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Circular 1222 Nitrogen Management Practices for Vegetable Production in Florida1 G. J. Hochmuth2 1. This document is Circular 1222; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First Published: April 2000. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. G.J. Hochmuth, professor, Horticultural Sciences Department, and Center Director, North Florida Research and Education Center, Quincy Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Introduction Nitrogen (N) is an important element for economic vegetable production, and is especially required for successful production on all mineral-soil farms in Florida. Nitrogen is required on mineral soils because such soils are largely sandy in nature, having low organic matter content. Thus, these soils retain little N against leaching so that frequent, intense rainfall, or excessive irrigation water, can leach N from the root zone and potentially into groundwater. Nitrogen management strategies should be used on vegetable farms to maximize the chances that N fertilizer will benefit crop yields and fruit quality, while minimizing the chances that N will be lost to the environment. This publication outlines some of the major N management practices, determined from research and grower experience, that can be used on vegetable farms to ensure that N fertilization results in economic vegetable production without serious negative impact on the environment. Water Management Water management on the vegetable farm is a critical component of N management. Regardless of the form or source of N fertilizer applied to the soil (nitrate, ammoniacal, urea, or easily mineralized organic N), the end-product, in a few days to a few weeks, is the nitrate ion, which easily leaches in sandy soils. Growers can do nothing to avoid frequent, intense rainfall received during the growing season; however, vegetable growers can use cultural practices and fertilizer management strategies that minimize the chances of N leaching. These practices and strategies include: 1. Using polyethylene mulch where practical, and applying N in the root zone under the mulch. 2. Reducing the water table level in subsurface-irrigated fields prior to anticipated heavy rainfall events so there is a soil reservoir for the rain water. This reduces the chances that a sudden rise in the water table will saturate the root zone, solubilize N fertilizer, and leach it from the field. 3. Scheduling N applications to coincide with crop N need and avoiding large applications of N at any given time, part of which might be leached before being used by the crop.
Nitrogen Management Practices for Vegetable Production in Florida 2 4. Avoiding the application of fertilizer or N-containing organic materials, such as sludge or manure, to noncropped areas such as drive roads, field alleys, or row middles. These areas are highly subject to leaching or erosional losses of nutrients, especially in polyethylene mulch culture, because large amounts of rain are shed from the mulched beds into these noncropped areas. 5. Minimizing the chances of large residues of N fertilizer being left in the field at seasons end due to overfertilization of a crop. When the crop is finished or when the mulch is removed, this remaining N is subject to leaching. A component of water management on the farm that is within grower control is irrigation. Although vegetable farms receive large amounts of water from rainfall, this water often comes at times when there are no crops in the field. Therefore, irrigation is required during most seasons for successful vegetable production. There are several important irrigation management practices that should be used to minimize N leaching. These include: 1. Where practical and economical, choosing the most efficient irrigation system. In terms of water use on the farm, efficiency is often greatest with drip irrigation, less with sprinkler irrigation, and least with subsurface irrigation. 2. Operating irrigation systems at peak efficiency with attention to proper design of the overall system, pump, and water delivery components. Repairing all leaks and checking periodically for uniformity of water application, replacing worn parts in the process. 3. With subsurface irrigation, monitoring of water tables with float indicator devices and minimizing the fluctuation of water tables that can scrub N from the root zone. 4. Adopting soil moisture indicator devices, such as tensiometers, or other moisture-status indicator tools. For most sand-soil farms in Florida, a soil moisture potential of -8 to -12 centibars in the root zone is optimum. Moisture potential values less negative than -8 centibars indicate soil water contents that generally exceed the water-holding capacity of the soil, thereby adding to the possibility of N leaching. 