Cotton Response to Polyhalite Compared to Other K and S Source Fertilizers in the Coastal Plain Soils of Virginia

Two studies were conducted in 2015 and 2016 to evaluate a mined mineral deposit of calcium-magnesium-potassium sulfate (polyhalite) as a granular potassium (K) fertilizer in upland cotton (Gossypium hirsutum). The objective of this study is to evaluate cotton response to K and S from polyhalite compared to MOP with and without S addition. The four K sources were muriate of potash (MOP) (0-0-60), polyhalite (0-0-14-19S-6Mg-17Ca), 50% MOP + 50% polyhalite blend, and a bulk blend formulation using MOP, magnesium sulfate (Kieserite), and calcium sulfate (gypsum). Each K source was applied at 34, 67, 101 and 134 kg K₂O ha^-1.  Soil S levels after harvest were significantly different and the differences can be attributed to S application rates applied with the application of polyhalite. No differences among treatments were observed in any of the biomass measurements taken during the study. At matchhead square total dry biomass ranged from 892 - 1,103 kg ha^-1 with the control treatment having the highest biomass production. Total dry biomass at cutout ranged from 6,594 - 8,009 kg ha^-1. Potassium uptake at matchhead square ranged from 31.5 – 39.0 kg K ha^-1, as well as 15.6 – 20.6 and 3.52 – 4.51 kg ha^-1, for Ca and Mg, respectively. ). Total uptake of S at cutout ranged from 12.6 – 29.0 kg S ha^-1 with the majority of the S found in the leaves. The most measurable response during the study was to S and increased uptake was measured at the matchhead square and cutout growth stages. For future studies a site with low soil test K and lower K saturation of the cation exchange complex will be needed to quantify polyhalite as a K source in cotton.