Best Management Practices
With the passage of the Federal Clean Water Act (FCWA) in 1972, states were required to assess the negative impacts of agricultural fertilizer additions on surface and ground water quality. Upon identification of the impaired water bodies, Florida has established the numeric criteria specific for the waterbodies and the reductions in applied nutrients required as per the Basin Management Action Plan, where excess of nutrients is found to be from agricultural sources. For vegetable production regions, water quality indicators are concentrations of nitrate, phosphate, and total dissolved solids. Best Management Practices (BMPs) are specific cultural practices aimed at reducing the load of specific nutrients entering ground and surface water while sustaining economical yields. BMPs are intended to be economically sound, cost effective, and environmentally friendly based on science. It is important to recognize that BMPs do not aim at becoming an obstacle to vegetable production. Instead, they should be viewed as a means to achieve horticultural and environmental sustainability. The BMPs that will apply to vegetable production in Florida are described in Water Quality/Quantity Best Management Practices for Florida Vegetable and Agronomic Crops, produced by the Florida Department of Agriculture and Consumer Services (FDACS). This manual was developed through a cooperative effort between state agencies, water management districts, and commodity groups, and under the scientific leadership of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS). The manual was adopted by reference in 2006 and by rule in Florida Statutes (5M-8 Florida Administrative Code) and was revised in 2015 (https://www.fdacs.gov/content/download/77230/file/vegAgCropBMP-loRes.pdf). Vegetable growers may contact their local UF/IFAS Extension agent for one-on-one consultation on (1) the benefits from joining the BMP program, (2) how to join it, (3) how to select the BMPs that apply to their operation, and (4) how to meet the requirements.
The vegetable BMP program has adopted the current UF/IFAS nutrient recommendations (UF/IFAS Standardized Nutrient Recommendations for Vegetable Crop Production in Florida, https://edis.ifas.ufl.edu/cv002), including irrigation management (see the new BMP manual on “Optimum Fertilizer Management”). At the field level, adequate fertilizer rates should be used together with proper irrigation-scheduling techniques, and crop nutritional status monitoring tools (leaf analysis, petiole sap testing) may also be employed as appropriate. In the BMP manual, adequate fertilizer rates may be achieved by combinations of UF/IFAS-recommended basal rates and supplemental nitrogen allowances to be added in case of leaching rainfall, when planting during cooler seasons, when tissue analysis shows any nutrient deficiency, or when the harvesting season is prolonged.
Vegetables are grown in various soil types throughout the state. These soil types include sandy and sandy loam soils, muck soils, and calcareous marl soils. Vegetables are produced predominantly on sandy soils throughout the Florida peninsula and on sandy and sandy loams in the Panhandle. Sandy soils have some advantages, such as ease of tillage, production of the earliest vegetable crops, and timely production operations, but also disadvantages, including the potential for leaching mobile nutrients such as nitrogen, potassium, and even phosphorus after heavy rains or excessive irrigation. For more information on soils, refer to Agricultural Soils of Florida (https://edis.ifas.ufl.edu/ss655). Therefore, sandy soils must be managed carefully regarding nutrient programs and irrigation scheduling. For more information, see Soil and Fertilizer Management for Vegetable Production in Florida (https://edis.ifas.ufl.edu/cv101).
A well-prepared planting bed is important for uniform stand establishment of vegetable crops. Previous crop residues and weeds should be plowed down well in advance of crop establishment. A 6-to-8-week period between plowing down of green cover crops and crop establishment is recommended to allow the decay of the residues. Freshly incorporated plant material promotes high levels of damping-off organisms, such as Pythium spp. and Rhizoctonia spp. Turning under plant residue well in advance of cropping reduces damping-off disease organisms. Land should be kept disked, if necessary, to keep new weed cover from developing prior to cropping.
In the Panhandle soils, chisel plowing may aid in breaking down subsurface hardpan in fields. For more information about soil preparation for commercial vegetable production, see Soil Preparation and Liming for Vegetable Gardens (https://edis.ifas.ufl.edu/vh024).
