Changes in Soil Chemical Properties Following Application of Polyhalite to Potato.

Polyhalite is a mined mineral containing sulfate salts of potassium, calcium, and magnesium with an approximate fertilizer value of 0-0-11.7(K)-19(S)-3.6(Mg)-12.1(Ca). The overall objective of this study was to determine the effects of polyhalite on potato yield and residual soil test K, S, Ca, Mg, and pH. This 2-year study was conducted on an acid, low organic matter Hubbard loamy sand soil. In year 1, six treatments replicated four times in a randomized complete block design were tested: 1) control (no K, S, Mg, Ca application); 2) 373 kg/ha K as polyhalite (Sirius Minerals, Plc), which also supplied 608 kg/ha S, 93 kg/ha Mg and 436 kg/ha Ca; 3) 373 kg/ha K as KCl (muriate of potash – MOP); 4) 373 kg/ha K as KCl plus gypsum and Epsom salts; 5) 280 kg/ha K as polyhalite and 93 kg/ha K as KCl; and 6) 93 kg/ha K as polyhalite and 280 kg/ha K as KCl. In year 2, the 4th treatment was changed to pelletized lime and Epsom salts. Soil samples (0-15 cm) were collected before treatment application and then again after harvest from each plot. Russet Burbank was the cultivar tested. Marketable yields with polyhalite were equal to or significantly higher than the control or with 100% MOP. Blends of polyhalite with MOP were as effective as polyhalite as the sole source of K. Without added K, soil test K decreased significantly following harvest compared with all other treatments receiving K. Soil test Ca and Mg were lowest in the MOP-only treatment. Soil test SO4-S was lowest in the control and MOP-only treatments, with increases as high as 65 mg/kg with 100% polyhalite. In year 1, soil pH decreased by about 0.4 units in all treatments. In year 2, a pH decrease of about 0.3 units was observed in all treatments except the one receiving pelletized lime, which stayed the same.