Sources of K fertilizer

K fertilizers are mined and refined from underground ore deposits, salt lakes and brines. North America, the former Soviet Union, and Europe account for almost 90% of world output. Known potash reserves are estimated to be equivalent to 9 billion t K2O. The Saskatchewan deposit in Canada is composed of about 42% KCl (sylvite), 53% NaCl (halite), some KClMgCl2.6H2O (carna-llite) and clay, and is the world’s largest high-grade potash deposit.

Potash refers to a variety of K-bearing minerals that are used for fertilizer including potassium chloride [KCl, or muriate of potash (MOP)], potassium sulfate [K2SO4, or sulfate of potash (SOP)], potassium-magnesium sulfate (K2SO4.Mg SO4, or sulfate of potash magnesia), potassium nitrate (KNO3, or saltpeter), and mixed sodium-potassium nitrate (NaNO3 + KNO3, or Chilean saltpeter).

KCl is separated from crude salt by thermal dissolution, flotation and crystallization processes that separate KCl from rock salt (mostly NaCl) or Mg Cl2 or kieserite (MgSO4.2H2O). This can also be achieved by a dry process known as ‘electrostatic differentiation’.

KCl is by far the most widely used K fertilizer for direct application and for the manufacture of N-P-K compound and bulk blend fertilizers. World standard fertilizer grade KCl (sometimes known as muriate of potash, MOP) contains 60–63% K2O and varies in colour from pink to brown or white depending on the mining and recovery processes used. Due to its high K content, KCl is the most economic K fertilizer and preferred for crop plants with large K and Cl requirements such as coconut and oil palm.

Sulphate of potash (SOP) (50% K2O + 17% S), KNO3 (13% N + 44% K20) and potassium-magnesium sulphate (22% K2O + 18% MgO + 22% S) or langbeinite are often included as part of mixtures, blends and compound fertilizers. They are usually more costly than KCl due to their complex manufacturing requirements. These materials are favoured by producers of chloride-sensitive crops (e.g. potato, tobacco, fruit and vegetables) with high market value that require other essential nutrients such as S, N and Mg.

This also applies to special K fertilizers such as potassium phosphates (K4 P2O7, KH2 PO4, K2 HPO4), potassium carbonates (K2CO3, KHCO3), potassium hydroxide (KOH), potassium thiosulfate (K2S2O3) and potassium polysulfide (KSx). These high purity low salt index fertilizers are produced and marketed on a limited basis for foliar application and fertigation.

All K fertilizers are water-soluble. They consist of K+ in combination with Cl-, SO42-, NO3-, PO43-, and P2O4-.

K fertilizers are usually available as fine crystals and in compacted or granulated forms. No detrimental effects of K fertilizers with regard to the environment are known.