Current UF/IFAS recommendations call for maintaining soil pH between 6.0 and 6.5 (Table 1); further discussion is in Soil pH Range for Optimum Commercial Vegetable Production (https://edis.ifas.ufl.edu/hs1207). If soil pH is too low, liming will be needed to correct the pH to the target range. A common problem in Florida has been overliming, resulting in high soil pH tying up micronutrients and phosphorus, limiting uptake by plants. Overliming can also reduce the accuracy with which a soil test can predict the supplemental applications of fertilizer nutrients based on the Crop Nutrient Requirement (CNR) philosophy. For more information about liming, see Liming of Agronomic Crops (https://edis.ifas.ufl.edu/aa128). Liming can not only adjust soil pH but also provide calcium and magnesium if dolomite (i.e., calcium magnesium carbonate) is used.
Irrigation water from wells in limestone aquifers is an additional source of liming material. The combination of liming and use of alkaline irrigation water has resulted in soil pH greater than 7.0 for many sandy soils in Florida. To measure the liming effect of irrigation, a water sample has to be analyzed for total bicarbonates and carbonates annually with the results converted to pounds of calcium carbonate per acre. Liming (Table 2), fertilization (Table 3), and irrigation programs are closely related to each other. To maximize overall production efficiency, soil and water testing in a critical BMP must be made a part of any nutrient management program. Elevated soil pHs can be adjusted to desired ranges by identifying the reason(s) behind the increases. More information on soil pH reduction can be found in Lowering Soil pH to Optimize Nutrient Management and Crop Production (https://edis.ifas.ufl.edu/ss651).
Fields where seepage irrigation is used, or fields prone to flooding, or fields where the soil profile is too shallow should be cropped using raised beds. Beds generally range from 3 to 8 inches in height, with high beds of 6 to 8 inches preferred where risk of flooding is high. Raised beds dry faster than nonbedded soils in addition to effectively suppressing the weeds. Raised beds promote early-season soil warming, especially when covered with black plastic mulch, resulting in somewhat early crops during cool seasons. Mulching requires a smooth, well-pressed bed for efficient heat transfer from black mulch to the soil. Adequate soil moisture is essential in forming a good bed for mulching using a bed press. Depending on the planting date and the sensitivity of the crop to heat stress, growers may consider using white or reflective plastic mulch instead of black mulch.
Commercial vegetable production requires intensive nutrient management for optimal production. Effective implementation of 4R nutrient stewardship principles—Right Source, Right Rate, Right Place, and Right Time—when applying nutrients to a crop is shown to enhance nutrient efficiencies and minimize nutrient loss to the environment. More information about the 4Rs is available in What is 4R Nutrient Stewardship? (https://edis.ifas.ufl.edu/hs1264) and The Four Rs of Fertilizer Management (https://edis.ifas.ufl.edu/ss624). For tomato production, more information is available in Implementing the Four Rs (4Rs) in Nutrient Stewardship for Tomato Production (https://edis.ifas.ufl.edu/hs1269).
Soil testing is the #1 BMP for nutrient management. There are 17 elements essential to plant growth (Table 4). The crop nutrient requirement (CNR) for a particular nutrient is defined as the total amount in lb/A of that element needed by the crop to produce optimum economic yield. The CNR can be satisfied from many sources, including soil, water, air, organic matter, or fertilizer.
The CNR for a crop has been determined from field calibrations and validation. The CNR is equivalent to the nutrient rate above which no significant increase in yield is expected. The CNR values derived from such experiments take into account factors such as the source, solubility, and availability in the soils. It is important to remember that nutrients are supplied to the crop from both the soil and fertilizers. Supplemental nutrients should be applied only when a properly calibrated soil test indicates a yield or quality response. Mehlich-3 is the standard soil extractant in Florida for all acid-mineral soils and calcareous soils of Miami-Dade County. For mineral soils with pH of =7.4, currently the ABDTPA procedure is used. For all vegetable production in muck soils, water extraction is used for phosphorus, and acetic acid for potassium, calcium, and magnesium. More information about Mehlich-3 is available in Extraction of Soil Nutrients Using Mehlich-3 Reagent for Acid-Mineral Soils of Florida (https://edis.ifas.ufl.edu/ss620).
