees were used as replicates. Each line of Hyve bags

contained the five fertilizer sources and was retrieved from the field on a given date. A

control treatment (15-2-18) fertilizer consisting of water-soluble N, P and K (ammonium

nitrate, concentrated superphosphate, and potassium chloride) was included as a

conventional standard. It was applied three times during the year (February, May and

September), while controlled-release materials (Table 3-2) were applied only once at the

beginning of the experiment.


.rV /'


Figure 3-2. Field study locations.



































Figure 3-3. Layout of mesh bags placement under citrus tree canopy.


Table 3-2. Controlled-release fertilizer specifications.
Source Formulation Release
(N-P205-K20) duration
(months)l


Principle source


N P20s
, AP, AP,CP


K20
KMgS, KS


CitriBlen@

Agrocote@
Type A
Agrocote@
Type C(D)
Agrocote@
Poly-S@


15-3-19

19-6-12

18-7-12

37-0-0


12

3-4

12-14

6


PSCU
AN, AP


AN


AP,CP KS


AN,AP AP, CP KS

PSCU -----


1Approximate at 210C soil temperature
2AN= ammonium nitrate; AP- ammonium phosphates; CP=calcium phosphate; PSCU=
polymer sulfur coated urea; KMgS= potassium-magnesium sulfate; KS= potassium
sulfate.

Six replicates of each fertilizer material were removed from the field after 14, 28,

42, 60, 90, 120, 150, 180, 240, 300 and 360 days. They were air-dried in the greenhouse









and then stored in plastic bags at room temperature for later analysis of urea-N, NO3-N,

and NH4-N in residual fertilizer granules. NO3-N, and NH4-N were analyzed using an air

segmented Rapid Flow Analyzer (RFA) unit. The concentration of urea-N was measured

using a colorimetric method (Bremner, 1982). Total nitrogen (TN) in residual fertilizer

granules was calculated by adding the three N forms detected in the granules. An

estimation of the TN released with time was calculated by subtracting the TN in the

residual fertilizer granules from the 3.5g N applied.

The average daily ambient temperature 60 cm above the ground and daily rainfall

(mm) for both locations were collected from the Florida Automated Weather Network

(FAWN). FAWN' s weather stations at both locations were located close to the

experimental sites. Treatments were arranged in a randomized complete block design.

Separation of means was accomplished with the general linear model procedure (PROC

GLM) and single degree of freedom contrasts at P I 0.05 (SAS Institute, 1999). Non-

linear regression curves were fitted to the N release data separately for each material at

each location to develop N release curves.

Results and Discussion

CRF Incubation and Nutrient Leaching Study

The pH of the leachate varied from 6.4 to 7. 1 in the non-amended soil. The soil

amended with CRFs maintained a pH between 5.0 and 6.9 until the fifth leaching event

(56 days) and then gradually decreased to 4.0 until the termination of the experiment.

This low pH was probably a result of the citric acid solution used in the study. However,

pH of the leachate from soil columns amended with a urea-based controlled release

fertilizer (Agrocote@ Poly-S@) increased from 6.0 to 7.0 during the initial three leaching

events and then decreased to pH 5.5. The initial increase in leachate pH was likely due to









hydrolysis of urea into ammonium carbonate through the action of the urease enzyme.

The pH of leachate then decreased as a result of production of nitrate through nitrification

(Paramavisan and Alva, 1997). N was recovered from the soil columns amended with

CRFs despite the low pH of 4.0 observed during the last leaching events, since their

nutrient release mechanism is not pH dependent. However, the lower N recovery from

Agrocote@ Poly-S@ was probably due to low pH since the rate of release from this type

of fertilizer is affected by pH and microbial activity.

Among the coated fertilizers, CitriBlen@ had the highest initial N release. This

result was expected since water-soluble N components are present in this blend. After the

completion of the twelve leaching events, the cumulative recoveries of total N in the

leachate were 90, 86, 85, 82 and 69% of the total N applied as CitriBlen@, Hydro@,

Agrocote@ Type C(D), Agrocote@ Type A and Agrocote@ Poly-S@ respectively.

Almost all N applied as Hydro@ was leached after the 1st week. The low recovery of

total N from the soil amended with Agrocote@ Poly-S@ could also be explained by

losses through denitrification, since minimum losses ofN due to NH3 VOlatilization were

obtained. Furthermore, it was likely that there was still some N left inside the prills after

the 270 day incubation. Table 3-3 shows the effect of N source on N release rate.

The total NH4-N recovered in twelve leachates was 46.6, 35.8, 33.8, 31.4 and

27.9% of the total N applied as Hydro@, CitriBlen@, Agrocote@ Type C(D), Agrocote@

Type A, and Agrocote@ Poly-S@ respectively (Figure 3-4, for each fertilizer, bars with

the same letter were not significantly different). The peak concentration of NH4-N for all

fertilizers but Agrocote@ Type C(D) was present after 7 days of incubation and then

decreased gradually (Figure 3-5). The decrease in leaching of NH4-N was likely due to










the transformation of NH4-N to NO3-N by nitrifieation. A gradual increase in leaching of

NH4-N observed from the first to the eighth leaching event from Agrocote@ Type C(D)

was likely due to its slower release characteristics compared with the other materials.

GO
SUrea
C B B I NH4-N
a~ 5 ~ II INO3-N A

S40 AII



2 0 llI







CitriBlen@ Agr.Type (A) Agr.Type C(D) Agr. Poly-S@ Hydro@

Figure 3-4. Cumulative leaching of N forms.

In the case of soils amended with urea-based CRFs, peak concentrations of urea

(1.7 and 3.0% of total N applied as CitriBlen@ and Agrocote@ Poly-S@ respectively)

occurred in the first leachate and then decreased significantly by the third leachate. As

shown in Figure 3-5, leaching of urea stopped after the third event as a result of the

hydrolysis of urea. Similar leaching behaviors were expected from both materials since

Agrocote@ Poly-S@ is the only urea-based component of CitriBlen@.

The total NO3-N recovered during the experiment accounted for 51.2, 50.5, 50.4,

39.4 and 35.5% of the total N applied as CitriBlen@, Agrocote@ Type C(D), Agrocote@

Type A, Hydro@, and Agrocote@ Poly-S@ respectively (Figure 3-4). For urea-based

CRFs, a gradual decrease in urea-N after the first leachate and a stable increase of NO3-N




















Urea-N













V~ I-I I I I

I I I I I
NH4-N


I


in the subsequent leachate fractions suggested that the urea-N released was being rapidly


hydrolyzed and nitrified (Figure 3-5).


200


150


100


5 0 1


0 50 100 150 200 250

Days Incubation

Figure 3-5. Leaching of N forms from soil columns.













Table 3-3. Effect of N source on total N released from four CRFs with time.



