Using P Soil Tests

February 19, 2013

Q: I’ve been told I might expect higher phosphorus and possibly higher potassium than normal this spring. Will the results of fall soil tests still be accurate or should I adjust for losses over winter?

Soil P tests for 2011, 2012In a long-term Nebraska experiment focusing on soil phosphorus, P levels did increase with the dry conditions in 2012. The fall 2011 test is typical of what each treatment had been averaging. The 2012 values are greater in all cases, although the amount varies. The P applied is given in the last column, and does not account for the increase in soiltest levels. Average yields for the experiment are about 145 bu/ac at this site (an eroded, Crofton silt loam soil, pH 6.0).

The results shown are from a single study at the Haskell Ag Laboratory at Concord and probably cannot be generalized. However, if P levels in soil tests taken last fall are much higher than expected, they should be used with caution.

In another study, soil potassium (K) levels increased about 50 ppm in 2012 as compared to 2011.

Charles Shapiro
Extension Soil Scientist - Crop Nutrition