Solubility of Polyhalite and Leaching As Affected By Soil Type Compared to Other K Fertilizers.

As mineral sources of potassium (K) have been depleting over the years, demands and prices of fertilizers have escalated. Thus, it is imperative to explore a new source of K based fertilizers for securing the sustainability of crop production.  Polyhalite has the potential, not only to be an alternative for traditional K fertilizers, but also to provide other nutrients including calcium (Ca), magnesium (Mg), and sulfur (S).  However, little data is available regarding its plant nutritional properties and comparison to other potassium (K) fertilizers.  The objective of this study was to characterize polyhalite in terms of total elemental content, solubility in water, salt index and mobility in three different soils (sandy, loamy and calcareous).  The results showed that uncalcited  polyhalite contains  11.3 ± 0.2, 11.0 ± 2.4, 3.8 ± 0.2 and 18.5 ± 3.2 % of the K, Ca, Mg, and S, respectively, with additional nutrients of Fe, Zn, Ni, Cu, and Mn  plus detectable quantities of trace elements in concentrations considered too low to contaminate soils.  The salt index (SI) determined using jackson method for  polyhalite was 68.5 ± 10.8, lower than muriate of potash (MOP, 128 ± 6) and sulfate of potash (SOP, 103 ± 2.2), and higher than sulfate of potash magnesia (SOPM, 49 ± 3.1).  The leaching of nutrients from soil columns amended with  polyhalite were more rapid and often higher for K, Ca, Mg, and S compared to MOP, SOP, and SOPM, but the rate and total amount of leaching varied by soil type. These results suggest that polyhalite in soils will supply nutrients to plants in sufficient quantities compared to other K source fertilizers MOP, SOP, and SOPM.