Total Nitrogen Released (mg)

Time (days)

N Source 7 14 28 42 56 84 112 140 180 210


Cumul.

240 270 (mg)


CitriBlen@ 113.6a 13.8b 28.7b 52.1a 24.9b 49.5a 38.9a

Agrocote@
Type A 62.2b 25.7a 38.9ab 53.9a 37.3a 53.3a 45.8a

Agrocote@
Type C(D) 20.90 14.7b 26.7b 43.0a 24.6b 45.2a 42.3a

Agrocote@
Poly-S@ 25.40 18.8b 44.7a 44.4a 25.1b 43.5a 39.9a

Statistical
Sigsnificance ** NS ** NS NS

1NS not significant, *- significant P<0.05, **- significant P<0.01 and ***=
2Means with the same letter within columns are not significantly different.


40.4b 23.5b 9.3b


22.4b 13.6b 8.5b


68.5a 54.3a 23.8a


26.3b 22.4b 7.3b


** ** ***


significant P<0.001.


1.6ab


2.8ab


5.2a


3.1ab


6.9a


5.4a


12.6a


9.2a


NS


403.1a


369.6b


381.6ab


309.90


**










Inorganic P added to soil that is not absorbed by plant roots or immobilized by

microorganisms can be adsorbed to mineral surfaces or precipitated as secondary P

compounds. Surface adsorption and precipitation reactions collectively are called Eixation

or retention (Havlin et., al 1999). Relatively little P leached from any fertilizer treatment

suggested P Eixation in the soil columns. P leaching was appreciably retarded with all

CRFs except CitriBlen@ (Figure 3-6). A high initial P release from CitriBlen@ was

expected since 40% of its P20s is water-soluble, then a similar lag period was observed

between the third and fifth leaching event. P leached from all CRFs generally increased

with time after the fifth leachate (56 days of incubation). Total P leached was higher for

the Hydro@ formulation than for the CRFs since it is a readily-soluble material.



40-





S25-



15-

10 C CitriBlen@
E J~--A- Agrocote@ Type A
r55 -v Agrocote@ Type C(D)
-0 Hydro@

0 50 100 150 200 250 300

Days Incubation


Figure 3-6. Phosphorus leached from soil columns.

The ionic composition of the fertilizer source and the charged component of the

soil have a significant influence on leaching losses of K source fertilizers. The soil used

in this experiment (Arredondo Eine sand) is composed of 960 g kgl sand and has a cation










exchange capacity (CEC) of 7.7 cmol(+) kgl (Sartain, 2002), thus the potential for K

leaching is great. Sartain (2002) reported significant K leaching from this soil. As shown

in Figure. 3-7, small differences in the total quantity of K leached relative to K sources

were obtained. Of all the materials studied, the Hydro@ formulation leached the largest

quantity of K. This result was expected since it is a water-soluble formulation. However,

a similar trend was obtained from one of the CRFs (CitriBlen@), likely due to the large

amount (80%) of water-soluble K components present in this blend. K recovered from

these materials (CitriBlen@ and Hydro@) was greater than the amount applied. It is likely

that the actual amount of K20 in these fertilizer granules was greater than the claimed

analysis. Agrocote@ Type A and Agrocote@ Type C(D) did not differ in quantity of K

leached. Similar K leaching might have been due to the same ionic composition of the K

source (K2SO4) and the same amount (83%) of K coated for slow-release.

120
A A

100 --
B








60




CitriBlen@ Agrocote@ Type (A) Agrocote@ Type C(D) Hydro@
Potassium Sources

Figure 3-7. Effect of K source on the quantity of K leached from soil columns.

Potassium release patterns are presented in Figure 3-8. Timing and quantity of K

leached was influenced by K source. Most of the K from Hydro@ leached during the 1 st










week and then decreased for the next two events, at which point all the applied K had

been leached. Similarly, CitriBlen@ leached 85% of the applied K after 1 week and then

declined during the rest of the experiment. Some retardation in quantity of K leached was

observed with Agrocote@ Type A and Type C(D). For Agrocote@ Type C(D), the initial

peak of K was delayed for longer (81 days). A slower release of K from this material was

expected since it has a thicker polymer coating. This result showed the influence of

coating technology on K release.


0 50 100 150 200 250 300

Days Incubation

Figure 3-8. Effect of K source on K leaching.

Field Mesh Bag Study

After 365 days in the field, the percentages of N released were 99, 95, 93 and 88%

of the total N applied as Agrocote@ Type A, CitriBlen@, Agrocote@ Poly-S@, and

Agrocote@ Type C(D), respectively at Immokalee, and 97, 90, 81, and 79% of the total N

applied as Agrocote@ Type A, CitriBlen@, Agrocote@ Poly-S@, and Agrocote@ Type

C(D), respectively at Lake Alfred (Figure 3-9). The entire N from the water-soluble


-0 CitriBlen@
-6- Agrocote@ Type
-9 Agrocote@ Type
-0 Hydro@


60 ~


A
C(D)


20 ~










formulation was released after the first rainfall it was exposed to, which was a 5-cm

event.


O 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400

Days in field

Figure 3-9. Nitrogen released (% of applied) with time.

The effect of N source, location and the interaction of these factors (N source x

location) on N release rates are shown in Table 3-4. The interaction of the two factors had

generally no effect on N release during the study. N sources had a significant influence

on the quantity of total N released.










Table 3-4. Effect of controlled-release nitrogen fertilizer type and location on N release
rates (% of applied).
Main Effect
Days in Fertilizer Location Source x Location
field Source
14 *** NS NS
28 ** NS NS
42 *** ** *
60 *** ** NS
90 *** ** NS
120 ****NS
150 *** *** *
180 ****NS
240******
300******
360 *** *** *
1Indicates whether main effects of fertilizer type, location, or fertilizer type x location
affected the data. NS, *, **, *** represent not significant, and significant where P< 0.05,
0.01 and 0.001, respectively;.

Single degree of freedom contrasts were used to compare means of total N released

among N sources (Table 3-5). Generally, N released from CitriBlen@ was significantly

different from the other N sources. Similar release patterns among N sources were

observed after 180 days in the field at Immokalee. When comparing Agrocote@ Type A

with Agrocote@ Type C(D), a highly significant difference in N release rates was found

during the entire study. Although both are resin coated materials, a slower N release rate

was expected from Agrocote@ Type C(D) since it has a thicker coating. However, when

Agrocote@ Type C(D) was compared with Agrocote@ Poly-S@ (polymer/sulfur coating),

no differences in N release patterns were observed periodically during the experiment.

This result suggested that polymer/sulfur coated fertilizers (PSCF) have the potential to

release N approaching polymer-coated fertilizers performance. This is an interesting

finding since PSCFs are produced at a much reduced cost and therefore are more

affordable to farmers










Table 3-5. Percentage of controlled-release nitrogen (CRN) fertilizers released with time.


