Principles of Potash Use |
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Sources of potash
| Fig 4: The K cycle |
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| Fig 5: Clay minerals and cations |
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Plant roots take-up potassium as the cation K+ from the soil solution and it is this, plus the 'readily available K', which is measured by soil analysis as being potentially available to the crop (Figure 4). Small quantities can be taken in from a foliar application but the amounts are usually insignificant in relation to overall potassium needs. (It is also a costly method.) Rainfall contributes very small amounts of potassium (less than 5 kg/ha/year). Fertilisers, manures and crop residues all add potassium to the soil as K+. Almost all potassium sources are soluble* and increase the concentration of K in the soil solution immediately they are applied. In some cases manures and crop residues may require biological breakdown for the K to be released, perhaps over 12 months.
Only small amounts of K are maintained in the soil solution 5-20 kg/ha K (6-24 kg K2O). The majority of potassium reserves in the soil are held by the negative charges on clay minerals and organic matter (Figure 5). The potassium may be held weakly or strongly according to the position in the clay lattice. K+ which is loosely held ('exchangeable K') is rapidly released for plant uptake, whilst the more firmly bonded reserve ('nonexchangeable K') is released more slowly. Individual soils have different capacities to hold potassium according to clay type and content and amount of soil organic matter. Sand soils have very limited reserves of exchangeable K.
The clay minerals themselves contain potassium as an integral part of their structure (mineral K) and this is released only over very long time periods by the breakdown of the minerals (weathering). In UK conditions the amount of K released by weathering of the clay minerals is too small to meet the needs of even average crops. The different pools of soil K shown in Figure 4 are defined in relation to the plant availability of the K and are not distinct. The amount of K in each pool varies with potash additions, crop uptake, N use, soil wetting and drying, cultivations, etc.
* except specific clay mineral fertilisers used in organic farming - see leaflet 23