7.3.6: Soil Testing for P

Soil test procedures for phosphorus are different than those for nitrogen. When testing for phosphorus, the soil is usually sampled to plow depth in the fall or in the early spring before tillage, and the sample is usually analyzed for phosphorus, potassium, sometimes other nutrients (such as calcium, magnesium, and micronutrients), and pH. The methods used to estimate available P vary from region to region and sometimes from state to state within a region. Although the relative test value for a given soil is usually similar according to different soil tests (for example, soil testing high in P by one procedure is generally also high by another procedure), the actual numbers can be different.

The various soil tests for P take into account a large portion of the available P contained in recently applied manures and the amount that will become available from the soil minerals. However, if there is a large amount of active organic matter in your soils from crop residues or manure additions in previous years, there may well be more available P for plants than indicated by the soil test. (While there is no comparable in-season test for P, the PSNT reflects the amount of N that may become available from decomposing organic matter.)

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