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The sulfur recommendation is the amount of supplemental fertilizer needed to meet the overall sulfur requirement. The yield goal is multiplied by the S_crop factor to calculate the crop sulfur requirement. The soil test sulfur is subtracted from the crop sulfur requirement to calculate the initial sulfur fertilizer recommendation. This assumes the sample accurately represents the field conditions. Adjustments or credits also may be deducted from the crop sulfur requirement. For example, sulfur applied in manure or compost or potential sulfur mineralized from soil organic matter.
Note, minimum rate: The soil test sulfur concentration may not accurately represent sulfur availability for certain crops in certain situations, especially soils with sandy texture and/or low organic matter. In these cases, ServiTech recommends a minimum sulfur fertilizer rate that overrides the calculated sulfur rate. This requires analysis to provide percent organic matter (OM%) and the cation exchange capacity (CEC) by summation. The organic matter percentage provides a threshold to trigger the minimum rate. The CEC is used to estimate the soil texture and adjust the minimum rate.
The "yield goal" is the final yield expected or projected for the upcoming growing season. Yield goals should be realistic for the particular field conditions. A good suggestion is to use the average yield of the previous five crops, then add five percent (5%).
The S_crop factor is the amount of total seasonal sulfur required to produce one unit of the particular crop (e.g., one bushel, one ton, etc.). The S_crop includes the amount of sulfur removed by harvesting one yield unit, but may be greater than that. For example, additional sulfur may be required for the vegetative growth necessary to produce the final harvested grain yield.
Sulfate and certain soluble sulfur compounds are available for root uptake, so some portion of soil test sulfur is deducted from the crop sulfur requirement. The soil test identifies the soluble sulfur concentration that would be found in the soil solution (i.e., water held in soil pores). This concentration (as parts per million, ppm S) is converted to a mass of sulfur (as pounds of sulfur per acre, lb S/ac) for calculating the sulfur recommendation. The conversion from a concentration to a mass depends on both an assumed soil bulk density and on the soil sample depth increment.
The depth increment is calculated by subtracting the distance from the top of the soil sample to the bottom of the soil sample. For example, a sample collected from the soil surface to an 8-inch depth would have an 8-inch depth increment (8-inch increment = 8 inches - 0 inches). A subsoil sample collected from an 8-inch depth to a depth of 24 inches would have a 16-inch depth increment (16-inch increment = 24 inches - 8 inches).
The density factor assumes the density of a typical soil of medial silt loam texture with 2.5% organic matter that is crushed to pas a 10-mesh screen. The bulk density of a crushed typical soil is 1.18 (73 lb/cu-ft). This compares to an undisturbed soil with a density of 1.32 (82 lb/cu-ft), resulting in an assumed weight of 3.6 million pounds per acre-foot. Thus, an acre-inch of soil would weigh 300,000 pounds (300, 000 = 3,600,000 ÷ 12-inches) or 0.3 million pounds. Multiplying the depth increment by "0.3" provides a factor to convert the concentration as "ppm" to a mass as "lb S/ac". The 0.3 factor may be adjusted for individual situations. For example, using a factor of 0.15 rather than 0.3 would give half-credit to the soil test sulfur.
A certain amount of plant-available sulfur may be mineralized from the soil organic matter over the growing season. The OM_adj factor is an estimated amount of sulfur (as "lb S/ac") that may be mineralized from each 1% of soil organic matter.
The following Crop Files discuss sulfur fertility:
¶ Table 1. |
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| Method: | Ammonium Acetate | Mehlich 3 ICP |
| Modus ID: | S-S-NH4AC.04 | S-S-M3.05 |