Nitrogen recommendations are based on research data and not on a soil test. A standard soil test provides soil pH, lime requirement (if needed), P, K, Ca, Mg, S, Cu, Mn, and Zn in mg/kg (ppm), and the recommendations are generated based on the interpretation specific for the extractant. More information about soil testing can be found in Developing a Soil Test Extractant: The Correlation and Calibration Processes (https://edis.ifas.ufl.edu/ss622) and Soil Testing for Plant-Available Nutrients—What Is It and Why Do We Use It? (https://edis.ifas.ufl.edu/ss621).
Plant Tissue Analysis
Analysis of plant tissues (e.g., leaves or petioles) for nutrient concentrations provides a good tool for monitoring nutrient status of a crop during the growing season. There are two main approaches to plant tissue testing: standard laboratory analysis and the plant sap testing procedures. Standard laboratory analysis involves analyzing the most recently matured leaf of the plant for an array of nutrients. The resulting analyses are compared against published adequate ranges for that particular crop. Laboratory results that fall outside the adequate range for that nutrient may indicate either a deficiency or possibly toxicity (especially in the case of micronutrients). The most recently matured leaf serves well for routine crop monitoring and diagnostic procedures for most nutrients. However, for immobile nutrients such as Ca, B, Zn, Fe, Mn, Cu, and Mo, younger leaves are generally preferred.
The second approach is use of plant sap quick-test kits that have been calibrated for N and K for several vegetables in Florida. These testing kits analyze fresh leaf petiole sap for N and K. Quick tests can be a valuable tool for on-the-spot monitoring of plant nutrient status. Diagnostic information for leaf and petiole sap testing can be found in Plant Tissue Analysis and Interpretation for Vegetable Crops in Florida (https://edis.ifas.ufl.edu/ep081) and Petiole Sap Testing for Vegetable Crops (https://edis.ifas.ufl.edu/cv004). However, standard plant tissue test at a laboratory is the primary tool for ground-truthing results to overcome inadequacies in field calibration of these alternate tools.
N, P, and K Sources
Nitrogen (N) is the most limiting nutrient in agriculture. The amount of nitrogen required by vegetable plants must be applied each growing season because residual N is lost to the environment through several pathways. Nitrogen requirements vary among crops (Table 5) and are not dependent on soil test results. All other nutrients must be applied based on soil test results as described above to comply with the BMP guidelines. The interpretations of Mehlich-3 (low, medium, and high) are shown in Table 6. UF/IFAS standardized nutrient recommendations based on Mehlich-3 testing include P2O5 and K2O (Table 7) and nutrient management using fertigation (Table 8). More information on Mehlich-3 can be found in Extraction of Soil Nutrients Using Mehlich-3 Reagent for Acid-Mineral Soils of Florida (https://edis.ifas.ufl.edu/ss620). Nutrient recommendations found in Tables 7 through 10 were determined in field rate studies considering a wide range of nutrient applications and various soil pH levels. Crop plant development, crop yield, and vegetable quality were considered in determining the optimum nutrient levels for UF/IFAS recommendations.
Nitrogen can be supplied in both nitrate and ammoniacal forms. Because the mineralization rate of conversion is reduced in cold, fumigated, or strongly acidic soils, it is recommended that under such conditions 25% to 50% of the N be supplied from nitrate sources. This ratio is not critical for unfumigated or warm soils.
Phosphorus (P) can be supplied from several sources, such as diammonium phosphate (DAP), monoammonium phosphate (MAP), or monopotassium phosphate, based on the soil pH and other factors. Initial soil reaction pH with DAP is about 8.5, which favors ammonia production and volatilization. This produced ammonia causes seedling injury and inhibits root growth. Adequate separation of seed and DAP is needed to eliminate any seedling damage. DAP should not be used on calcareous or high-pH soils. MAP’s reaction pH is 3.5, so it does not have the above problems and is better suited for acidic and lower-pH soils.
Potassium (K) can also be supplied from several sources, including potassium chloride (muriate of potash—60%), potassium sulfate (sulfate of potash—50%), potassium nitrate, and potassium-magnesium sulfate (Sul-Po-Mag). If adhering to amounts of K fertilizer recommended by soil tests, there should be no concern about the K source or its relative salt index. However, when applying chloride-containing sources, crop sensitivities must be considered.
