Assessing the Effect of Polyhalite on Soil Environments.

Calcium amendments such as gypsum (Ca₂SO₄.2H₂O) are regularly used as conditioners to help support nutrient retention or remediate degraded soils. Polyhalite (K₂SO₄MgSO₄2CaSO₄.2H₂O, commercially known as ‘POLY4’ fertiliser) is a mineral which contains 17% CaO, 14% K₂O, 6 % MgO and 19% S. It provides the potential to supply K and Ca to soils and to affect soil physical properties. Previous work using fast throughput Brazil Tests on a wide range of soils mixed with 0, 2.5, 5.0 and 50.0 t/ha powdered polyhalite revealed differences in the mechanical behaviour of treated soils. Mechanically weak soil (Insch Series, NE Scotland) saw an increase of 70% of tensile strength whereas a hard-setting soil (Sulaimani, Iraq) and a wastewater-treated dispersive soil (Israel) were weakened 16.4% and 18.2% respectively. Subsequently polyhalite was compared with gypsum at 0, 55, 110, 220 and 440kg Ca/ha using two soils. The soils demonstrated differing responses wherein a response to gypsum did not confer a response to polyhalite. Now, current work at the mesocosm scale has investigated the impact of polyhalite on soil susceptibility to surface erosion and nutrient leaching. Chemical and physical characteristics of two soils with either 0 or 110 kg Ca/ha were measured after undergoing three extreme rainfall simulations. After each simulation, there was no effect of polyhalite addition in either soil for macroporosity, sorptivity, penetration resistance, aggregate stability or surface erosion after comparison with the control. However differences in nutrient leaching were noted, in particular DOC and PO₄-P. The addition of polyhalite to soil at higher amendment application rates suggests small scale changes to soil structure but the soil response is not consistent. At larger scales and application at the fertiliser level there is little response of soil physical properties detectable in this series of experiments. However its impact on nutrient leaching and soil behaviour in the presence of crops now provides interest for future study.