Variability in Soil Test Potassium and Crop Yield

The main objectives of this research are 1) to study the variability in soil-test K and corn response to K fertilization and 2) to evaluate soil tests for K currently used in the Midwest and a new soil test for K with potential to improve the predictability of crop response to K fertilization. In the 2000 season, the second year of the project, soil samples and grain yields were collected from three long-term, conventional trials at three research farms and from three on-farm strip trials. These trials added to a similar number of tests conducted in 1999. In on-farm trials, there was large variation in soil test K within and across fields. Corn grain yield responses were larger in the low-testing areas and were progressively smaller as soil test levels increased. Small responses were observed, however, in areas testing in the “high” category.

In conventional trials at research farms, large yield responses were observed, as high as 70 to 80 bu/A in some cases, in soils testing medium or higher in soil test K. Current recommendations are based on the expectation of small and infrequent responses in the medium soil test category. Correlations between NH₄OAc and Mehlich 3 soil test procedures showed that both tests extracted similar amounts of K, which were well correlated across most soils, except the Kenyon series. Marked differences exist between the amount of K extracted by the NH₄OAc or Mehlich 3 tests and the sodium tetraphenyl boron test. Differences are small for some soils, but large for others. Understanding these differences will require further study. Research in this study is being used to re-evaluate current university fertilizer recommendations for K. IA-09F