DIF


CRN Type2


COntrast3


4 1 vs. rest 2 vs. 3 3 vs. 4


IMM4
LAL


26.4
25.7


15.9
22.3

33.6
26.8

50.6
30.6

56.2
37.5

65.3
58.0

82.7
67.9

89.2
75.4

91.7
85.1

97.1
92.8

98.6
94.6

99.0
96.5


7.6
15.2

22.4
26.2

27.0
29.2

49.4
32.8

56.6
45.0

61.8
53.6

66.4
62.3

79.1
66.2

85.4
72.9

86.3
75.2

93.0
80.6


IMM 35.5
LAL 34.9


17.9
8.2

36.0
14.7

33.9
22.5

46.3
42.1

54.6
42.8

62.8
42.6

67.4
51.5

83.9
61.2

90.7
71.5

88.8
78.8


IMM
LAL


51.8
35.3


IMM 59.4
LAL 45.4


IMM
LAL
120
IMM
LAL
150
IMM
LAL
180
IMM
LAL
240
IMM
LAL
300
IMM
LAL
360
IMM
LAL

1Days in field


64.3
59.9

76.5
58.2

81.8
68.7

85.7
77.6

92.2
83.9

94.9
87.3

94.7
89.7


2CRN type: 1, 2, 3 and 4 represent CitriBlen@, Agrocote@ Type A, Agrocote@ Type
3C(D) and Agrocote@ Poly-S@, respectively.
3Single degree of freedom contrasts were generated using SAS GLM Proc. NS, *, **, ***
represent not significant, and significant where P< 0.05, 0.01 and 0.001, respectively.
4IMM and LAL represent Immokalee and Lake Alfred, respectively.









Location had a significant influence on N release rate. In general, slower release

rates and less N released during the 365-day experimental period were observed at Lake

Alfred. For this study temperature and rainfall were considered as the potential

environmental factors that caused differential N release rates between locations. The 12-

month average temperatures were 22.3 and 22.0 C for Immokalee and Lake Alfred,

respectively. Figure 3-10 compares daily average temperatures during the study for both

locations. Temperature trends were very similar between locations. Sporadic lower

temperatures were observed after 250 days at Lake Alfred. However, at this point most N

had been released from all sources. Since little difference was found in annual average

temperature and temperature trends between locations, it was concluded that temperature

was not the main reason for differential N release rates between locations.

Total amount of rainfall and irrigation were used as an estimation of the amount of

water received by the fertilizers during the experiment. The trees were irrigated using

under-canopy microsprinklers (one emitter per tree). Irrigation was scheduled based on

rainfall and season (spring/fall or winter) and it was normally applied when there was no

rain and delayed when rainfall occurred. At Immokalee, Maxi-j et green j ets with a

delivery rate of 6.05 x 10-2 m3 h-l were used to irrigate three times per week for 4 hours

per application. At Lake Alfred, Maxi-j et violet j ets with a delivery rate of 5.79 x 10-2 m3

h-l were used to irrigate twice per week for 4 hours per application. An estimation of the

total amount of water received by the fertilizers through irrigation was calculated based

on these parameters. It was found that approximately twice as much irrigation water was

delivered at Immokalee (23.22 m3) COmpared with Lake Alfred (11.06 m3), thus the

fertilizer bags at Immokalee were exposed to more irrigation.












30 -







a, Ilil
0 50: 10 15 20 5 305
EDay
Fiur 3-0 Copaisnoavrg dal tme atr(C between loaions












graifl did0 probpablyisn flu venete Nal relea e rates e(C between locations.Lesadlo rN

relase rates observedi at Lake Alfcto i ored wer prsuaby ueomre frequTentt intermitent


drysecingo erti lzerg mterialsrifl a vr iia between wetigbyiriation or rainfall. iia eut

wesre found bys Kochbaet al. (1990). or anfl vet t moale(2 an

Furs)thermorLae, diffrence ( riny orcard orienti a feun o ationmahvels cotibtd t

dainfferent obby nlune N release patters between locations. Ats Immkaee rows were not-ot







oriented while at Lake Alfred they were oriented in an east-west direction (Figure 3-12).

In citrus orchards, more sunlight is intercepted by trees planted in rows oriented north-

south than east-west (Tucker et al., 1994). Thus, the amount of sunlight that was

intercepted by the fertilizer bags on the ground was greater in rows planted in an east-







































Lake Alfred
100 --





60 -


40 -


20-



O 50 100 150 200 250 300 350

Days


Figure 3-1 1.Comparison of rainfall distribution (mm) between locations.


North South East West



Figure 3-12. Citrus orchard orientation at Immokalee and Lake Alfred, respectively.


42



west direction than north-south. This occurrence resulted in more frequent drying periods


of the fertilizer granules which extended the time for N release through the coating.


~~L~qr: i:q;;
1.


ri" ~i p r 4


-~--
rr \..I~?E.IJ:
.s ~
--',- lilt

~: .~'L-hE~
.
L









Nitrogen Release Curves

Regression coefficients, R2 and P values of the non-linear regression equations are

provided in Table 3-6 and 3-7 for Immokalee and Lake Alfred, respectively. The R2

values for all the equations at both locations were close to unity, and all relationships

were statistically significant at the P < 0.0001. This result indicated that the equations

provided a good approximation of the N release rate (% of applied) for a given time in

the field. Figures 3-13 and 3-14 show N release curves for all materials at both locations.

Table 3-6. Regression analysis of estimated N release rate from different N sources
against time using an exponential rise to a maximum model (Immokalee).
N Source Yo a b R2 P-value
CitriBlen@l 15.16 80.90 0.011 0.99 <0.0001
Agrocote@ 99.85 0.014 0.99 <0.0001
Type A2
Agrocote@ 98.79 0.007 0.98 <0.0001
Type C(D)2
Agrocote@ 93.70 0.009 0.98 <0.0001
Poly-S@)~2
Y Yo + a (1- exp-bx) where X = time, Yo = mean value of %/NR when t equals zero and
a, and b are rebgression coeffcients.
2Y a (1-exp x) where X time and a, and b are regression coeffcients.

Table 3-7. Regression analysis of estimated N release rate from different N sources
against time using an exponential rise to a maximum model (Lake Alfred).
N Source Yo a b R2 P-value
CitriBlen@l 18.38 78.40 0.007 0.98 <0.0001
Agrocote@ 7.11 97.98 0.008 0.99 <0.0001
Type Al
Agrocote@ 95.61 0.005 0.97 <0.0001
Type C(D)2
Agrocote@ 8.52 76.87 0.007 0.99 <0.0001
Poly-S@)~
lY Yo + a (1- exp-bx) where X = time, Yo = mean value of %/NR when t equals zero and
a, and b are regression coeffcients.
2Y a (1-exp- x) where X time and a, and b are regression coeffcients.















