A successful soil fertility program for wheat requires knowledge of a field’s yield potential and a recent soil test. The soil test will provide current levels of phosphorus and potassium in the soil and the soil pH. Soil pH will assist in determining the need for micronutrients and other soil amendments—most importantly, lime. When the proper soil pH is maintained, adequate levels of micronutrients and secondary nutrients should be released by the soil organic matter. The target soil pH should be above 6.0 and below 6.8 for western Ohio (subsoils derived from limestone) and above 6.5 and below 6.8 for eastern Ohio (subsoils derived from shale and sandstone). The lime test index or buffer pH on the soil test should be used for lime recommendations. These recommendations are for mineral soils with adequate drainage containing 1%–5% organic matter. Organic soils (organic matter > 20%) and sandy soils (CEC < 6) will require different recommendations.
Refer to Extension bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa, available online at extensionpubs.osu.edu/tri-statefertilizer-recommendations-for-corn-soybean-wheatand-alfalfa, for nitrogen, phosphorus and potassium recommendations. The following discussion of these nutrients has been adapted from this publication.
Nitrogen (N)
Nitrogen rates are based on yield potential and not on soil analysis. Nitrogen recommendations are given in Table 6.4 or may be calculated by the following equation: N rate (lb N/acre) = (1.33 x Yield Potential) - 13
For the calculated nitrogen rate, a small portion should be applied in the fall and the rest after green-up. Applying starter N up to 25 pounds per acre can promote autumn growth and tillering with timely planted winter wheat. Spring recommendations should be the total nitrogen required less the amount applied in the fall. For example, a wheat crop with a 90-bushel-per-acre yield goal would require 110 pounds nitrogen per acre (Table 6.4). If the grower applied 20 pounds in the fall, the remaining 90 pounds should be applied in the spring. No credits are given for previous crops.
Yields are generally not affected when the initial spring nitrogen is applied between green-up and Feekes GS (growth stage) 6 (first node visible). Nitrogen losses may be severe on applications prior to green-up and may cause significant yield reductions, regardless of nitrogen source. Significant yield losses may also occur if initial spring applications are delayed until after Feekes GS 6.
Split Applications. Split application may improve nitrogen efficiency; however, in most years, yield gains from a split application have not been large enough to offset the application cost of a second trip across a field. A split spring application program may be a benefit in poorly drained fields that are prone to nitrogen loss and in years that the potential for nitrogen loss is great. Years that have a potential for nitrogen loss generally have a warmer than normal winter followed by a warm and wet April. Delaying initial nitrogen application until closer to Feekes GS 6 would have the same effect as a split application without sacrificing yields. In a split application program, the larger proportion of the nitrogen should be in the second application by Feekes GS 6.
Nitrogen Source. Nitrogen source is not a concern unless conditions are conducive to nitrogen loss. In general, urea-ammonium nitrate solutions have the greatest potential for loss, followed by urea and then ammonium sulfate the least. Risk for nitrogen loss potential is the greatest for early applications and decreases as plants approach Feekes GS 6. Fields prone to wet conditions would also be susceptible to nitrogen loss. If nitrogen loss is not a concern, economics and application equipment should determine nitrogen source.
Phosphorus (P)
Phosphorus should be applied before planting when the soil-test level is below 50 ppm (Mehlich-3). Recommendations are determined by yield goal and soil-test level (Table 6.5). Phosphorus and fall-applied nitrogen are often applied as diammonium phosphate (DAP) or monoammonium phosphate (MAP).
Potassium (K)
Potassium recommendations are based upon yield potential, soil CEC, and the soil-test level (Table 6.6). Soils with a CEC >5 meq/100 g (loams and clays) have a greater risk of potassium becoming unavailable to the crop and require more potash than soils with a CEC <5 meq/100 g (sands).
Sulfur (S)
Sandy soils and soils low in organic matter may respond to sulfur fertilizer. Medium- to fine-textured soils with adequate organic matter generally have not produced higher yields with supplemental sulfur. Current research has shown limited yield increase on these soils. However, atmospheric depositions have decreased over past decades as sulfur emissions from manufacturing processes have diminished, which may cause these soils to be deficient in the future. Sulfur rates have not been established as a result of soils generally not being deficient; however, 20 to 40 pounds per acre of sulfur mixed with topdress nitrogen should be adequate for soils suspected of being deficient. Suitable sulfur fertilizers include ammonium sulfate, ammonium thiosulfate, and gypsum.
Manganese (Mn)
Manganese deficiency has rarely been seen in Ohio wheat fields. Generally, the whole field is not deficient, and the deficiency is found in pockets and small areas of a given field. Deficient soils have generally occurred where soil pH is above 7.0. Deficient plants will have reduced tillers, appear weak and thin, and have leaves with interveinal chlorosis or white specks and blotches. Foliar applications of 4 pounds per acre of manganese (generally manganese sulfate) is often the best practice for mineral soils with a history of manganese deficiency, which may be added to spring applications of urea-ammonium nitrate.