Efficient Use of Nitrogen.

There are two keys to efficient use of nitrogen fertilizer in crop production:

1. Manage to minimize losses between fertilizer application and crop uptake.  In humid regions this means applying the fertilizer near the time of maximum crop uptake.

2. Apply only as much fertilizer as the crop needs.  This is difficult to accomplish, because what the crop needs varies widely--from year to year, field to field, and place to place within a field.  Soil, topography, management history, and weather interact to control how much of the soil's organic N will be released, and how much of the N that is released is lost.  To maximize nitrogen fertilizer efficiency, spatially dense diagnosis of optimal N rate is needed.  I will discuss several approaches that meet this criterion.  At a lower tier, field-specific and year-specific diagnostics can move us in the right direction relative to current practice.