Variability in Soil Test Potassium and Crop Yield

Work was completed on the four year study on "Potassium Uptake, Recycling, and Soil-Test Temporal Variability in Soybean and Corn". The initially planned project was improved upon by also evaluating P recycling. Main findings were that K recycling to soil from standing soybean plants at physiological maturity in the fall and from residue until early the following spring was much more complete and faster than for corn. About 65 to 70% of soybean plant K (except grain) was recycled by early December, and by early spring 80 to 90% was recycled to soil. In corn, the K release from plant tissue and residue was more gradual and about 45% of the K remained in the residue by early spring. Much less rainfall was needed to recycle a certain amount of K in soybean than in corn. There was a significant, increasing, linear relationship between K loss from residue and the additional soil-test K measured in the spring compared with the fall. Phosphorus loss followed a similar pattern to K from physiological maturity until harvest, but there was no significant residue P recycled to the soil from the fall until early spring for any crop, no relationship with rainfall, and no relationship between P recycled and soil-test P change from fall to spring. These results have great value to better understand very high temporal variability and uncertainty of soil-test K. IA-09F