Immokalee

-/






-P


Lake Alfr-ed


I I I I I


Lake Alfed



o/
/a



/O




-4( 0 Release Data
-Fitted curve release data


l i l l i l l


44



CitriBlen@


100







2 0


60


V


0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400


Agrocote@ Type A


Immokalee

s-






0/




d-


100 1-


.980


60





20


0


0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400


Days in field


Figure 3-13. Nitrogen release curves for CitriBlen@ and Agrocote@ Type A at
Immokalee and Lake Alfred, respectively.

































































Days in field


Figure 3-14. Nitrogen release curves for Agrocote@ Type C(D) and Agrocote@ Poly-S@
at Immokalee and Lake Alfred, respectively.


Inunokalee

o




o//

/







O


Lake Alfred









-~ 7





-d O Release Data
-Fitted curve release data


Agrocote@ Type C(D)


Inunokalee

-, '

or










o


Lake Alfred








/ -





/6
-


100


.s 80


S60


S40


20


0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400


Agrocote@ Poly-S@


I


I I


_I


100


4s 0




S0


0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400









Conclusions

CRF Incubation and Nutrient Leaching Study

N source had a significant effect on N released to the soil solution. Among the

controlled-release formulations, N release followed the order: CitriBlen@ > Agrocote@

Type C(D) > Agrocote@ Type A > Agrocote@ Poly-S@, and the proportion of NO3-N in

twelve leachate fractions was greater than either urea-N or NH4-N. This result indicated

that nitrification was taking place and thus suggested that the system was

microbiologically active, mimicking the conditions of a natural soil system. Little P

recovered (~30% of applied) from any fertilizer treatment suggested P fixation in the soil

due to chemical reactions. Incorporation of P in the soil columns also made P more

vulnerable to fixation.

Fertilizer source had a significant effect on quantity and timing of K released to the

soil. The low CEC of the soil also accelerated K leaching. Rapid release of N, P and K

from the Hydro@ formulation was due to its large water solubility. This study

demonstrated that the time to transfer a given fraction of N, P and K through a membrane

was considerably longer with CRFs applications than water-soluble fertilizers. Also, N, P

and K release patterns varied depending on composition and thickness of the coating.

Field Mesh Bag Study

CRFs showed different intensities and patterns of N release due to differences in

coating material and technology. The N release patterns measured were similar to those

claimed by the manufacturer. Environmental conditions were more favorable for N

release at Immokalee than at Lake Alfred. Despite differences in total amount of N

released between locations, N release patterns at both locations followed the same order:

Agrocote@ Type A > CitriBlen@ > Agrocote@ Poly-S@ > Agrocote@ Type C (D).









Potential factors affecting these differences were quantity and frequency of irrigation and

rainfall, and orchard orientation. It is suspected that intermittent drying of fertilizer

granules between wetting by irrigation or rainfall extended the time for N release.

CitriBlen@, a complete N-P-K controlled- release fertilizer composed mostly of

coated nutrients is made and marketed exclusively for mature Florida citrus as a single

annual application material. CitriBlen@ was developed to gradually release nutrients in

such a way that matches tree nutritional requirements and thus increases nutrient uptake

efficiency while reducing nutrient losses to the environment. In addition, CitriBlen@

nutrient release mechanism is temperature dependent and therefore it potentially provides

nutrients to the tree anytime growth is induced as a result of warm growing conditions.

Citrus trees require the highest amount of nutrients for each year from late winter

through early summer when flowering and fruit development compete with the spring

flush of growth. After the flower-fruitlet shedding process is completed in May-June, the

tree is left with only the fruit it can satisfactorily support to maturity. Fewer nutrients are

required for fruit development after this period, with the best fruit quality being obtained

with moderately low nutritional levels, mainly N, during fall and early winter. Based on

these nutrient requirements, current UF-IFAS citrus fertilizer guidelines recommend that

2/3 of the tree nutritional requirements should be made available between March and

June 15th (105 day period) and the remaining 1/3 can be applied after September 15th.

This study demonstrated that N release patterns from CitriBlen@ matched tree

nutritional requirements recommended by BMPs. The dashed line in Figure 3-15 shows

that after 105 days in the field approximately 70 and 60% of total N applied as







48


CitriBlen@ was released at Immokalee and Lake Alfred, respectively. Then a gradual

release of the remaining N was observed until termination of the 1-yr field experiment.


100-


80 O


40 --
o


Immokalee
20 - Lake Alfred



0 50 100 150 200 250 300 350

Days in field
Figure 3-15. CitriBlen@ N release curves for Immokalee and Lake Alfred.


400















CHAPTER 4
EVALUATION OF CITRIBLEN@ ON FRUIT PRODUCTION AND FOLIAR
NUTRIENT STATUS OF MATURE CITRUS TREES

Introduction

A structured fertilization management program is needed to ensure high yields and

optimal fruit quality while minimizing environmental impacts and costs. Leaf analysis is

the best indicator of proper fertilization in long-term crops such as citrus. It is a useful

management tool for making fertilization decisions since the composition of the plant

tissue reflects prior fertilization and production practices (Tucker et al., 1995). It can also

assist with diagnostic problems within the orchard or reveal symptomless nutritional

problems. Leaves have to be properly sampled, handled, processed and analyzed to

ensure that analytical results are meaningful and can be used as guidelines for managing

citrus nutritional programs. In Florida, 4- to 6-month-old spring flush leaves are sampled

following the procedure described by Obreza et al. (1992).

CRFs have the potential to produce comparable or improved fruit yield relative to

water-soluble fertilizer. Many studies have evaluated the effects of CRFs on production

of both young and mature citrus trees. Increased fruit yield has been reported using

controlled-release sources of N compared with water soluble sources (Koo, 1986; Alva

and Paramasivam, 1998; Obreza et al., 1999). The economic feasibility however of using

CRFs exclusively to produce citrus needs to be further evaluated.

The obj ectives of this study were:









1. Compare leaf nutrient status of commercial orange trees subj ected to a controlled-
release nutrient management program with trees fertilized using a conventional
water-soluble fertilizer program.

2. Evaluate the economic feasibility of using a controlled-release fertilizer program
relative to a conventional water-soluble nutrient management program for
commercial orange production.

Materials and Methods

Three commercial citrus orchards in southwest Florida (Collier and Hendry

counties) and one in central Florida (Polk county) were used to assess the potential use of

CitriBlen@ on mature citrus production and nutrition. All management practices were

done on a commercial basis. The orchards used in this study represent the two maj or

sections of the citrus industry. Three of the four sites were located on poorly drained soils

on the flatwoods (site A, B, and C). The fourth site (D) located in central Florida was

planted on well drained sandy Entisols. Characteristics of the studied citrus orchards are

described below.