Ca, Mg, and S Sources
The secondary nutrients calcium (Ca), magnesium (Mg), and sulfur (S) are generally sufficient in Florida soils. Calcium is ubiquitous in Florida soils due to their genesis from limestone base. Depending on the location, Ca tends to occur very close to or on the surface and typically within the root zone. Therefore, Ca is not interpreted on soil tests. Irrigation water also generally contains dissolved Ca, so maintaining optimum moisture levels in the soil via irrigation will ensure Ca supply to the roots. Calcium is not mobile in the plant; therefore, foliar sprays of Ca are not likely to correct deficiencies. It is difficult to place enough foliar-applied Ca at the growing point of the plant on a timely basis.
Magnesium deficiency may be a problem for vegetable production; however, when the Mehlich-3 soil-test index for Mg is below 20 ppm, 35 lb Mg/A will satisfy the crop Mg requirement. If lime is also needed, Mg can be added by using dolomite as the liming material. If no lime is needed, then the Mg requirement can be satisfied through use of magnesium sulfate or Sul-Po-Mag. Blending of the Mg source with other fertilizer(s) to be applied to the soil is an excellent way of ensuring uniform application of Mg to the soil.
Although S deficiencies are not common in Florida soils, sulfur deficiency may occur in sandy soils low in organic matter. If a Mehlich-3 soil test determines that the S level is <6 mg/kg, or ppm, then S deficiency may be diagnosed and can be corrected by using S-containing fertilizers, such as magnesium sulfate, ammonium sulfate, potassium sulfate, or potassium-magnesium sulfate. Using one of these materials in the fertilizer blends at levels sufficient to supply 20 lb S/A or higher should prevent S deficiencies.
It has been common in Florida vegetable production to routinely apply a micronutrient package. This practice has been justified on the basis that these nutrients were inexpensive and their application appeared to be insurance for high yields. In addition, there was little research data and a lack of soil-test calibrations to guide judicious application of micronutrient fertilizers. Confounding the problem has been the vegetable industry’s use of micronutrient-containing pesticides for disease control.
Copper (Cu), manganese (Mn), and zinc (Zn) from pesticides have tended to accumulate in the soil. This situation has forced some vegetable producers to overlime in an effort to reduce availability and avoid micronutrient toxicities. Table 10 provides guidelines for the above micronutrient on sufficiencies, toxicities, and soil pH dependencies. It is unlikely that micronutrient fertilizers will be needed on old vegetable land, especially where micronutrients are being applied regularly via recommended pesticides. A standard soil-test report includes micronutrients also.
Manures and Composts
Waste organic products, including animal manures and composted organic matter, contain nutrients that can be recovered by crops. These materials applied to the soil gradually decompose, releasing nutrients for vegetable crops to utilize. The application of these materials has to comply with food safety requirements, such as those of the Produce Safety Alliance (PSA). The key to proper use of organic materials as fertilizers comes in the knowledge of the nutrient content and the decomposition rate of the material. Growers contemplating using organic materials as fertilizers should have an analysis of the material before determining the rate of application. Sludge is not permitted for land application in vegetable production. Decomposition rates of organic materials are relatively rapid in warm, sandy soils in Florida. Residual nutrient levels in soils after the crop season are generally limited. Usually application rates of organic wastes are determined largely by the N content, which will result in inadvertent P applications as well. Excessive rates of organic waste materials can contribute to groundwater or surface water pollution; therefore, it is important to understand the nutrient content and the decomposition rate of the organic waste material and the P-holding capacity of the soil. For more information about using manure for vegetable production, see Using Composted Poultry Manure (Litter) in Mulched Vegetable Production (https://edis.ifas.ufl.edu/ss506) and Introduction to Organic Crop Production (https://edis.ifas.ufl.edu/cv118).
As a soil amendment, compost improves soil's physical, chemical, and biological properties, thus making soil more productive. To eliminate or minimize human and plant pathogens, nematodes, and weed seeds, the composting temperature must be kept in a range from 131°F to 170°F for 3 days in an in-vessel or static aerated pile. Nitrogen in compost is basically organic. Thus, before being mineralized, compost N is not as readily bioavailable as synthetic N fertilizers. Compost N mineralization rate varies with feedstock, soil characteristics, and composting conditions. Compost N fertilizer releases only 5% to 30% bioavailable N to crops in the first year. Contrarily, compost P and K are as bioavailable as chemical fertilizers. Composting converts raw organic materials to humus-stable forms and hence minimizes possible adverse impacts on the environment.
