Sulfate of potash (SOP) is available in three main agricultural grades: standard, granular, and soluble.
Standard SOP is used for direct application on hardy crops and in the manufacturing of compound fertilizers. It contains 50% potassium oxide, 45% sulfur trioxide, and a maximum of 1% chloride. It appears as fine crystals with a typical particle size range of 0.21 mm to 1.68 mm.
Granular SOP is the most important and most widely used grade in the United States and many other parts of the world. This grade is used in bulk blends, for mechanized spreading, and for manual application on crops that have even soil nutrient distribution. It contains 50% potassium oxide, 45% sulfur trioxide, and a maximum of 1% chloride. It appears as small granules with a typical particle size range of 0.84 mm to 3.36 mm. Granular SOP is produced by mechanically compressing the product, and then breaking and screening it to achieve the desired particle size. Granular grade can also be produced by granulating the material and using a binding agent. The price premium for granular SOP is $20 to $30 per tonne.
Soluble SOP is used in open field fertigation, foliar feeding, and greenhouse and hydroponic systems. It contains 52% potassium oxide, 45% sulfur trioxide, and maximum 0.5% chloride. It appears as a fine powder, which dissolves rapidly in water, with a typical particle size range of 0.106 mm to 0.30 mm. This form of SOP holds less than 5% of the SOP market and sells for a substantial premium to the other forms of SOP.