Polysulphate

Polysulfate refers to a natural mineral fertiliser extracted from polyhalite deposits, a hydrated evaporitic mineral containing potassium, magnesium, calcium and sulphur in sulphate form. This product delivers four major or secondary nutrients simultaneously, all in plant-available sulphate forms. Unlike potassium chloride (MOP), it contains no chloride, making it suitable for crops sensitive to this anion: tobacco, fruit, vegetables, vines and seed potatoes. Typical fertiliser-grade contents are approximately 14% K₂O, 19% sulphur, 6% magnesium and 17% calcium, though these figures may vary depending on ore purity and processing undertaken^[1][2][3][4].

Extraction and geological origin

Polyhalite forms in marine evaporitic basins, often associated with sylvinite (conventional potash ore) and kieserite. Exploitation involves underground mining followed by crushing and particle-size screening. No chemical transformation is required, giving polysulfate a favourable energy balance compared with industrially refined potash fertilisers. The principal commercially exploited industrial deposit lies in Great Britain, in the county of Yorkshire, where Anglo American has developed the Woodsmith project. Other polyhalite deposits exist in Germany, the United States and Poland, but their exploitation remains limited or at evaluation stage.

Agronomic uses and fertiliser properties

Polysulfate offers value for soil fertilisation where several secondary and major nutrients must be supplied together. Sulphur, increasingly deficient in European soils owing to reduced atmospheric deposition, is present as sulphate (SO₄²⁻), immediately available for plant nutrition. Magnesium plays a role in chlorophyll formation, whilst calcium contributes to cell wall stability and soil structure. The absence of chloride constitutes the dominant commercial argument against standard MOP. By way of comparison, potassium sulphate (SOP) shows a salinity index of 46.1 against 116.3 for MOP on a material basis, illustrating the better tolerance of chloride-sensitive crops for sulphate formulations^[1]. Polysulfate may be applied directly to soil, incorporated into bulk blend mixtures or used as a raw material for complex NPK fertiliser formulation. Its dissolution rate in soil, slower than that of pure SOP, allows gradual release of nutritional elements over several weeks.

Market and trade

Polysulfate remains a niche product in international potash fertiliser trade. Its large-scale commercialisation is relatively recent, with the British deposit coming into production over the past two decades. The product positions in the premium segment of the potash market, in indirect competition with refined SOP and MOP for applications where chloride is excluded. Traded volumes remain modest compared with the hundreds of millions of tonnes annually of potassium chloride and DAP. Export logistics from the United Kingdom follow conventional solid bulk shipping routes. Market evolution will depend on uptake by European and Brazilian farmers of this potash alternative, as well as on producers' ability to establish reliable price benchmarks in a universe dominated by annual potash contracts and MOP indices at Vancouver.

Regulatory framework

Within the European Union, polysulfate falls under the fertilising products regulation, which sets market placement rules, safety criteria and labelling requirements. As a chemically unprocessed mineral product, it may be placed on the market as an organo-mineral or mineral fertiliser depending on classification. Applicable component material criteria (CMC) mainly concern contaminant levels. Cadmium, for which European regulation sets increasingly strict thresholds for phosphate fertilisers, poses no particular difficulty for polysulfate owing to the absence of phosphorus in its composition. Granulometric requirements and minimum fertiliser unit contents must be met for compliant labelling under the product function categories (PFC) defined by the applicable regulation^[4].