Evaluation of Polyhalite As a Source of Potassium and Sulfur for a Corn-Soybean Rotation in Minnesota.

Polyhalite is a fertilizer source which contains K, S, Mg, and Ca which could be used as a nutrient source for crops in Minnesota on soils deficient in K or S. The objective of this research was to determine if polyhalite can be incorporated in standard fertility program for a two-year corn (Zea mays L.)-soybean (Glycine max var. Merr.) rotation. Field studies were established at two locations in Minnesota (Saint Charles and Staples) with K soil test <120 mg K kg-1 (0-15 cm sample extracted with 1M NH4OAC) and where a response to S was expected. Treatments consisted of polyhalite, KCl, and a mixture of polyhalite and KCl applied at four rates of K (0, 37, 74, 111 kg K ha-1), and gypsum applied at four rates of S (0, 16, 32, 48 kg S ha-1) plus a single rate of KCl (111 kg K ha-1). Corn was grown at each site in 2015 and then rotated to soybean in 2016. Corn ear leaf K and S concentration was increased similarly by polyhalite and KCl or gypsum when the same rates of K or S were applied ha-1. The amount of K and S in the corn stover was influenced by the application of S. Corn grain yield was increased by K and S at Saint Charles and by S at Staples. When there was a response to K or S there was no difference in yield based on whether the source of fertilizer was polyhalite or KCl or polyhalite or gypsum. The results indicated that, depending on product cost, polyhalite can be substituted for KCl or gypsum to supply K and S to crops in Minnesota.