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The nitrogen recommendation is the amount of supplemental fertilizer needed to meet the overall nitrogen requirement. The yield goal is multiplied by the N_crop factor to calculate the crop nitrogen requirement. The soil test nitrogen is subtracted from the nitrogen requirement to calculate the nitrogen recommendation. This assumes the sample accurately represents the field conditions. Adjustments or credits also may be deducted from the nitrogen requirement. This may include applied manure or compost, crop grown the previous season, or potential nitrogen mineralized from soil organic matter (see Crop File 1.02.051, Soil Test Nitrogen Credits).
Note: the calculated recommendation is only a recommended rate. The final application strategy depends on the particular nitrogen material, potential use of nitogen enhancement products, application method and equipment, and timing of individual applications. The calculated rate may be reduced for efficiencies obtained from particular sources, application methods, or application timings. Such reductions depend on individual situations or conditions, so must be determined on a site-specific basis.
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 N_crop factor is the amount of total seasonal nitrogen required to produce one unit of the particular crop (e.g., one bushel, one ton, etc.). The N_crop includes the amount of nitrogen removed by harvesting one yield unit, but may be greater. For example, additional nitrogen may be required for the vegetative growth necessary to produce the final harvested grain yield. There may also be inefficiencies or background nitrogen losses due to the growth habit of the particular crop that require additional nitrogen. Nitrogen requirements will be closer to the harvested crop removal in high efficiency production systems.
Nitrate is the preferred form of nitrogen for root uptake, so some portion of soil test nitrate-nitrogen (NO3-N) is deducted from the nitrogen requirement. Routine soil testing identifies the nitrate-nitrogen concentration that would be found in the soil solution (i.e., water held in soil pores). This concentration (as parts per million, ppm NO3-N) is converted to a mass of nitrogen (as pounds of nitrogen per acre, lb N/ac) for calculating the nitrogen recommendation. The conversion from a concentration to a mass depends on both an assumed soil bulk density and on the soil sample depth increment. Exchangeable ammonium-nitrogen (NH4-N) may also be included in the calculation and is handled in the same way as nitrate-nitrogen.
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 N/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 nitrogen.
There are various adjustments that can be applied as "credits" in the recommendation calculation. This includes things like manure to be applied, previous legume crops, and/or organic matter mineralization. These adjustments are discussed in more detail in Crop File 1.02.051, Soil Test Nitrogen Credits.
The following Crop Files discuss nitrogen fertility: