Potassium and Trends of Potassium and Phosphorus Deficiency
Potassium chloride (often called muriate of potash) is the cheapest fertiliser per unit of potassium and by far the most widely used. Potassium sulphate contains 20 per cent less potassium than the chloride and is used for special purposes only, mainly for small fruit and tobacco leaf production.
The areas recognised as needing potassic fertilisers to maintain reasonably high-producing pastures are growing larger year by year. This spread of potassium-deficient areas may be caused in part by the losses of potassium under pastoral farming and in cash cropping; recent research into soil fertility, however, is leading to better recognition of long-standing potassium deficiencies. The soils most liable to become potassium deficient are those highly leached or in areas with a high annual rainfall. These are the soils with a severe initial phosphorus deficiency as well. Hence, where potassic fertilisers must be used, phosphatic fertilisers are usually also needed. In practice, therefore, potassic fertilisers are mainly applied as potash-phosphate mixtures either as potassic serpentine superphosphate or, less commonly, as potassic superphosphate stabilised by 3–6 per cent of ground limestone. There are, however, differences in the long-term fertility trends of phosphorus and potassium. While regular applications of phosphatic fertilisers result in a gradual rise in the level of available phosphorus, regular applications of potassic fertiliser do not have the same beneficial, long-term effect. On the contrary, the light rates of application of potassic fertilisers commonly used may not completely arrest a downward trend in potassium fertility. For this reason intensively top-dressed areas may gradually become less deficient in phosphorus and more deficient in potassium. The ratio of use of phosphatic fertilisers to potassic fertilisers is therefore expected to become narrower.