Fertilizer rate and placement must be considered together. Banding low amounts of fertilizer too close to plants can result in the same or greater amount of damage as broadcasting excessive amounts of fertilizer on the field. Because P is immobile in soils, it should be banded alongside the plant rows. Micronutrients can be broadcast with the P and incorporated in the bed area. In calcareous soils, micronutrients, such as Fe, Mn, and B, should be banded or applied foliarly. Because N and K are easily prone to leaching in sandy soils, they must be managed properly to maximize crop uptake. Both N and K should be split in unmulched production systems to minimize losses below the root zone. Hence, one-third to one-half of the N and K may be applied to the soil at planting or shortly after that time. The remaining fertilizer can be applied in one or two applications during the early part of the growing season. Split applications also will help reduce the potential for fertilizer burn, which is defined as leaf scorch resulting from overfertilization. In mulched beds with fertigation, both and N and K should be applied in 10–14 equal split installments for efficient uptake by plant roots and minimized leaching.
When using plastic mulch, fertilizer placement depends on the type of irrigation system (seepage or drip) and on whether drip tubing or the liquid fertilizer injection wheels are to be used. With seepage irrigation, all P and micronutrients should be incorporated in the bed. Apply 10% to 20% (but not more) of the N and K with the P. The remaining N and K should be placed in narrow bands on the bed shoulders, the number of which depends on the crop and number of rows per bed. These bands should be placed in shallow (2-to-2½-inch-deep) grooves. This placement requires that adequate bed moisture be maintained so that capillarity is not broken. Otherwise, fertilizer will not move to the root zone. Excess moisture can result in fertilizer leaching. Fertilizer and water management programs are linked. Maximum fertilizer efficiency is achieved only with close attention to water management.
In cases where supplemental sidedressing of mulched crops is needed, applications of liquid fertilizer can be made through the mulch with a liquid fertilizer injection wheel. This implement is mounted on a tool bar and, using 30 to 40 psi pressure, injects fertilizer through a hole pierced in the mulch.
Supplemental Fertilizer Applications and BMPs
In practice, supplemental fertilizer applications, when growing conditions require doing so, allow vegetable growers to stay within BMP guidelines while numerically applying fertilizer rates higher than the standard UF/IFAS-recommended rates. Possible conditions that may require supplemental fertilizer applications are leaching rains, cooler planting seasons, and extended harvest periods. Applying additional fertilizer under the following four circumstances is part of the current UF/IFAS fertilizer recommendations and thus BMPs: (1) If grown on bare ground with seepage irrigation, a 30 lb/A of N and/or 20 lb/A of K2O supplemental application is allowed after a leaching rain, defined as when it rains at least 3 inches in 3 days, or 4 inches in 7 days; (2) potatoes planted in cooler seasons may receive a supplemental application of 25 lb/acre P2O5; (3) if nutrient levels in the leaf or in the petiole fall below the sufficiency ranges, the supplemental amount allowed for bare-ground production is 30 lb/A of N and/or 20 lb/A of K2O, and for drip-irrigated crops, 1.5 to 2.0 lb/A/day for N and/or K2O for one week; or (4) for economic reasons, the harvest period has to be longer than the typical harvest period. When the results of tissue analysis or petiole testing are below the sufficiency ranges, a supplemental 30 lb/A N and/or 20 lb/A of K2O may be made for each additional harvest for bare-ground production. For drip-irrigated crops, the supplemental fertilizer application is 1.5 to 2.0 lb/A/day for N and/or K2O until the next harvest.
Common irrigation systems used for fertigation include drip, sprinkler, and pivot systems. Advantages of fertigation over conventional fertilizing methods are (1) more efficient delivery of nutrients, (2) more precise localized application, (3) more flexible control of application rate and timing, and (4) lower application cost. Liquid and water-soluble fertilizers are more commonly used for fertigation than dry fertilizers. The most common liquid N fertilizers for fertigation are ammonium nitrate (20-0-0), calcium ammonium nitrate (17-0-0), and urea ammonium nitrate (32-0-0). Complete fertilizers (e.g., 8-8-8 and 4-10-10) are also commonly used. For commercial vegetable production in south Florida, a formula of 4-0-8 or 3-0-10 is the most common in fertigation. To develop a more precise fertilizer application strategy, growers can request a custom blend at a local fertilizer dealer based on soil test results and crop nutrient requirements. For more information, consult Fertigation Nutrient Sources and Application Considerations for Citrus (https://journals.flvc.org/edis/article/view/108095).
