Abstract

The potential of polyhalite, a natural mineral consisting of K₂O, SO₃, MgO, and CaO at 14%, 48%, 6%, and 17%, respectively, to become a slow-release multi-nutrient fertilizer was the focus of the present study. Fertilizer solubility, and hence the potential availability of its various constituent minerals, was investigated in both a laboratory test and a field experiment. In the laboratory test, the potassium (K) and magnesium (Mg) salts were completely dissolved, while the calcium (Ca) salts displayed significantly limited solubility. In the field experiment, under repeated cycles of wetting that simulate successive rainfall events, all polyhalite constituents were between 75-100% dissolved following 300 mm of accumulating water application, in the order of K>Mg>S>Ca. Significant differences in solubility occurred with the rising amounts of water application up to 800 mm. While K and Mg displayed no or negligible residues, S dissolution was very gradual, and Ca residues remained constant. Supported by previous studies, the present study demonstrates that polyhalite dissolution in water takes longer and requires larger water volumes compared to other soluble fertilizers. This advantage reduces possible risks of damage due to over-fertilization and enables the application of polyhalite as a slow-release substance.

Keywords: Calcium; fertilizer; magnesium; polyhalite; potassium; sulfur.