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

Three studies were conducted in 2015-2017 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 30, 60, 90 and 120 lb K₂O ac^-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 as a standalone fertilizer or in a bulk blend. Responses were limited in 2015 with tissue and uptake of S being significantly different among fertilizer formulations. No yield response was observed in 2015. In 2016, there was a yield response at the location with polyhalite and 50% MOP + 50% polyhalite having the highest yields. The site at 2016 was a very low yielding site as drought and tropical storms limited lint yield. In 2017, uptake of S was different among fertilizer formulations with polyhalite having greater tissue S concentrations and S uptake at the 1^st square growth stage. Nutrients from polyhalite are plant available during the growing season for cotton, and polyhallite seems to be a good source of S without applying N on coastal plain soils.