The basic components for a fertigation system include a fertilizer tank, an injector, a filter, a pressure regulator, a pressure gauge, and a backflow prevention device. All components must be resistant to corrosion. In most situations, N and K are the nutrients injected through the irrigation tube. Split applications of N and K through the irrigation system offer a means to capture management potential and reduce leaching losses. Other nutrients, such as P, are usually applied to the soil rather than by injection. This is because chemical precipitation can occur with these nutrients and the high calcium carbonate content of our irrigation water in Florida.
Nutrient management through irrigation tubes involves precise scheduling of N and K applications. Application rates are determined by crop growth and resulting nutrient demand. Demand early in the season is small, and thus rates of application are small, usually on the order of ½ to ¾ lb of N or K2O per acre per day. As the crop grows, nutrient demand increases rapidly, so that for some vegetable crops such as tomato the demand might be as high as 2 lb of N or K2O per day. Schedules of N and K application have been developed for most vegetables produced with drip irrigation in Florida (Table 7).
Irrigation water with high and alkaline pH can be acidified using injections of sulfuric acid, phosphoric acid, N-phuric acid, hydrochloric acid, urea-sulfate, etc.
Foliar fertilization should be used as the last resort for correcting a nutrient deficiency (Table 11). The plant leaf is structured in such a way that it naturally resists infiltration of fertilizer. Foliar fertilization is most appropriate for micronutrients but not appropriate for macronutrients, such as N, P, and K. In certain situations, temporary deficiencies of Mn, Fe, Cu, or Zn can be corrected by foliar application. For example, micronutrients should be foliar applied in the following situations: (1) In winter when soils are cool and roots cannot extract adequate micronutrients; and (2) in high-pH soils (marl and Rockdale soils) that immobilize broadcast micronutrients. There is a fine line between adequate and toxic amounts of micronutrients. Indiscriminate application of micronutrients may reduce plant growth and yields because of the toxicity. The micronutrients can accumulate in the soil and may cause yield and economic losses in vegetable production. If you are not sure if your crop requires micronutrients or how much you should apply, contact your local UF/IFAS Extension agent.
The 5th R, Right Irrigation
Fertilization and irrigation go hand in hand, with fertilizers included in irrigation schedules and systems. Water is the solvent of all nutrients and the carrier of almost every pollutant. Keeping moisture and fertilizer primarily in the root zone by managing irrigation inputs and drainage minimizes nutrient-related impacts. Irrigating in excess of the soil’s water-holding capacity leads to increased runoff or leaching and may result in greater production costs or smaller marketable yields. Similarly, insufficient water supply to crops can reduce nutrient bioavailability for vegetable production. For more information, see Implementing the Five Rs of Nutrient Stewardship for Fertigation in Florida’s Vegetable Production (https://edis.ifas.ufl.edu/publication/HS1386).
Table 1. A general guideline to crop tolerance of mineral soil acidity.1
Table 2. Liming materials.
Table 3. Effect of some fertilizer materials on soil pH.
Table 4. Nutrient elements required by plants.
Table 5. Target pH and nitrogen (N) fertilization recommendations for selected vegetable crops in mineral soils of Florida.
Table 6. Soil test interpretation for Mehlich-3 extractions for vegetable crops in Florida.
Table 7. Phosphorus (P, expressed as P2O5) and potassium (K, expressed as K2O) fertigation recommendations for selected vegetable crops in mineral soils for Florida based on low, medium, and high soil-test index using the Mehlich-3 soil extractant method. (For details, refer to UF/IFAS Standardized Nutrient Recommendations for Vegetable Crop Production in Florida, https://edis.ifas.ufl.edu/cv002.)
Table 8. Fertigation1 and supplemental fertilizer1 recommendations on mineral soils testing low in potassium (K2O) based on the Mehlich-3 soil extraction method.
Table 9. Soil test guidelines for micronutrients.
Table 10. Foliar fertilizer sources and rates for vegetable production in Florida.