Site A

Characteristics of site A are shown in Table 4-1. Beginning in 2000, CRF

(CitriBlen@) was applied once per year (late March) at a rate of 101 kg N ha-l yr- while

the soluble conventional fertilizer was applied four times (March, June, August and

October) at a rate of 202 kg N ha-l year- Blocks 1 and 3 received the conventional

water-soluble treatment during the first 2 years, and then CitriBlen@ was applied for the

rest of the experiment. Blocks 2 and 4 received CitriBlen@ and the conventional water-

soluble fertilizer, respectively, during the entire experiment.









Table 4-1. Characteristics of site A.
Variables Block number/(total area of the block in the study)
Block 1 (25 ha) Block 2 (38 ha) Block 3 (34 ha) Block 4 (18 ha)
Scion Hamlin Hamlin Hamlin Hamlin
Rootstock Swingle Cleopatra Cleopatra Carrizo
Year
1987 1987 1987 1987
planted
Tree
density 373 299 299 299
(no. ha- )
Spacing3.7 x 7.3 4.6 x 7.3 4.6 x 7.3 4.6 x 7.3
(m.)
Percentage
15 20 11 5
of resets

Site B

Scion/rootstock combinations at site B are shown in Table 4-2. Two treatments

were applied to each block. The water-soluble N standard was applied three times per

year (late March, June and September) at an N rate depending on fruit production. Trees

received 202 kg N hal year- in 2002 and 224 kg N ha-l year- from 2003 through 2005.

CitriBlen@ was applied once per year in late March at a rate of 50% of the total N

applied to the standard plots.

Table 4-2. Characteristics of site B.

Varibles Block number/(total area of the block in the study)
Block 1 (113 ha) Block 2 (30 ha)
Scion Valencia Hamlin
Rootstock Carrizo Carrizo

Site C

An 8-ha fertilizer source comparison was conducted on mature Valencia orange

trees budded on Swingle citrumelo (Citrus paradisi Macf. x Poncirus trifoliate (L.) Raf.)

rootstock planted in 1990 in a commercial citrus orchard. Tree rows were spaced 7.3 m

apart, with 3.0 m between trees within each row (448 trees ha- ). Percentage of non-









bearing resets was negligible. Conventional fertilization practices consisted of a water-

soluble fertilizer compound applied three times per year (late March, June and

September) at a rate of 224 kg N ha-l year- from 2000 to 2002. Then CitriBlen@ was

applied once per year at a rate of 101 kg N ha-l year- from 2003 to 2005.

Site D

The orchard characteristics are shown in Table 4-3. Percentage of non-bearing

resets was negligible. Three fertilizer treatments were applied in this orchard. A

fertigation program that consisted of 202 kg N hal year- split in 4 applications per year

(February, March, August, and October) was applied to blocks 1 and 4. A conventional

dry water-soluble fertilizer compound was applied to blocks 2 and 3 under the same

specifications as the liquid program. CitriBlen@ was applied to block 5 once per year

(late March) at a rate of 101 kg N hal year-

Table 4-3. Characteristics of site D.
Variables Block number/(total area of the block in the study)
#1 (11 ha) #2 (11 ha) #3 (11 ha) #4 (12 ha) #5 (8 ha)
Scion Valencia Valencia Valencia Valencia Valencia
Cleopatra / Carrizo Cleopatra / Carrizo Cleopatra
Rootstock
Carrizo Carrizo
Year
1997 1998 1997 1998 1997
Planted
Trees
287 287 287 287 287
(ha- )
Spacing4.6 x 7.6 4.6 x 7.6 4.6 x 7.6 4.6 x 7.6 4.6 x 7.6
(m.)

Leaf Sampling of Commercial Citrus Orchards

Leaf tissue was sampled at sites A, B and D. Treatment blocks were partitioned

into management units of approximately 8 ha and about 20 trees were sampled within

each management unit. About 100 4-month-old spring flush leaves from each

management unit were collected from non-fruiting twigs in late August 2004 and 2005.









Leaves at the edge of orchard blocks were avoided when sampling because of possible

surface contamination that could lead to measurement errors. The leaves were dried at 70

C for 3 days and then finely-ground. Samples were sent to a commercial agricultural

laboratory for analysis of total N, P, K, Ca and Mg concentrations. Statistical analysis

was performed on the leaf tissue data independently for each site and sampling date using

the Statistical Analysis System (SAS) software (SAS Institute, 1999).

Economics of CitriBlen@ Use on Commercial Mature Citrus Trees

A partial budget analysis compared the costs and benefits of using a CRF

(CitriBlen@) program with a conventional water-soluble fertilizer program for mature

orange trees. Partial budgeting is a planning and decision-making tool used to compare

the costs and benefits of alternatives faced by a farm business. It focuses only on the

changes in income and expenses that would result from implementing a specific

alternative while all aspects of farm profits that are unchanged by the decision are

ignored (Roth and Hyde, 2002). It is based on the principle that a small change in a farm

business eliminates or reduces some costs and returns, adds costs, and/or adds revenues.

Costs of the fertilization programs were estimated by adding the fertilizer product

and application cost for each orchard. Fertilizer cost ($ ha- ) was calculated by

multiplying the price of fertilizer ($ kg-l product) by its application rate (kg product ha- ).

Thus, fertilization costs varied based on fertilizer rate, source, and application frequency.

Fruit yield data for each block were obtained from the growers for sites A, C and D in

terms of quantity (box ha- )l and quality (pound-solids box- )2


i One box is equal to 41kg for oranges.

2 Pound-solids per box is an expression of total soluble solids per unit weight of fruit and is the basis on
which a grower gets paid for his fruit.









The standard water-soluble fertilization program was taken as 202 kg N ha-l year-

split in four applications per year, while the CitriBlen@ program consisted of 101 kg N

ha-l year- applied once per year. The CitriBlen@ (15-3-19-2.5Mg) price was obtained

from the distributor. The price of the standard water-soluble formulation (15-2-15-

2.4Mg) and the average cost of a single dry fertilizer application including labor and

equipment were taken from Muraro et al. (2004). Even though the two fertilizer products

did not have exactly the same P and K analysis, they were considered to be equal for this

economic analysis.

Results and Discussion

Leaf Sampling of Commercial Citrus Orchards

Results are described independently for each study site due to differences in

scion/rootstock combinations and treatment applications.

Site A

No statistical analysis was performed on these data due to differences in rootstock

types between blocks. Mean leaf nutrient concentrations are summarized in Table 4-4.

