Fruit Crops Fact Sheet: Fertilization of Young Citrus Trees
http://edis.ifas.ufl.edu/ ( Publisher's URL )
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Permanent Link: http://ufdc.ufl.edu/IR00001656/00001
 Material Information
Title: Fruit Crops Fact Sheet: Fertilization of Young Citrus Trees
Physical Description: Fact Sheet
Creator: Ferguson, James J.
Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication: Gainesville, Fla.
Publication Date: 1989
Acquisition: Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer.
Publication Status: Published
General Note: "First printed May 1989; reviewed May 1999."
General Note: "FC79"
 Record Information
Source Institution: University of Florida Institutional Repository
Holding Location: University of Florida
Rights Management: All rights reserved by the submitter.
System ID: IR00001656:00001


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1.This document is FC79, one of a series of the Department of Horticultural Science, Florida Cooperative Extension Service, Ins titute of Food and Agricultural Sciences, University of Florida. First printed May 1989; reviewed May 1999. Please visit the EDIS Web site at http://edis.ifas.ufl.edu 2.J. J. Ferguson, Associate Professor, Extension Horticulturist, and F. S. Davies, Professor, Horticulturist, Cooperative Exte nsion Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.The Institute of Food and A g ricultural Sciences is an equal opportunit y /affirmative action emplo y er authorized to provide research, educational information and other services onl y to individuals and institutions that function without re g ard to race, color, sex, a g e, handicap, or national ori g in. For information on obtainin g other extension publications, contact y our count y Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and A g ricultural Sciences / Universit y of Florida / Christine Ta y lor Waddill, DeanFigure 1 Ferti g ation of y oun g citrus tree, usin g low volume irri g ation s y stem.FC79Fruit Crops Fact Sheet: Fertilization of Young Citrus Trees1 J. J. Fer g uson and F. S. Davies2During the past 3 years approximately 10 millionestablished groves. Controlled or slow release materials young citrus trees have been planted in Florida, withcan be applied less frequently, and fertigation, the additional thousands of new acres being planned orapplication of soluble fer tilizers thr ough irrigation developed (Figure 1). Fertilization is a major limitingsystems, can stimulate growth comparable to that obtained factor in the growth of these young trees, primarilywith readily soluble fer tilizers, but these practices can because of Flordas relatively infertile soils. Citrusreduce application frequency and associated energy costs. growers have traditionally applied readily soluble granular fertilizers 4 to 6 times per year to ensure a conti nuous supply of nutrients to the limited root systems of young trees. Such repeated applications demand a heavy investment in energy, time, labor, and machinery; can increase soil compaction; and may contribute to contamination of groundwater. An add itional expense is the fertilization of isolated y oung trees replanted in The purpose of this fact sheet is to review briefly general concepts of fertilization, discuss current fertilization practices for y oung citrus trees, and consider how controlled release fertilizers and fertigation can reduce application frequency and energy costs.General Concepts of Fertilization and Soil FertilityPlant growth and development depend on the availability of 15 different nutrient elements. The major or macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, magnesium, sulfur and calcium. The minor or micronutrients include boron, molybdenum, iron, manganese, copper and zinc. However, since two of the major or macronutrients, nitrogen and potassium, leach readily from soil, these elements are usually the ones most limiting to growth of young citrus trees in Florida. Phosphorus, another macronutrient, and the minor elements are of less importance, particularly in replant situations where they may have accumulated in grove soils over years of fertilization. Natural deposits of phosphorus in reclaimed mine lands planted to citrus may also contribute to high soil levels of this element.


Fruit Crops Fact Sheet: Fertilization of Youn g Citrus Trees Pa g e 2Ma y 1999However, phosphorus levels are quite low in many heavy rains move fertilizer thr ough or away from the root flatwoods soils and should be carefully monitored. zone. The key to success for any fertilizer program is a readily available soluble supply of essential nutrient elements. Availability depends on timely application of fertilizer and the capacity of soil particles to retain and release nutrients. Sandy soils are relatively infertile and lack this capacity to retain nutrents. Frequent applications of fertilizer are n ecessary to ensure that essential elements are available. Solubility is initially more important for granular than liquid fertilizers but in either case adequate soil moisture levels must be continually maintained to move nutrients to roots where uptake occurs. Citrus trees utilize organic and inorganic forms of nitrogen, but nitrate (NO) and ammonium nitrogen3(NH) are the most common forms. It is well documented4 +that the source of these ions (electrically charged particles) is not important. However, their solubility in the root zone is important. Plant roots will not "disti nguish" nitrogen applied as a granular material and made soluble using irrigation from nitrogen applied as a liquid fertilizer. Fertilizers move by mass flow in the soil solution to roots, as the plants lose water through transpiration. Once nutrients reach the root, they are taken up by an active process powered by energy derived from respiration (energy released from the breakdown of plant compounds). Flooding, excessive irrigation, or low soil temperatures (less than 50 ( F) decrease respiration and reduce nutrient uptake.Current Fertilization PracticesCurrent recommendations for young trees (Table 1)controlled release fer tilizers that are applied less frequently call for the application of approximately 5.6, 9.2, and 12.5than standard materials. Use of controlled release pounds/year of a balanced fer tilizer which contains 0.4,fertilizers in newly planted bedded groves with steep 0.7, and 1.0 pounds nitrogen to 1-, 2and 3-year old trees,banks and minimal sod cover should be monitored to respectively. These figures are average values within theprevent fertilizer losses due to r unoff during heavy rains. given ranges for number of applications, pounds ofSince sulphur-coated fer tilizers may increase acidity of the complete fertilizer, and p ounds of nitrogen. Low rates ofsoil solution, pH should be monitored and adjusted when fertilizer are applied with a high frequency to ensure eventhese fertilizers are used. distribution within the limited root zone and to aviod root damage from excess salt concentrations in localized areas. Although studies have shown that 2 to 3 applications of granular fertilizer are sufficient for adequate growth during the first year, more frequent applications are recommended to ensure optimum fertilization under all Florida conditions with all application methods. Additional applications are seen as insurance against uneven fertilizer distribution when mechanical spreaders are used and whenControlled Release FertilizersControlled or slow release fertilizers are available in a variety of formulations with different analyses. These materials contain fertilizer granules coated with compounds that control the rate of release of nutrients. This rate of release is moderated by soil temperature and moisture and may vary from product to product. Controlled release fertilizers can be broadcast, incorporated after planting, applied as a pre-plant treatment, and can be utilized al ong with a fertigation program to ensure uniform distribution of nutrients throughout the rapidly enlarging root zone of young trees. Current research indicates that controlled release materials applied at recommended rates two times per year can stimulate growth comparable to that of young trees fertilized six times per year with standard materials. Although controlled release fer tilizers usually cost more than standard or readily soluble fertilizers, application rates, energy, and associated costs can be reduced. For example, when application frequency is reduced from six times per year to two times per year, application costs can be reduced by approximately 60% for both solid plantings (@140 trees/acre) and resets (5 trees/acre). When controlled release fertilizers are used, total fertilization costs (fertilizer cost + application cost) are generally lower than total fertilization costs for standard fertilizers for a small number of resets per acre but not for solid plantings. Careful placement and retention of fertilizer within the root zone of young trees becomes more important withFertigationFertigation (the application of soluble fertilizers through irrigation systems) has proven an efficient, effective means of fertilizing citrus trees in many areas of the world. With the widespread use of low volume irrigation in Florida, fertigation has been widely adopted


