Soybean Yield Responses to Polyhalite and Other Potassium Fertilizer Salts.

Field trials were conducted in 2013 and 2014 with soybean [Glycine max (L.) Merr.] on a calcareous soil in south Texas to evaluate the effectiveness of polyhalite (POLY, a potassium, calcium magnesium sulfate salt) compared to other sources of K fertilization [potassium chloride - MOP, potassium sulfate - SOP, and potassium magnesium sulfate – SOPM (only in 2014)] applied at four rates (50, 100, 150, and 250 Kg K₂O/ha). Grain yields were significantly increased by K fertilization (P<0.001) in both years. Yields were generally higher in 2014 (average 3634 kg/ha) compared to 2013 (average 2297 kg/ha) possibly due to relatively shorter growing season and predominantly dry weather conditions. In 2013, grain yield was higher for all rates above 50 kg ha^-1 not K source. Pod number per plant was higher for both POLY and SOP than control and MOP. Seed number per plant was not influenced by any source of fertilizer. Interaction between source and rate of K₂O was found to be significant for average seed weight and grain protein concentration. In 2014, grain yield was influenced by both rate and source of K₂O. Higher rates resulted in higher bean yields. Single degree of freedom contrast indicated that yields of POLY treatments were higher than those of MOP and SOPM. However, pod number per plant, seed number per pod and average seed weight were not influenced by either source or rate of K₂O. Grain K and protein contents were significantly increased by K fertilization (P<0.01).  Grain protein contents ranged from 253 to 376 mg/g and were generally highest at the highest K fertilization rate for each K source. In general, growth, yield and quality responses to polyhalite were similar or better than the other K sources.