Adequate soil test levels are always important to optimum crop production, but in a drought year, high P and K soil test levels are critical. Dr. Jay Johnson's long-term (over 25 years) fertility studies at Springfield, Ohio, provide an excellent base to study interactions of P and K. These plots have been in a corn and soybean rotation under conventional tillage and are one of the longest-running fertility studies in the U.S. In a very dry year, 1999, these plots dramatically demonstrated the importance of maintaining high soil tests, and the importance of the interaction between P and K. When K was low (blue bars), there was a slight negative response to P. At higher K soil test levels, increasing P soil tests increased yields. The long-term data set shows that maintaining higher soil tests generally supports optimum yield levels. Usually there was an advantage to the higher soil test levels. But in 3 years when extemely dry weather persisted, the response was similar to 1999.

For further information, contact Dr. Jay Johnson, The Ohio State University. This research has been supported in part by the Potash & Phosphate Institute and the Foundation for Agronomic Research.