Building Soils for Better Crops, Third Edition

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 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 (table 21.1). Although the relative test value for a given soil is usually similar according to different soil tests (for example, a high P-testing soil by one procedure is generally also high by another procedure), the actual numbers can be different (table 21.2).

Table 21.1: Phosphorus Soil Tests Used in Different Regions
RegionSoil Test Solutions Used for P
Arid and semiarid Midwest, West, and NorthwestOlsenAB-DTPA
Humid Midwest, mid-Atlantic, Southeast, and eastern CanadaMehlich 3Bray 1 (also called Bray P-1 or Bray-Kurtz P)
North Central and MidwestBray 1 (also called Bray P-1 or Bray-Kurtz P)
Southeast and mid-AtlanticMehlich 1
Northeast (New York and most of New England), some labs in Idaho and WashingtonMorgan or Modified MorganMehlich 3
Source: Modified from Allen, Johnson, and Unruh et al. (1994).

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. (On the other hand, the PSNT reflects the amount of N that may become available from decomposing organic matter.)

Table 21.2: Interpretation Ranges for Different P Soil Tests
 LowOptimumHighVery High
Bray 1 (Bray P-1)0–1515–2424–31>31
Mehlich 10–25 25–50>50
Mehlich 30–1515–3124–31>31
AB-DTPA (for irrigated crops)0–78–1112–15>15

Note: Units are in parts per million phosphorus (ppm P), and ranges used for recommendations may vary from state to state.

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