Similar leafN concentration trends were observed for both sampling dates. Trees that

received only the conventional water-soluble fertilizer (Block 4) had numerically the

highest leaf N content. Since water-soluble N was applied to those trees only a few weeks

before sampling, leaf N concentration was expected to be the highest among blocks. A

variation in leaf nutrient concentrations was observed from 2004 to 2005. Yearly

variations in macronutrient concentrations have been confirmed in other long-term

studies.









Table 4-4. Effect of soluble and controlled-release fertilizers on N, P, K, Ca, and Mg (site
A).
Mean leaf element concentration (%)
Block N P K Mg Ca
08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05

1 2.84 2.53 0.13 0.16 1.09 1.45 0.31 0.32 4.50 4.21

2 2.69 2.20 0.14 0.16 1.39 1.74 0.38 0.40 3.97 4.76

3 2.70 2.22 0.13 0.16 1.45 1.78 0.36 0.42 4.10 4.75

4 2.92 2.54 0.13 0.15 1.38 1.60 0.36 0.36 4.41 4.80


In fact, a consistent pattern from year to year appears to be the exception rather

than the rule. Various phenological factors such as light intensity, temperature, relative

humidity and water availability interact with edaphic and physiological factors in such a

way as to produce profound changes in leaf composition from one year to the next (Smith

1966). Lower leaf N occurred when the rainfall was about 3 5% greater than the previous

year. Perhaps because of increased vegetative growth of the trees, the concentration of N

was lower in the leaves during this year.

Generally all treatments resulted in leaf P, K, Ca and Mg status in the optimum

range according to current guidelines (Table 4-5). Overall, trees that were planted on the

same rootstock type (blocks 2 and 3) showed similar leaf concentrations for all nutrients.

This result suggested that rootstock type may have influenced leaf nutrient status.

Table 4-5. Leaf analysis standards for mature, bearing citrus trees based on 4 to 6-month
old spring-cycle leaves from nonfruiting terminals.
Element Deficient Low Optimum High Excessive
Nitrogen (N) (%) <2.2 2.2-2.4 2.5-2.8 2.9-3.2 >3.3
Phosphorus (P) (%) <0.09 0.09-0.11 0.12-0.17 0.18-0.29 >0.30
Potassium (K) (%) <0.7 0.7-1.1 1.2-1.7 1.8-2.3 >2.4
Calcium (Ca) (%) <1.5 1.5-2.9 3.0-5.0 5.1-6.9 >7.0
Magnesium (Mg) (%) <0.20 0.20-0.29 0.30-0.50 0.51-0.70 >0.80
Adapted from Obreza et al. (1992).










Site B

The effect of fertilizer source on leaf nutrient concentration is shown in Table 4-6.

For Block 1, leaf nutrient concentrations did not differ between treatments on any

sampling date, except for K and Mg in the first year. Trees treated with the standard

water-soluble fertilizer had the highest leaf N. However, leaf N concentrations of

CitriBlen@-treated trees were within the high range (Table 4-5) and then decreased to the

optimum range. Leaf P, K, Mg and Ca were within optimum ranges through the entire

study regardless of treatment.

For Block 2, leaf nutrient concentrations did not differ between treatments either

year. CitriBlen@-treated trees had the highest leaf N in August 2005, while the

conventional standard had the highest leaf N in August 2004 and the lowest in August

2005. These results suggested that the N in the water-soluble fertilizer had short residual

effects on leaf N compared with that in CitriBlen@. A study by Zekri and Koo (1992)

showed similar results. All treatments resulted in leaf P, K, Mg and Ca status in the

optimum or high range according to guidelines (Table 4-5).

Table 4-6. Effect of soluble and controlled-release fertilizers on N, P, K, Ca, and Mg (site
B).
Mean leaf element concentration (%)
Source N P K Mg Ca
------------------------------------Blockl ---------------------------------------

08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05

CitriBlen 2.90 2.54 0.15 0.18 1.32 1.45 0.47 0.48 3.62 3.70
Std. Sol. 2.96 2.60 0.15 0.17 1.45* 1.48 0.43* 0.45 3.52 3.68

------------------------------------Block2---------------------------------------

08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05
CitriBlen 2.52 2.61 0.15 0.19 1.35 1.26 0.44 0.41 3.97 4.13
Std. Sol. 2.64 2.49 0.14 0.17 1.37 1.32 0.42 0.41 3.70 4.40
Significant at P<0.05.









Site D

These data were not statistically analyzed since treatments were replicated on trees

that had different rootstock types. Rootstock selection influences the concentration of

maj or elements in leaves appreciably (Smith, 1966). A summary of the leaf mineral status

for each block is shown in Table 4-7. Generally, leaf N content was within the low range

of 2.2 to 2.4% regardless of the fertilizer source in 2004, but was optimum in 2005.

Compared with analysis standards (Table 4-5), leaf P, K, Mg and Ca were usually within

the optimum or high range in both years regardless of fertilizer treatment. CitriBlen@-

treated trees (Block 5) showed numerically higher leaf P and K in both years than the

conventional dry and liquid fertilization programs.

Table 4-7. Effect of a dry, liquid and controlled-release fertilization program on N, P, K,
Ca, and Mg (site D).
Mean leaf element concentration (%)
Block N P K Mg Ca
08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05 08/04 08/05

1 2.27 2.56 0.12 0.15 1.23 1.67 0.40 0.39 4.54 3.94

2 2.57 2.74 0.12 0.14 1.23 1.30 0.53 0.54 4.37 4.08

3 2.35 2.64 0.12 0.16 1.35 1.65 0.43 0.49 4.06 4.70

4 2.31 2.70 0.12 0.15 1.23 1.43 0.47 0.47 4.32 4.08

5 2.18 2.65 0.13 0.17 1.44 1.74 0.38 0.39 4.61 4.49


Economics of CitriBlen@ Use on Commercial Mature Citrus Trees

Commercially-obtained fruit yield and juice quality data collected from each

orchard are shown in the appendix (Table A-1). No statistical analysis was performed on

these data due to the lack of replicate plots for each fertilization program. Any

conclusions on yield effects from these data can be misleading since CitriBlen@-treated





































$ per ha


trees were planted on a less productive rootstock (Cleopatra) than trees subj ected to a

water-soluble fertilization program (Carrizo). Differences in rootstock type are likely to

influence fruit yield.

A partial budget analysis was used to evaluate the effects from changes on

fertilization costs on changes in net income. The partial budget, with a detailed

description of the positive and negative impacts and net change in income, is shown in

Table 4-8. Reduced costs listed the fertilization costs that were no longer incurred when

the CitriBlen@ program was initiated. Added costs included additional expenses that

occurred when the CitriBlen@ program took place.

Table 4-8. Partial budget for CitriBlen@ fertilization program.
Proposed Change
Replacing a water-soluble formulation (15-2-15-2.4Mg) with CitriBlen@
(15-3-19-2.5Mg)


POSITIVE IMPACTS

Reduced Costs

$0.20 kg-l (15-2-15)
@ 202 kg N ha- yrl Std.
Sol.


