Agronomic Crop Fertilization
Proper fertilization is very important when growing field or forage crops. Development of fertilization programs for Florida requires that consideration be given to nutrient requirements for each crop. Soil nutrient levels, reactions of the soil with added nutrients, and the ability of the soil to retain and deliver nutrients and water are also important factors. These factors are used to determine rates, placement, and timing for fertilizer applications.
Soil tests are very important for fertilization to determine the specific needs of the soil in which the crop will be planted. Suggested fertilization rates are based on soil tests currently used by the University of Florida Soil Testing Laboratory in Gainesville. Fertilization rates are based on broadcast application while lower rates would be suitable in a band application in or near the row. Soil samples should be taken at a depth of 6-8 inches, except when sampling to determine the need for gypsum by peanuts; in this case, samples should be taken to a depth of 3″ from the potential pegging zone of the plant.
Soil Nutrient Recommendations
Nitrogen– Recommendations for N fertilization are not based on soil tests. Fertilizer N is not normally needed for field crops grown on organic soils. Nitrogen can be leached from sandy soils by heavy rainfall. If this occurs, it may be desirable to apply more N as an adjustment to leaching or to use smaller but more frequent applications in an attempt to minimize leaching losses.
Phosphorus– Many fields used for agronomic crops have received annual applications of P-containing fertilizers for many years. As a result, the P levels of these soils are high or very high as measured by a soil test. This element may not be needed until indicated by a soil test.
Magnesium– If the magnesium soil test is below target level and dolomitic limestone was not applied, soluble Mg should be included in the basic application. Application of at least 10lb/Acre of Mg is recommended.
Sulfur– Indications are that at least 20lb/Acre of S should be applied annually in the fertilizers for field crop production. Due to low emissions from industrial establishments, very little sulfur is added to Florida soils in rain water. Fertilizer materials that contain relatively high percentages of sulfur include ammonium sulfate, ordinary superphosphate, sulfate of potash, sulfate of potash-magnesia, agricultural gypsum, and magnesium sulfate.
Calcium– If the soil pH is appropriate for the field crop in question, there will be sufficient Ca for proper plant nutrition.
Micronutrients– Iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), boron (B), and molybdenum (Mo) are the essential micronutrients. Deficiencies of one or more of the micronutrients have been noted on some crops in all sections of Florida. Soil tests for micronutrients are available and should be used where deficiencies are expected. Local experience should serve as the primary guide as to the need for these nutrients.
Application of Micronutrients
Micronutrients may be applied to the soil or to the growing plant. Mo may also be applied as a seed treatment. Soil applications usually involve mixing and then applying the micronutrients with fertilizers or pesticides. Since small amounts are used, mixtures can be applied more uniformly and economically than in a separate operation.
Legume Seed Treatment– Mo may be added to soybean and other legume seed to be planted on mineral soils that have not been limited to a pH of 6.0.
Soil Applications– Solubility of Mn, Zn, Cu, and Fe decreases rapidly as pH increases about 6.0, causing a deficiency. Over liming can provoke micronutrient deficiencies. The best soil application of these nutrients is in a band together with elemental S. The resulting low pH in the band will keep the nutrients soluble throughout the growing season. Care must be taken not to apply excess amounts of the materials or toxicities may result.
Micronutrient Mixtures– Producers should watch for signs of deficiency, determine which nutrient is needed (by responses to single-element applications), and subsequently apply the needed element. However, many homogenized fertilizers have a micronutrient package that will meet the requirements for most plants unless soil levels are very low. Low amounts of these nutrients can be spread uniformly if each pellet of fertilizer has a uniform amount of micronutrients mixed in.
Foliar Applications– Will normally result in a rapid response if a deficiency exists. Also, a response can be obtained with less material than in soil applications. However, there is a danger of burning the foliage if excessive rates are applied. Since there is often more foliage burning with mixtures of certain pesticides and nutrient sprays, they should be applied separately.
Timing and Placement of Fertilizer– Mixing seed with fertilizer and broadcasting both in the same operation has given fair to good stands of many crops. However, legume seed should not be mixed with fertilizers. If this method is used, spread the mixture as quickly as possible. Conventional seeding methods, drilling or broadcasting on a well-prepared seedbed are preferred, since seed can be more uniformly distributed.
You can learn more about agronomic crop fertilization by contacting your local UF/IFAS Extension Agricultural Agent.
Chris Vann- Extension Agent- Agriculture/FCS