Fruit Crops Fact Sheet: Fertilization of Youn g Citrus Trees Pa g e 3Ma y 1999in fertilization programs for both y oung trees and matureautomated and computer controlled from remote locations, groves.thus eliminating labor and equipment costs involved in A typical fertigation system consists of a storage tank for the liquid fertilizer, a unit for injecting fertilizer intoFertigation also has some disadvantages and may not the system, and a back-flow prevention device mandatedbe the right system for everyone. Cost for liquid versus by state law. Fertilizer may be injected using a Venturigranular materials are comparable, but the initial tube which relies on pressure differences between theinvestment for storage tanks, valves and injectors, and irrigation line and the storage tank to drive the injectionback-flow prevention devices may be moderate to high. system or by mechanical or electrical systems calibrated toSome liquid formulations do not contain all necessary distribute a predetermined unit of fertilizer/unit of waternutrients requiring additional foliar or granular applied. The Venturi method requires no external powerapplications. Fertigation systems must be properly source or energy expenditure and is low in cost, but cannotmanaged to be effective. Constant monitoring of irrigation be adjusted as accurately as a mechanical or electricallines, emitters and fertilizer injection systems is injector and usually cannot supply fertilizer at uniformrecommended to ensure uniform coverage. During times of rates under high demand cond itions.high rainfall where the soil becomes saturated, fertigation water, are leached or are not rapidly taken up by the rootsOperation of Fertigation SystemsSystem design and uniforrn emitter output and position are the keys to proper design of fertigation systems. The amount of fer tilizer applied is directly related to the amount of water applied, fer tilizer analysis, and fertilizer injection rate. Small variations in sprinkler or emitter output will greatly affect am ount of fer tilizer applied per tree, particularly on a seasonal basis. Emitters should be pos itioned for optimal distribution of nutrients to the limited root zone of young trees. Ongoing maintenance, including the use of screen or media filters, can prevent problems associated with clogging of em itters and damage to tubing. Considerable controversy exists over proper fertigation rates and frequencies. Rates usually correspond to those used for granular application but frequency of application may range from weekly to bimonthly to monthly. Current research suggests that tree growth is similar whether materials are applied at weekly or monthly intervals provided that rates of irrigation and fertilization are equivalent. However, frequent applications guarantee that nutrients are being supplied to the tree and ensure against losses from excessive rainfall or irrigation. Since most growers will fertigate when they irrigate, there is no disadvantage to making frequent applications from an energy or cost basis. Fertigation has many advantages over other fertilization systems. Fertilizer can be applied at any time during the year and at the frequency desired. If properly utilized, fertigation can also reduce fertilizer rates because nutrients can be applied when needed in a soluble form directly in the root zone. Fertigation systems also may be fertilizer spreading. becomes inefficient because nutrients run off in irrigation due to low oxygen conditions. Fertigation may increase total dissolved salts (TDS) levels of irrigation water and when used in combination with water already high in TDS may cause damage to trees. TDS levels above 900 ppm may cause damage to leaves and should be avoided.


Fruit Crops Fact Sheet: Fertilization of Youn g Citrus Trees Pa g e 4Ma y 1999 Table 1. Su gg ested fertilization of y oun g citrus trees up to 7 y ears of a g e. Years in GroveYearTree* ( Ran g e ) per tree Number of Applications eachPounds per Application perPounds N per Application First5 60.75 1.250.06 0.10 Second4 51.75 2.250.14 0.18 Third3 43.0 4.00.24 0.32 Fourth 3 4 3.5 4.50.28 0.36 Fifth 3 4 4.0 5.00.32 0.40 Sixth 3 4 4.5 5.50.36 0.44 Seventh 3 4 5.0 6.00.40 0.48 *Use 8-8-8-1.6-0.4-0.2-0.025 mixure or e q uivalent ( adapted from Bulletin 536D )