5. Scheduling water applications according to crop water needs. Water requirements for several crops have been well documented and these requirements should be used as targets when scheduling irrigation, then fined-tuned using soil-moisture sensors (see Chapter 8 in Hochmuth and Maynard, 1998). Fertilizer Management Optimum fertilization can be accomplished by using the correct amount of N, and applying the N according to a proper schedule and method of placement. The goal is to ensure that the applied N benefits crop yield and vegetable quality, while not negatively impacting the environment. Some growers might be tempted to apply N in excess of what is needed for best yields, because the extra N is thought to reduce overall risk associated with crop production. However, this extra N also represents reduced profits when it does not contribute to greater yields or improved fruit quality. Furthermore, recent research with several vegetable crops and strawberries has documented reduced yields and reduced fruit quality with excess N. In addition, excess N can lead to more disease, and for tomato, to more damage from insects such as thrips. The following N management practices and philosophies can be used on vegetable farms to ensure the greatest benefit from N fertilization while minimizing negative impacts on the environment: 1. Knowing the crop nutrient requirement (CNR) for N and targeting this amount for total crop N fertilization. Current N recommendations for vegetables in Florida are presented in Table 1, Table 2, and Table 3 and, in several cases, are supported by more than 40 years of research and on-farm trials. Supplemental N (30 to 40 lb per acre) should be applied when an impending N deficiency is predicted by leaf or petiole sap testing, or when leaching rainfall occurs (3 inches in 3 days or 4 inches in 7 days). 2. Setting realistic yield goals. Some growers continue to think that N fertilization is the key to greater yields and, therefore, set yield goals at unrealistic and rarely achieved levels. Once
Nitrogen Management Practices for Vegetable Production in Florida 3 optimum N-fertilization programs, determined by research, have been set, then weather and market conditions usually determine the fluctuations in marketable yield from year to year. 3. Using polyethylene mulch, where practical, to protect N from leaching. 4. Selecting controlled-release N fertilizers when practical and economical. 5. Calibrating fertilizer applicators accurately and making adjustments to equipment so that the correct amount of N is applied in the correct position of the root zone or production bed, near the root system. 6. Applying N at periods during the growing season when crop N uptake is most active. Avoiding late-season applications of N when crop uptake rate from the soil is diminishing. Using mid-season broadcasting on unmulched crops only after roots have expanded into the row middles where they can intercept applied N. 7. Using fertigation where possible to spoon-feed N to crops during the season. N-application schedules have been published for vegetables grown with drip irrigation (see Hochmuth and Maynard, 1998 and Hochmuth and Smajstrla, 1997). 8. Managing irrigation water properly to avoid leaching and to keep water and N in the root zone. Following guidelines under the subtopic Water Management above. Production of crops without irrigation is risky in Florida, not only economically but environmentally as well. Well-fertilized but drought-stricken crops will leave large amounts of unused N in the soil, subject to leaching when rain returns. Water tables also must be carefully managed for mulched, subsurface-irrigated crops. If the water table drops too far, any banded N in the bed may be left in overly dry soil, not used by the crop, but susceptible to leaching by rain when the mulch is removed. Optimally managed irrigation will reduce the risk of N leaching and increase the probability of successful crops with the N fertilization programs outlined herein. 9. Using tissue-testing or petiole sap testing (Table 4) to monitor crop-N status and to determine adjustments needed in the N-fertilization program. Guidelines for plant tissue testing have been published for Florida vegetables (Hochmuth et al., 1991). Soil Management Sandy soils in Florida are low in organic matter content, because the organic matter is oxidized (decayed) rapidly in Floridas warm, humid environment. Floridas environment makes it difficult to maintain increased organic matter content in these sandy soils. When N-containing organic matter materials, such as animal manures or composts, are used to raise the soils organic matter content, then consideration also must be given to the potential loss of N upon organic matter decay, which continues during the summer rainy season, for example. Therefore, N-containing organic materials should be land-applied with attention to their N concentration, rate of application, likely N mineralization (decay) rate after application, and growth pattern and N needs of the particular crops to be grown. Cover crops between growing seasons should be used whenever feasible in vegetable production, planted immediately after the vegetable crop is terminated and the soil tilled. Cover crops will capture some of the N left behind by the vegetable crop, and return that N (and organic matter) to the soil for use by subsequent crops. Literature Hochmuth, G.J., and E.A. Hanlon. 1995. IFAS standardized fertilization recommendations for vegetable crops. Fla. Coop. Ext. Serv. Circ. 1152. Hochmuth, G. 1996. Commercial vegetable fertilization guide. Fla. Coop. Ext. Serv. Circ. 225D.
