Variability in Soil Test Potassium and Crop Yield

A need to update Iowa soil-test potassium (K) interpretations was first suggested during the mid 1990s by an increasing frequency of K deficiency symptoms in corn. These symptoms occurred mainly when spring rainfall was below normal, but were observed in other conditions—even in some soils that tested optimum according to current interpretations. Also, field experiments designed to evaluate K placement methods often showed larger than expected yield response in soils testing optimum, and smaller responses in soils testing high. In previous decades, deficiency symptoms and large yield responses were seldom observed for the optimum category.

The primary 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 as well as a new soil test for K with potential to improve the predictability of crop response to K fertilization. This research is based on evaluation of conventional, long-term trials at three research farms and several on-farm, replicated strip trials harvested with yield monitors.

Based upon the results to date, K recommendations were changed in November 2002. Recommended rates of K were increased and categories shifted upward, reflecting crop responsiveness to K fertilization observed at higher levels where once little was expected. The new recommendations have been published and are being shared with producers and crop advisers at numerous meetings.

These changes are, however, a temporary fix. This research has demonstrated that the probability of crop response to K varies with several conditions, one of the more influential ones being soil type. As this research continues, such important factors will continue to be examined with further revisions targeted for release in two to three years. IA-09F