Variability in Soil Test Potassium and Crop Yield

We advanced at summarizing, for publication, a project to study the impact of genetic rootworm resistance on corn response to K. Work also continued on five long-term trials with corn-soybean rotations managed with and without tillage to understand soil-test K (STK) temporal variability and relationships among K rates, placement, STK, and grain yield. A summary was completed of 16 years for the no-till treatment, which included measuring removed K.

Potassium had inconsistent or small effects on grain K concentration (means of 0.32% for corn and 1.63% for soybean), but increased yield and K removal with STK less than 150 to 180 ppm. Yield was poorly correlated with grain K concentration, but was linearly correlated with removal. STK decreased 2.5 to 3.8 ppm per yr. STK and K removal were well correlated only over the long term. There was a large stratification of STK and non-exchangeable K, and non-exchangeable K partially explained STK variation across K rates and years.

Work was also completed for a third year of two trials that evaluated interactions among hybrid (rootworm susceptible or resistant) and N-K fertilization in corn. Analysis completed for one site showed a positive N-K-hybrid interaction (higher yield and response to N with adequate K for the resistant hybrid). Results of tissue tests for grain and leaves are being studied. Finally, progress was made on the study of K recycling with corn residue at seven trials. We observed significant K loss from standing plants or residue from physiological maturity until the following spring. About 70% of the plant K was lost by late March, and precipitation increased K loss. IA-09F