Optimizing Potassium Fertility Levels for No-till Soybean Production.

Current potassium (K) recommendations for soybean made by The University of Tennessee recommend 80 lb/acre of K₂O for low testing soils and 40 lb/acre of K₂O for medium testing soils; no K fertilizer is recommended for high and very high testing soils. Because of continuous yield increases resultant from new soybean cultivars and better management practices, the current K recommendations may need to be updated. The objective of this study was to evaluate the effectiveness of the current K recommendations for soybean in TN. This study was conducted on low testing soils near Jackson and Milan, TN from 2008 through 2009. Six treatments were evaluated at both locations each year. The first five treatments were 0, 40, 80, 120, and 160 lb/acre of K₂O. The sixth treatment was a K rate based on a soil testing lab’s recommendation. A randomized complete block design was used with four replicates. Soybean ‘Pioneer 94M80’ was planted in 30-inch rows at both locations each year. Yield responses to K varied with years, but showed similar tendencies across locations and years. The 2-year averages at Jackson showed that applications of 40, 80, 120, and 160 lb/acre of K₂O increased yield by 6.6%, 7.9%, 6.9%, and 3.8%, respectively, compared with the zero K control. There was an 8.5% yield increment on average with treatment six which was recommended by a soil-testing lab. At Milan, applying 40, 80, 120, and 160 lb/acre of K₂O increased yield by 4.7%, 5.8%, 7.2%, and 3.0%, respectively, relative to zero K. A 5.0% yield increase was observed with treatment six. Using quadratic regression of yields with K rates in treatments 1 to 5, we found 90 lb/a K₂O was the optimal K application rate at Jackson and 94 lb/acre K₂O was optimal at Milan based on the 2-yr averages.