7) Soil Fertility

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 (e.g. sulfur) should be released by the soil organic matter. The proper soil pH for western Ohio (subsoils derived from limestone) should be above 6.0 and below 7.0, and above 6.5 and below 7.0 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. Lime recommendations are available from the Ohio State University Extension fact sheet AGF-505-07 Soil Acidity and Liming for Agronomic Production or bulletin 472 Ohio Agronomy Guide 14th Edition. These recommendations are for mineral soils with adequate drainage containing 1 to 5% organic matter. Organic soils (organic matter > 20%) and sandy soils (CEC < 6) will require different recommendations.

A) Nitrogen: rates are based on yield potential and not on soil analysis. Total nitrogen recommendations are given in Table 1 or may be calculated by the following equation:

40 + [1.75 x (yield potential – 50)]

For the corresponding rate, part of it should be applied in the fall and therest after greenup. Generally, 20 to 30 pounds of fall applied nitrogen should be adequate for early fall and spring growth. Spring recommendations should be the total nitrogen required less the amount applied in the fall. No credits are given for previous crops. For example, a wheat crop with a 90 bu/A yield goal would require 110 pounds of nitrogen (Table 1). If the grower applied 30 pounds in the fall, the remaining 80 pounds should be applied in the spring.

Table 1. Nitrogen recommendations for wheat.

Yield potential (bu/A) / Nitrogen Rate (lb/A)
60 / 60
70 / 75
80 / 90
90 / 110
100 / 130

Yields are generally not affected when the initial spring nitrogen is applied between greenup and early stem elongation (Table 2). Nitrogen losses may be severe on applications prior to greenup and may cause significant yield reductions, regardless of nitrogen source (Table 3). Significant yield losses may also occur if initial spring applications are delayed until after early stem elongation (Table 3).

Table 2. Grain yield response to spring N (70-80 lb/A) at different application times -- Custar (2000 - 2006) and S. Charleston, Ohio (2005 - 2006).

Application Time / Location
Custar / S. Charleston
bu/acre
Greenup / 82.2 / 93.5
Early Stem Elongation (Feekes GS 6) / 85.5 / 94.5
Average / 83.8 / 94.0

Table 3. Grain yield response to spring nitrogen (70 lb/A) applied at different growth stages – Custar, Ohio (Lentz, 2003)

Application Time / Nitrogen Source / Average
Urea / Urea-ammonium
Nitrate Solution / Ammonium
Sulfate
bu/acre
Pregreenup (2 wks before greenup) / 55.0 / 54.0 / 62.8 / 57.3
Greenup / 76.3 / 74.1 / 83.9 / 78.1
Early Stem Elongation / 82.3 / 76.0 / 77.9 / 78.7
Late Stem elongation / 70.2 / 68.7 / 73.0 / 70.6
Average / 70.9 / 68.2 / 74.4

Split applications and nitrogen source. In most years, yield gains from a split application have not been large enough to offset the 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 also 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 early stem elongation would have the same effect as a split application without sacrificing yields (Table 2).

Nitrogen source is not a concern unless conditions are conducive for nitrogen loss. In general, urea-ammonium nitrate solutions have the greatest potential for loss, then urea, and ammonium sulfate the least. Risk for nitrogen loss potential is the greatest for early applications and decreases as plants approach early stem elongation. Fields prone to wet conditions would also be susceptible to nitrogen loss. If nitrogen loss is not a concern, economics should determine nitrogen source.

In summary, initial spring application should be applied between greenup and early stem elongation. Waiting until early stem elongation may increase yields slightly but the small gain is offset by the risk of wet conditions at elongation time. If these wet conditions delay application until late stem elongation or later, a yield decrease may occur. Nitrogen source should be dependent upon the risk of nitrogen loss conditions and cost.

B. Phosphorus: should be applied before planting when the soil test level is below 50 ppm. Actual phosphorus recommendations are determined by the yield goal and soil test level (Table 4). Phosphorus and fall applied nitrogen are often applied as diammonium phosphate (DAP) or monoammonium phosphate (MAP).

Table. 4. Phosphorus recommendations for wheat at various yield potentials and soil test levels.

