Evaluation of Polyhalite As a Nutrient Source for Irrigated Crop Production.

Polyhalite is a naturally occurring mineral consisting of sulfate forms of potassium, magnesium, and calcium with a chemical formula of K2SO4-MgSO4-2CaSO4-2H2O and an approximate fertilizer value of 0-0-11.7(K)-19(S)-3.6(Mg)-12.1(Ca). Because of relatively large deposits worldwide, there is interest in whether polyhalite can be used as an economical nutrient source for crop production. The overall objective of this study was to determine the effectiveness of polyhalite as a nutrient source for irrigated potato production in Minnesota. This study was conducted on an acid, low organic matter Hubbard loamy sand soil with low soil test K, Ca, Mg, and S. Six treatments varying in fertilizer source were tested: 1) control (no K, S, Mg, Ca application); 2) 450 kg/ha K as polyhalite (Sirius Minerals, Plc), which also supplied 610 kg/ha S, 95 kg/ha Mg and 435 kg/ha Ca; 3) 450 kg/ha K as KCl (Muriate of Potash – MOP); 4) 450 kg/ha K as KCl plus gypsum and Epsom salts to provide the same amount of Ca and Mg as treatment 2; 5) 335 kg/ha K as polyhalite and 115 kg/ha K as KCl; and 6) 225 kg/ha K as polyhalite and 225 kg/ha K as KCl. Russet Burbank was the cultivar tested. Marketable yields were significantly higher with polyhalite than the control or with 100% MOP. Blends of polyhalite with MOP were as effective as polyhalite as the sole source of K. Because of high inputs of Ca and Mg from irrigation/rainwater, yield increases with polyhalite over MOP alone were likely due to the added S. Based on yield response as well as tissue and soil tests, polyhalite appears to be an effective source of both K and S and is comparable to a combined application of MOP, gypsum, and Epsom salts.