Nitrogen Management Practices for Vegetable Production in Florida 4 Hochmuth, G., D. Maynard, C. Vavrina, and E. Hanlon. 1991. Plant tissue analysis and interpretation for vegetable crops in Florida. Fla. Coop. Ext. Serv. Special Series SS-VEC-42. Hochmuth, G.J., and A.G. Smajstrla. 1997. Fertilizer application and management for micro (or drip) irrigated vegetables in Florida. Fla. Coop. Ext. Serv. Circ. 1181. Hochmuth, G.J., and D.N. Maynard (eds.) 1998. Vegetable production guide for Florida. Fla. Coop. Ext. Serv. Circ. SP170. Hochmuth, G. 1994. Plant petiole sap-testing guide for vegetable crops. Fla. Coop. Ext. Serv. Circ. 1144. Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with eggplant in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-751 (11 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with muskmelon in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-754 (11 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with pepper in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-753 (16 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with potato in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-756 (22 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with snapbean in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-757 (15 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with squash in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-750 (9 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with strawberry in Florida. Fla. Coop. Ext. Fact Sheet HS-752 (18 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with sweet corn in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-758 (14 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with a tomato in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-759 (21 pp.) Hochmuth, G., and K. Cordasco. 1998. A summary of N, P, and K research with watermelon in Florida. Fla. Coop. Ext. Serv. Fact Sheet HS-755 (19 pp.)
Nitrogen Management Practices for Vegetable Production in Florida 5 Table 1. Nitrogen recommendations for vegetable crop production on sandy mineral soils in Florida. Crop N recommendations (lb/acre)z Bean, snap, lima, pole 100 Beet 120 Broccoli, cauliflower, Brussels sprout 175 Cabbage, collard, Chinese cabbage 175 Carrot 175 Celery 200 Cucumber 150 Eggplant 200 Lettuce, crisphead, romaine, endive, escarole 200 Muskmelon 150 Mustard, kale, turnip 120 Okra 120 Onion, bulb 150 Onion, bunching, leek 120 Parsley 120 Pea, southern, snow, English 60 Pepper, bell, specialty 200 Potato 200 Radish 90 Spinach 90 Squash, summer, winter, pumpkin 150 Strawberry 150 Sweet corn 200 Sweet potato 60 Tomato, slicing, cherry, plum 200 Watermelon 150 z Rate of N application for crop growing in one acre (43,560 sq ft) using the typical row (bed) spacings summarized in Table 2. Amounts of fertilizer used per acre will vary with changes in amount of linear bed feet of crop in acre, as shown in Table 3.