$18 ha- @ 4 applications yr



Total reduced costs


NEGATIVE IMPACTS
$ per ha
Added Costs

$0.77 kg-l (15-3-19)
269.3 @ 101 kg N ha- yr- CitriBlen@



72.0 $18 ha-l @ 1 application yr-



341.3 Total additional costs


518.5



18.0



536.5


536.5

(195.2)


Total positive impacts 341.3 Total negative impacts

Change in net income
(Total positive impacts) minus (Total negative impacts)









Assuming no change in yield, economic benefits of using CitriBlen@ as one

application per year at half the N rate applied as opposed to applying fertilizer at the full

rate over four applications were not sufficient to offset its higher cost. A study by Obreza

and Rouse (2004), however, on bearing Hamlin orange trees in a commercial citrus

orchard demonstrated that a resin/polymer-sulfur coated urea (Poly-S) mixture applied

once per year at 101 kg N ha-l year- yielded about 4 pound solids per tree more than the

standard water-soluble N in 5 years at the 202 kg N ha-l year- rate split in three

applications (Figure 4-1, adapted from Obreza and Rouse (2004)). The resin/Poly-S

mixture evaluated in this trial served as the forerunner to the suite of CitriBlen@.



I / ~Resin coated


;r 80 As esn/Poly-S mixture


a, 75 -
Water-soluble N
70 -

Poly-S coated
65 -


50 100 150 200
N rate (kg/ha)
Figure 4-1. Response of Hamlin orange trees to controlled-release and water soluble
fertilizers.

A partial budget analysis constructed assuming that CitriBlen@ had a similar yield

effect to that mentioned above when it was applied in large scale to the commercial citrus

orchards is shown in Table 4-9. Since fruit production does not increase sufficiently to

offset the higher cost of CitriBlen@, economic incentives will be needed to encourage









growers to utilize CRF sources on mature citrus trees. One option is for state regulatory

agencies to designate CRFs a Best Management Practice (BMP) and provide cost-share

funds. If the cost difference ($195.20 per ha) was totally supported by regulatory

agencies funds, the cost-share program would require about $13.1 million annually,

assuming that the CitriBlen@ fertilization program was adopted for all orange-producing

orchards located in the vulnerable soils of the central Florida ridge (Polk, Highlands and

Lake county). The cost-share program cost would be a relatively small cost to maintain

the Florida orange production with a potential value of $508.5 million annually.

Table 4-9. Partial budget for CitriBlen@ fertilization program assuming an increase in
yield.
Proposed Change
Replacing a water-soluble formulation (15-2-15-2.4Mg) with CitriBlen@
(15-3-19-2.5Mg)

POSITIVE IMPACTS NEGATIVE IMPACTS
$ per ha $ per ha
Reduced Costs Added Costs
$0.20 kg-' (15-2-15) $0.77 kg-l (15-3-19)
@ 202 kg N ha lyr Std. 269.3 @ 101 kg N ha- yr -CitriBlen@ 518.5
Sol.

$18 ha-l @4 applications yil 72.0 $18 ha-l @ 1 application yil 18.0


Total reduced costs 341.3 Total additional costs 536.5
Added Returns
Increase in yield 136.0
0.8 p.s. til @ 6 p.s. bxl
0.13 bx trl @ $2.89 bxl
$0.38 til @ 358 tr ha-l


Total Added Returns 136.0

Total positive impacts 477.3 Total negative impacts 536.5

Change in net income (59.2)
(Total positive impacts) minus (Total negative impacts)









Furthermore, there are environmetnal benefits that should be considered when

comparing the value of using CitriBlen@ with the standard soluble fertilization practices.

Those benefits include: 1) CitriBlen@ gradually releases nutrients matching plant

demands and consequently maximizes nutrient uptake efficiency. Therefore, there is less

opportunity for nutrient losses to the environment, and 2) The CitriBlen@ fertilization

program requires fewer field operations and a lower N rate. One trip per year through the

field may result in less soil compaction. Likewise, heavy fertilizer loads may significantly

affect soil physical, chemical and biological reactions. With a reduced fertilizer load, the

potential for soil degradation or structural damage is minimized.

Conclusions

Leaf Sampling of Commercial Citrus Orchards

For site B, no differences in leaf nutrient concentration were found due to N source

except for K and Mg in Block 1. Variability in leaf nutrient status was found among

different rootstock types regardless of N source, while the opposite was observed when

the same rootstock was used. This result suggested that leaf mineral patterns might have

been modified by rootstock selection. Generally, leafN concentration was numerically

higher when water-soluble fertilizers were applied compared with CitriBlen@

applications. However, leaf N concentrations with the CitriBlen@ treatment were usually

within the optimum range according to accepted standards (Table 4-5).

Leaf P, K, Mg and Ca concentrations were always in the optimum or high range for

any treatment at any given time. However, generally CitriBlen@-treated trees had

numerically higher leaf P, K, and Mg than the water-soluble treated trees. This study

demonstrated that CitriBlen@ had the potential to maintain leaf nutrient status within the










optimum range with only one application per year at a N rate reduced by one half

compared with that of water-soluble fertilizer.

Economics of CitriBlen@ Use on Commercial Mature Citrus Trees

Conclusions drawn from the provided yield data regarding the CitriBlen@ impact

on fruit production are misleading since fertilizer treatments were applied on trees that

had rootstocks with different productivity potential. A partial budget analysis that

compared costs between the two fertilization programs showed a negative change in net

income. This finding indicated that using CitriBlen@ exclusively to produce mature citrus

is economically not feasible because of too high fertilizer costs.

However, if the state regulatory agencies designated CRFs a Best Management

Practice (BMP) and provided cost-share funds, the implementation of a CRF program for

orange production would become economically attractive for citrus growers. A cost-share

program for CRF use would benefit growers and regulatory agencies by helping them

meet their production and environmental goals while providing better water quality to

Florida citizens and reducing environmental hazards.















CHAPTER 5
CONCLUSIONS

Providing sufficient N fertilization to citrus is critical to achieve high fruit quality

and yields. However, with Florida citrus grown mainly under conditions of extremely

sandy soils and high-volume rainfall, a structured fertilization program is needed to

maximize N uptake efficiency and minimize environmental hazards. Excessive use of

water-soluble N fertilizer can potentially lead to groundwater contamination. Controlled-

release fertilizers (CRF) can be utilized as a management tool to supply nutrients during

an extended period of time while reducing potential nutrient losses to the environment.

Four studies were conducted to evaluate the effectiveness of polymer coated fertilizers in

matching citrus nutrient requirements and achieving optimal fruit production and foliar

nutrition.

