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Competency Area 6: Nutrient Management and Planning

PO 56. List the three options for P management as defined by USDA-NRCS (national level).

  1. Agronomic soil test
  2. Environmental P threshold soil test level
  3. P index

If an agronomic soil test is done, manure or biosolids can be added in amounts where expected plant P uptake and removal is equal to the P that will be added.

Soil test P levels in surface soil can be used to determine whether or not the recommendation should be P-based.  If soil P test levels are below the critical level, then manure and biosolids applications can be N-based.  If soil test levels exceed the critical value, then recommendations are P-based.  A second higher critical value precludes manure or biosolids application.

A P index can be developed for fields, which integrates factors relating to potential P loss to evaluate the relative risk of P application to a field.  These include the P that might be transported from manure or biosolids and soil, the potential to transport that P to a body of water, how much rainfall might occur, and whether or not best management practices are in place.  The amount of manure or biosolids that can be applied is a function of the P index value.  If it is low enough, manure or biosolid application rates can be N-based.