Nitrogen Management Practices for Vegetable Production in Florida 6 Table 2. Typical bed (row) spacings for vegetables. Crop Bed (row) spacing Number of rows (per bed) Bean, snap, lima 30 inches 1 Broccoli, cauliflower, Brussels sprout 6 ft (mulched) 2 Cabbage, collard, Chinese cabbage, kale 6 ft (mulched) 2 Carrot 4 ft 2-3 Celery 4 ft 2 Cucumber 6 ft (mulched) 2 Eggplant 6 ft (mulched) 1 Lettuce, crisphead, romaine, endive, escarole 4 ft 2 Muskmelon 5 ft 1 Okra 6 ft (mulched) 2 Onion 6 ft 4 Pea, southern 30 inches 1 Pepper, bell, specialty 6 ft (mulched) 2 Potato 42 inches 1 Squash, summer 6 ft (mulched) 2 Strawberry 4 ft (mulched) 2 Sweet corn 36 inches 1 Sweet potato 42 inches 1 Tomato, slicing, cherry, plum 6 ft (mulched) 1 Watermelon 8 ft 1 For the following crops, see footnotey Mustard Turnip Parsley Pea, snow, English Radish Spinach
Nitrogen Management Practices for Vegetable Production in Florida 7 Table 2. Typical bed (row) spacings for vegetables. Crop Bed (row) spacing Number of rows (per bed) y These crops are generally produced on wide (40 to 48-inch) beds on 6-ft centers with 4 to 6 multiple rows. Some of the crops are also sown in broadcast-fashion on the bed. Table 3. Conversion of fertilizer rates in lb/A to lb/100 linear bed feet (LBF). Bed spacing (ft)z Recommended N rate (lb/acre) 20254050607580100120140160180 200 Pounds (lb) of N to apply per 100 LBF 3 0.140.170.280.350.410.520.550.690.830.961.101.24 1.38 4 0.180.230.370.460.550.690.730.921.101.291.471.65 1.84 5 0.230.290.460.570.690.860.921.151.381.611.842.07 2.30 6 0.280.340.550.690.831.031.101.381.651.932.202.48 2.77 8 0.370.460.730.921.101.381.471.842.202.572.943.31 3.67 z The number of linear bed feet (LBF) for any cropping pattern is equal to 43,560 sq ft divided by the row (bed) spacing (center-to-center). Table 4. Guidelines for plant petiole fresh sap testing for vegetables grown in Florida. Sufficiency ranges for fresh petiole sap concentration (ppm) Crop Crop developmental stage NO3-N K Broccoli and collard Six-leaf stage 800-1000 NRz One week prior to first harvest 500-800 First harvest 300-500 Cucumber First blossom 800-1000 NRz Fruits three inches long 600-800 First harvest 400-600 Eggplant First fruit (two inches long) 1200-1600 4500-5000 First harvest 1000-1200 4000-4500
Nitrogen Management Practices for Vegetable Production in Florida 8 Table 4. Guidelines for plant petiole fresh sap testing for vegetables grown in Florida. Sufficiency ranges for fresh petiole sap concentration (ppm) Crop Crop developmental stage NO3-N K Mid harvest 800-1000 3500-4000 Muskmelon First blossom 1000-1200 NRz Fruits two inches long 800-1000 First harvest 700-800 Pepper First flower buds 1400-1600 3200-3500 First open flowers 1400-1600 3000-3200 Fruits half-grown 1200-1400 3000-3200 First harvest 800-1000 2400-3000 Second harvest 500-800 2000-2400 Potato Plants eight inches tall 1200-1400 4500-5000 First open flowers 1000-1400 4500-5000 50% flowers open 1000-1200 4000-4500 100% flowers open 900-1200 3500-4000 Tops falling over 600-900 2500-3000 Squash First blossom 900-1000 NRz First harvest 800-900 Strawberry November 800-900 3000-3500 December 600-800 3000-3500 January 600-800 2500-3000 February 300-500 2000-2500 March 200-500 1800-2500
Nitrogen Management Practices for Vegetable Production in Florida 9 Table 4. Guidelines for plant petiole fresh sap testing for vegetables grown in Florida. Sufficiency ranges for fresh petiole sap concentration (ppm) Crop Crop developmental stage NO3-N K April 200-500 1500-2000 Tomato (field) First buds 1000-1200 3500-4000 First open flowers 600-800 3500-4000 Fruits one inch in diameter 400-600 3000-3500 Fruits two inches in diameter 400-600 3000-3500 First harvest 300-400 2500-3000 Second harvest 200-400 2000-2500 Tomato (greenhouse) Transplant to second fruit cluster 1000-1200 4500-5000 Second cluster to fifth fruit cluster 800-1000 4000-5000 Harvest season (Dec.-June) 700-900 3500-4000 Watermelon Vines 6 inches in length 1200-1500 4000-5000 Fruits 2 inches in length 1000-1200 4000-5000 Fruits one-half mature 800-1000 3500-4000 At first harvest 600-800 3000-3500 zNR No recommended ranges have been developed.