Nitrogen is released as ammonium- N from organic matter in the soil through mineralization. Mineralization is a microbial process that is favored by conditions favorable to high corn yield; thus, the estimated credit for N from organic matter is related to expected yield. When a soil test for organic matter is not available, 1 percent organic matter is assumed for coarse soils and soils in the Panhandle, and 2 percent is assumed for other soils. The maximum soil organic matter content used in the algorithm is capped at 3 percent organic matter since few Nebraska soils above this level were represented in the database used to develop the equation.

Preceding legume crops result in improved N supply to the corn crop because legume crop residues decompose faster than cereal crop residues and cause less soil and fertilizer N immobilization or tie- up. When corn follows a legume in rotation, the N rates are reduced by the legume N credit. The soybean credit of 35 lb/ac N for coarse soils is a revision based on recent research in Holt County unless soybean yield was less than 30 bu/ac when the credit is 1 lb of N per bushel harvested. Soybeans are good scavengers of soil nitrate; therefore, residual soil nitrate- N after soybean harvest is often between 3 to 4 ppm nitrate- N. Soil sampling for nitrate- N following soybean is only recommended if organic amendments were applied within the previous two years or if the soybean crop yield was poor due to hail, weather, or insect damage. Soybeans do not add N directly to the soil. In most cases, soybean doesn’t leave a positive soil N balance. On average 55 percent of soybean N uptake is from the air and 45 percent from the soil. Soybean scavenges soil nitrate efficiently and does not add N to the soil. The apparent N credit from soybean is due to greater availability of mineralized soil N. High C:N ratio residue from a preceding corn crop will immobilize mineralized soil N during residue decomposition, making it unavailable for crop use. Low C:N ratio residue from a soybean crop immobilizes less mineralized soil N, leaving it more available for the following crop. The recommended credits were established empirically through the findings that corn needs less N when grown in rotation with soybeans.

Irrigation water often contains a significant amount of nitrate- N that is readily available to corn. When the season total amount of N supplied in irrigation water exceeds 15 lb N per acre it should be deducted from the recommended N. For each foot of effective irrigation water applied, one ppm nitrate- N in water is equal to 2.7 lb N per acre. Irrigation amounts vary from year to year, and the N credit for irrigation should be based on the three- year average irrigation amount up to the corn R3 (milk) stage. Overall, in Nebraska 65 percent of the total irrigation amount is applied by August 1. Long term, average amounts of irrigation are estimated to be 8 in/yr in eastern Nebraska, 9 in/yr in central Nebraska, 12 in/yr in west central Nebraska, and 20 in/yr at the western Nebraska border with Wyoming (simplified from Sharma and Irmak, 2012)