Yield Potential (bu/A) / Soil Test (ppm)
15 / 20 / 25-40 / 45 / 50
lb P2O5/A
60 / 90 / 65 / 40 / 20 / 0
70 / 95 / 70 / 45 / 20 / 0
80 / 100 / 75 / 50 / 25 / 0
90 / 105 / 80 / 55 / 30 / 0
100 / 115 / 90 / 65 / 30 / 0

C. Potash: recommendations are based upon the yield goal, soil cation exchange capacity (CEC) and the soil test level (Tables5 and 6). Soils with larger CEC values have a greater chance of potassium becoming unavailable to the crop, and require more potash than low CEC soils. Table 5 recommendations only account for grain removal of potassium by the crop. Recommendations should be greater in fields where the straw may be baled and removed (Table 6).

Table 5. Potash recommendations for wheat at various yield potentials, CEC, and soil test levels – only grain removed not straw.

Yield Potential (bu/acre) / Soil CEC / Soil test K (ppm)
25 / 50 / 75 / 100 / 125 / 150 / 175
60 / lb K2O/acre
10 / 155 / 115 / 80 / 40 / 40 / 0 / 0
15 / 195 / 150 / 110 / 65 / 40 / 25 / 0
20 / 240 / 190 / 140 / 90 / 40 / 40 / 0
80 / lb K2O/acre
10 / 160 / 125 / 85 / 50 / 50 / 0 / 0
15 / 205 / 160 / 115 / 70 / 50 / 30 / 0
20 / 250 / 200 / 150 / 100 / 50 / 50 / 0
100 / lb K2O/acre
10 / 170 / 130 / 95 / 55 / 55 / 0 / 0
15 / 210 / 165 / 125 / 80 / 55 / 35 / 0
20 / 260 / 205 / 155 / 105 / 55 / 55 / 0

Table 6. Potash recommendations for wheat at various yield potentials, CEC, and soil test levels – grain and straw removed.

Yield Potential (bu/acre) / Soil CEC / Soil test K (ppm)
25 / 50 / 75 / 100 / 125 / 150 / 175
60 / lb K2O/acre
10 / 210 / 170 / 135 / 100 / 100 / 0 / 0
15 / 250 / 205 / 160 / 120 / 100 / 60 / 0
20 / 300 / 250 / 200 / 150 / 100 / 100 / 0
80 / lb K2O/acre
10 / 235 / 200 / 160 / 120 / 120 / 0 / 0
15 / 275 / 230 / 190 / 145 / 120 / 80 / 0
20 / 320 / 270 / 220 / 170 / 120 / 120 / 0
100 / lb K2O/acre
10 / 260 / 225 / 185 / 150 / 150 / 0 / 0
15 / 300 / 260 / 215 / 170 / 150 / 95 / 0
20 / 350 / 300 / 250 / 200 / 150 / 150 / 0

D. Sulfur: historically data has not supported the need for sulfur on medium to fine textured soils with adequate organic matter. However, atmospheric depositions have decreased over past decades as sulfur emissions from manufacturing processes have diminished, casting doubt whether Ohio soils still contain adequate sulfur levels for optimum wheat production. Studies were repeated in 2005 and 2006 to determine if wheat yields may respond to supplemental sulfur (Table 7). Yields were similar to non-sulfur and sulfur treatments confirming that typical Ohio soils would have minimal response to sulfur fertilizer.

Table 7. Grain yield response to spring N (80 lb/A) and sulfur ( 20 or 40 lb/A) -- Lentz and Mullen, 2005 - 2006.

Fertilizer Source / Location
Custar / S. Charleston
bu/acre
Urea / 82.2 / 94.4
Urea-gypsum 20 lb S/A / 81.3 / 95.6
Urea-gypsum 40 lb S/A / 83.7 / 93.7
Average / 82.4 / 94.6

E) Manganese. Wheat is almost as sensitive to manganese deficiencies as is soybean and occurs in the same areas of fields. Deficiency symptoms are usually not severe enough to be seen but will reduce yield. Maintaining soil pH between 6.0 and 7.0 will usually eliminate the problem. For fields that have a history of manganese problems, manganese sulfate can be applied in a band or in contact with seeds at planting or mix 4 pounds per acre with urea-ammonium nitrate solution at spring topdress.