Short-term Stability of Soil Test Phosphorus in Agricultural Fields

The spatial and temporal stability of soil test values is important to the use of soil testing for nutrient management. A study was conducted to evaluate the spatial and temporal stability of soil test phosphorus (P). Five sites ranging in size fiom 9.1 to 10.8 acres were soil sampled in the same locations in a 60 X 60 ft. grid either three or four times over a two year period. Bray 1 -P values were similar or decreased over time while Olsen-P values at two of five sites decreased. One site showed no pattern and two sites had cyclic patterns where the spring sample values were greater than the fall. The spatial pattern of soil test values during a two year rotation for Bray 1 -P and Olsen-P was very stable. The changes in distribution in soil test P categories over a hvo year period resulted in a shift to lower soil test categories. The decreases in soil test P were probably caused by plant P uptake in combination with no application of P fertilizer during the study.