Rianne Wagner, Matt Ruark and Shawn P. Conley
Recently, questions have been raised about potential yield reductions in soybeans due to chloride (Cl⁻) toxicity associated with spring applications of muriate of potash (MOP). While ongoing research at the University of Minnesota is examining the correlation between potassium (K) application timing and Cl⁻ toxicity, this latest research does not indicate significant reductions in yield that would warrant altering existing application timing recommendations here in Wisconsin.
Minnesota’s research found that spring applications of more than 100 pounds of K2O per acre resulted in an average yield reduction of 1 bushel per acre across all sites, compared to fall applications of the same rate (Kaiser, 2024). To put this rate into context of UW’s fertilizer guidelines found in A2809, a rate of 100 lb/ac of K2O would be recommended for a 70 bu/ac yield goal when soil tests fall within the optimum range. If concerned about this minor effect, farmers can simply shift potash applications to the fall. Alternatively, if interested in applying sulfur, switching to potassium sulfate (0-0-50-18) is another viable option. Another strategy would be to “double up” MOP application ahead of corn in order to fertilize both the corn and soybean crops in rotation. No negative effects on corn yield were observed from spring potash applications in Minnesota (Kaiser, 2024).
Other research in Minnesota showed more extreme yield reduction of 2 to 15 bushels per acre only when MOP application rates exceeded 800 pounds per acre (applying 480 lb/ac of K2O) (Kaiser, 2023). These findings clearly demonstrate that yield drags due to excessive Cl– can occur. However, for most farmers who maintain soil test levels in the optimum range, rates of MOP should never be that high. These results may call into question the idea of rapidly increasing soil test K when levels are in the very low or low categories. Even then, A2809’s recommendations only go as high as 185 lb/ac of K2O for a 100 bu/ac yield goal when soils test in the very low category. Additionally, the bigger effects of high MOP rates may be seen on soils in western Minnesota and North Dakota due to limited rainfall keeping Cl– concentrations relatively high in the root zone. With this in mind, the current potassium application guidelines should not be a cause for concern in Wisconsin.
Regarding K application timing and rates in Wisconsin, ongoing research at UW-Madison aims to better understand in-season K management. The goal of this Wisconsin Soybean Marketing Board funded work is to help growers address yield-limiting K deficiencies in their soybean crop through updating K fertilizer recommendations, developing a dynamic critical K concentration curve for leaf and petiole sufficiency to improve tissue test accuracy, and evaluating the ability to correct K deficiencies in-season with applications of foliar and granular K fertilizer. With this work currently underway, we look forward to sharing our preliminary findings in the coming year.
Literature cited:
Kaiser, D. (2023, May 30). Chloride and soybean: What to know about potash fertilizer management. https://blog-crop-news.extension.umn.edu/2023/05/chloride-and-soybean-what-to-know-about.html
Kaiser, D. (2024, March 31). Timing of K application for corn and soybean production. AFREC. https://mnsoilfertility.com/wp-content/uploads/2024/10/KAISER-2023-Timing-of-K-application-for-corn-and-soybean-production-K-timing-Study-R2023-C.pdf
Laboksi, C. A. M., & Peters, J. B. (2012). Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin (A2809).
