Evaluation of Important Agronomic Properties of Polyhalite Fertilizer Derived from the Zechstein Mineral Deposit in the United Kingdom.

While evaluated as a potential fertilizer in the United States in the 1930’s, polyhalite as a commercial fertilizer is only recently emerging with the discovery of large polyhalite beds in Zechstein deposits in the southern North Sea basin in the United Kingdom.  While the fertilizer potential of polyhalite derives from its multi-nutrient composition of K, Ca, Mg, and S in the chemical formulation K₂SO₄.MgSO₄.2CaSO₄.2H₂O, little is known about important properties of polyhalite fertilizer from this deposit and its effect on soil properties.  The objective of this study was to quantify the nutrient content, solubility, salt index (SI), and residual effects on soil pH and soil electrical conductivity (EC) of polyhalite from the Zechstein deposit.  Polyhalite obtained from the Zechstein deposit was evaluated for nutrient composition using a standard acid digest from 135 samples, solubility in water at 25 °C from 12 samples using the ISO standard solubility test, and SI from 7 samples using a modified Jackson method.  Additionally, post-harvest soil pH and EC from 32 fertilizer crop trials evaluating polyhalite compared to other K source fertilizers over a range of application rates were summarized.  Nutrient composition of polyhalite from the Zechstein deposit averaged 14.3+/- 0.1 % K₂O, 19.9+/- 0.4 % CaO, 6.3+/- 0.1 % MgO, 20.3+/- 0.3 % S, with these levels corresponding to a relatively high purity level for polyhalite in the range of 92%.  Polyhalite solubility in water at 25 °C was 26 +/- 1.2 g L^-1.   Salt index averaged 76.7 +/- 9.4.  When polyhalite was applied at K₂O recommended application rates or higher, no significant effect of polyhalite fertilizer application on soil pH was observed as would be expected for a fertilizer derived from evaporate deposits containing primarily sulphate components.  Effects of polyhalite on soil EC will also be presented.