Nitrogen Availability.

Soil organic matter nitrogen (SOMN) remains a significant source of nitrogen (N) in contemporary agricultural systems, despite the inclusion of inorganic N fertilizers, organic N sources like manures, N-fixing legumes, and direct deposition of atmospheric N.  The dual and often contradictory goals of maximizing N use efficiency (NUE) from each of these sources, and minimizing the environmental loss of both dissolved and gaseous N requires accurate predictions of plant available N (PAN) from SOMN.  This need has stimulated a decades-long research effort to develop and validate an N availability index that is sufficiently robust to be used as a routine soil test.  As is shown in the Nitrogen Availability chapter of the ASA Monograph “Nitrogen in Agricultural Systems,” there are numerous biological and chemical indices of N availability that are capable of identifying relative differences in SOMN mineralization in soil.  This is the case for both field-based assessments and when results from long-term aerobic incubation are used to validate the N index.  However, it does not appear that any of these indices rise to the standard of a soil test, due in part to insufficient standardization.