Sulphate of potash (SOP)

Potassium sulphate (SOP), with the formula K₂SO₄, serves as a major potassic source that delivers both potassium and sulphur in sulphate form, both of which are readily taken up by plants. It contains 50 % K₂O and 18 % S, making it a particularly valued binary fertiliser for outputs sensitive to the chloride ion^[1][2]. Its salt index, assessed at 46.1 on a product basis, remains markedly lower than that of potassium chloride (MOP) at 116.3, which curbs the risk of osmotic stress in soils^[1][2]. The stability of this salt under humid atmospheric conditions allows for storage without appreciable deterioration^[1][2].

Manufacture and origins

SOP production follows several routes. Solar evaporation of brines, notably from the Great Salt Lake in Utah, yields potassium sulphate as either a primary or co-product^[3]. Certain facilities also process subsurface brines through solar evaporation followed by flotation^[3]. SOP may also arise from the reaction of potassium chloride with sulphuric acid, or from the extraction of complex minerals such as langbeinite (double sulphate of potassium and magnesium)^[3]. In the United States, over 60 % of potash output consists of sulphate forms (SOP and potassic-magnesian sulphate), intended for specific crops that do not tolerate chloride^[3].

Agronomic uses and precautions

SOP's main outlet lies in the fertilisation of quality crops: fruit, vegetables, tobacco, vines, citrus, storage potatoes, as well as certain protected crops^[1][2]. The absence of the chloride ion makes it essential for chloride-sensitive species. The presence of sulphur in sulphate form, directly usable by plants, provides an additional advantage where deficiency of this element occurs^[2]. Care should nonetheless be taken to avoid blending SOP with calcium ammonium nitrate (CAN) or urea, as incompatibilities may trigger undesirable reactions^[1][2]. Internationally recognised technical specifications generally set a maximum moisture content of 1 % by mass and a sodium content (expressed as NaCl) not exceeding 2.0 % on a dry product basis^[1].

Market and trade

SOP holds a niche position within the global potash market, which is dominated by potassium chloride (MOP). In the United States, the fertiliser industry absorbed roughly 85 % of potash sales, with the remainder directed to chemical and industrial uses^[3]. World consumption of fertiliser potash reached 40.6 Mt of K₂O in 2024; it was estimated at 41.6 Mt for 2025, with a projection of 45.3 Mt for 2029^[3]. Asia and South America show the most pronounced growth^[3]. The potash market is characterised by strong geographical concentration: the three leading producer countries account for approximately 70 % of global supply^[4]. Average FOB mine prices in the United States, across all products, ranged between 1,120 and 1,790 USD/t of K₂O equivalent depending on the year, whilst MOP traded at lower levels, from 600 to 980 USD/t over the same period^[3]. Global potash resources are estimated at roughly 250 billion tonnes, with no possible substitute for potassium as an essential nutrient^[5].