The most commonly used potassium fertilisers are Muriate of Potash (potassium chloride), Sulfate of Potash (potassium sulfate) and Potassium Nitrate.

Coarse crystalline, granular or prilled grades of these products are available for dry application to the soil. Higher purity (Solution Grade) products, which usually have a smaller incitecpivotfertilisers.com.au Incitec Pivot Fertilisers (IPF) is a business of Incitec Pivot Limited Ltd ABN 42 004 080 264), 28 Freshwater Place, Southbank Vic 3006. Free call 1800 009 832. Fertcare® is a registered trademark of Australian Fertiliser Services Association, Inc.. Agritopic – Potassium Page 7 of 14 particle size so they dissolve quickly, are preferred for application in solution, e.g. for use in fertigation programs. Potassium sulfate is the least soluble of the three products.

Reflecting its lower solubility, Sulfate of Potash also has the lowest Salt Index of the three products and is the least likely to cause root burn. The Salt Index compares the increase in osmotic potential brought about by the use of a fertiliser compared to sodium nitrate, which is given a value of 100. It provides a guide to but is not the sole determinant of the likelihood that a fertiliser will burn plant roots. All three-products supply potassium, so the differences in the Salt Index come down to the concentration (potassium chloride has the highest potassium content) and the behaviour of the companion ions (chloride, nitrate and sulfate) in the soil. Chloride (Cl- ) and nitrate (NO3 - ) are monovalent, so at the same potassium rate, there will be twice as many of their ions as there will be for sulfate (SO4 2- ), which is divalent, i.e. has two positive charges. In addition, chloride and nitrate are mobile in the soil, while sulfate is more likely to be sorbed onto soil colloids. Chloride may also have direct toxic effects on plants.

Sulfate of Potash, having a lower Salt Index than Muriate of Potash, and is often preferred in crops that are sensitive to chloride or fertiliser burn, where the soil is saline, and where poorquality irrigation water is used. Sulfate of Potash is often used in planting fertilisers for French bean, and for shallow-rooted tree crops such as avocado and macadamia.