CRF Incubation and Nutrient Leaching Study

The goal of this study was to determine the cumulative N, P and K released from

coated fertilizers with time in a short-term laboratory incubation. Fertilizer material had

an effect on the quantity of N, P and K released to the soil solution. This differential

release of nutrients was likely influenced by the composition and thickness of the coating

material. Rapid release of N, P and K from the Hydro@ formulation was due to its high

water solubility. Among the controlled-release formulations, N release followed the

order: Citriblen@ > Agrocote@ Type C(D) > Agrocote@ Type A > Agrocote@ Poly-S@.

Low recovery of P (~30% of applied) from any fertilizer treatment was probably

due to P fixation in the soil columns. Some retardation of P release was observed from









the CRFs. Relative release of P from the fertilizers followed the same order as N release.

P release patterns from Citriblen@ were similar to those of its components (Agrocote@

Type A, Agrocote@ Type C(D) and Hydro@), with a high initial P release due to its

water-soluble component and then a gradual release until termination of the experiment.

The soil used in this experiment had a great potential for K leaching due to its low

CEC. Citriblen@ released 85% of the applied K after 1 week of incubation and then a

gradual release of the remaining portion was observed. This release pattern was likely

due to the large amount (80%) of water-soluble K components present in this blend.

When comparing Agrocote@ Type A with Agrocote@ Type C(D), similar release

patterns were found, with a slower release of K from Agrocote@ Type C(D) likely due to

its thicker coating.

Field Mesh Bag Study

The obj ective of this study was to measure the N release characteristics of polymer

coated fertilizers and a standard water-soluble fertilizer applied to a bearing citrus

orchard. Differential N release among CRFs was likely due to differences in coating

material and technology. The N release patterns measured were similar to those claimed

by the manufacturer. The entire N from the water-soluble formulation was released after

the first rainfall. Despite differences in total amount of N released between locations, N

release patterns at both locations followed the same order: Agrocote@ Type A >

CitriBlen@ > Agrocote@ Poly-S@ > Agrocote@ Type C (D).

Environmental conditions were more favorable for N release at Immokalee than at

Lake Alfred. Quantity and frequency of irrigation and rainfall and orchard orientation

probably influenced the differential N release between locations. Slower release rates and









less N released during the 1-yr field experiment at Lake Alfred were probably due to a

more frequent drying of fertilizer granules between wettings by rain or irrigation.

It was found that Citriblen@, a complete N-P-K controlled- release coated blend

that is made and marketed exclusively for mature Florida citrus as a one-application per

year fertilizer, matched tree nutritional requirements recommended by current UF-IFAS

citrus fertilizer guidelines. These recommendations indicate that 2/3 of the tree nutritional

requirements should be made available between March and June 15th (105 day period),

and the remaining 1/3 can be applied after September 15th. About 70 and 60% of total N

applied as Citriblen@ was released after 105 days in the field at Immokalee and Lake

Alfred, respectively, and then a gradual release of the residual N was observed.

This finding indicated that Citriblen@ can potentially increase N uptake efficiency

while reducing leaching losses since only the portion of the N needed by the tree is

available at a given time. Furthermore, the nutrient release mechanism of Citriblen@ is

temperature dependent and therefore it potentially provides nutrients to the tree anytime

growth is induced as a result of warm growing conditions.

Leaf Sampling of Commercial Citrus Orchards

Three commercial citrus orchards were used to compare the effects of a CRF

program with a standard water-soluble program on leaf nutrient status of mature orange

trees. Leaf N, P, K, Ca and Mg concentrations were usually within the optimum or high

range according to guidelines regardless the fertilization program. However, trees

receiving the Citriblen@ program had numerically higher leaf P, K, and Mg

concentrations than the water-soluble treated trees. Furthermore, results suggested that

Citriblen@ can potentially produce leaf mineral concentrations within the optimum range









with only one application per year at a N rate reduced by one half compared with that of

conventional water-soluble fertilizer programs.

Economics of Citriblen@ Use on Commercial Mature Citrus Trees

The obj ective of this study was to compare the costs and benefits of using

Citriblen@ and a conventional water-soluble fertilizer program on commercial orange

orchards. A partial budget analysis used to evaluate the positive and negative impacts of

using Citriblen@ compared with a standard water-soluble fertilizer program indicated a

negative net change in income. Break-even prices required to cover Citriblen@

fertilization costs were higher than the current on-tree per box market prices.

These results suggested that using Citriblen@ exclusively to produce mature citrus

is economically not feasible because of excessive fertilizer costs. The use of CRFs would

be unattractive to citrus growers unless they were designated as a BMP and regulatory

agencies provided cost-share funds. A fully funded cost-share program would require

annually about $13.1 million. This amount would be a relatively small cost to maintain

the Florida orange production with a potential value of $508.5 millions annually.

Environmental benefits should also be considered when evaluating the use of Citriblen@

in a fertilization program. By using Citriblen@, N uptake efficiency is maximized and

leaching losses to the groundwater are potentially reduced.
















APPENDIX
COMMERCIAL YIELD DATA

Table A-1. Historic commercial fruit yield data for sites A, C and D.
Fertilizer Source
CitriBlen@ Std. water-soluble
Crop Juice Juice
Yield Yield
Year quality quality
(box/ha) (p.s./box) (box/ha) (p.s./box)
-----------------------------------------St A----------------------------------------

Hamlin / Cleopatra Hamlin / Carrizo
2000-01 1,236 6.59 1,312 6.21
2001-02 1,260 5.71 1,092 5.08
2002-03 912 5.62 954 4.98
2003-04 1,371 5.55 1,846 5.24
2004-05 578 6.30 1,344 6.56

-----------------------------------------StC----------------------------------------

Valencia / Swingle
1999-00 --- --- 891 6.85
2000-01 --- --- 1,202 7.28
2001-02 --- --- 912 6.61
2002-03 1,096 6.99----
2003-04 831 6.86----
2004-05 709 7.67----

-----------------------------------------StD----------------------------------------

Valencia / Cleopatra Val./Carr. Val./Cleo- Val./Carr. Val./Cleo-
Carr. Carr.
2001-02 68 5.13 78 53 5.36 5.20
2002-03 81 5.71 103 102 5.79 5.61
2004-05 96 5.48 146 128 5.43 5.39
















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BIOGRAPHICAL SKETCH

Carolina Medina was born in Guayaquil, Ecuador, on April 30, 1981. She attended

the Polytechnic School of the Littoral in Guayaquil, Ecuador, for two years and then

moved to Gainesville as a transfer student where she received her bachelor' s degree in

agricultural operations management from the University of Florida in 2003. She

continued further studies in the Soil and Water Science Department at the University of

Florida, obtaining the Master of Science degree in May 2006. After earning her degree,

Carolina would like to continue her work towards a Ph.D. degree.