Variability in Soil Test Potassium and Crop Yield

The objectives of this research are to: (1) study the within-field variability in soil-test K and corn response to K fertilization, (2) evaluate a modified K soil test that has the potential to improve the predictability of crop response to K fertilization, and (3) study the relationships among yield response, K removal in grain, and long-term changes in yields and soil-test K.

Soil, plant tissue, and grain samples were taken from three conventional plot trials and three field-scale strip trials. The conventional trials compared several rates of K fertilizer and the strip trials compared fixed-rate and variable-rate fertilization. The widely used ammonium acetate method and the Mehlich-3 extraction procedure were performed on all soil samples. Analytical results from the ammonium acetate and Mehlich-3 methods showed large variation in K levels within and across fields. Correlations of these methods will be made, and a new analytical procedure, the tetraphenyl-boron K test, will also be investigated. Plant analysis of leaf tissue, although not yet completed, initially showed very large differences in K concentrations. Grain samples have yet to be analyzed.

Preliminary yield data are exciting and clearly show the need for this research. Large yield responses were observed at two conventional plot trials and at two strip trials. In these trials, soil test K (both ammonium acetate and Mehlich-3) varied from low to very high. As expected, large yield responses (up to 20 bu/A) were observed in soils testing low. However, variation in responses observed in soils testing optimum (medium) or high was extremely large. For example, in one strip trial there was no or little response even though soils tested from the upper optimum to the very high classes, and the overall yield level was acceptable (about 150 bu/A). In two other trials, however, there were large responses even though soils tested high by the ammonium acetate method. Future analysis of yield response, plant nutrient concentrations, and analytical results from all three soil test procedures is expected to provide insight into these observations. A better understanding of the factors affecting soil test results and crop response will provide farmers with better tools to manage K in Iowa. IA-09F