Crops can acquire N from a number of sources including (1) the inorganic soil N forms nitrate (NO3) and ammonium (NH4), (2) N mineralized from soil organic matter, crop residues and animal wastes (3) N present in the irrigation water, and (4) fertilizers.
The recommended rates are based on the total N requirement adjusted for yield potential, soil test NO3-N and NH4-N in the top 12 inches, and the previous crop. The recommendations assume an average of 60 lb N/acre of mineralized N which is accounted for in the table.