Nitrogen—Availability Indices

Numerous soil, climate, and management variables affect the yield and hence the nitrogen (N) needs of the crop. Some of these variables, particularly yearly weather patterns, are nearly impossible to predict in advance. Management, including economics of N fertilizer use, is important variables that are difficult to deal with quantitatively. Nitrogen mineralization from organic matter may be assessed directly by biological or chemical methods, or as is the usual case, by past knowledge gained from field trials. It is well established that in climates where extensive leaching, de-nitrification, or both do not normally occur before planting and during the growing season, the residual inorganic N in the root zone is approximately equivalent in availability to fertilizer N and should be taken into account in fertilizer N recommendations. Laboratory indices of soil N availability that have been proposed can readily be subdivided into biological and chemical tests.