Potash for Organic Growers

Potash - a naturally available nutrient

Potash is found in plant-available form as potassium (K) salts such as potassium chloride, sulphate, nitrate etc. These natural deposits are generally the result of the drying out of seas millions of years ago. In soils and plants these salts, which are all water soluble, separate into the potassium cation K⁺ and the relevant anion Cl^-, SO₄^2-, NO₃^- etc. Potash in manures is also mainly (70-90%) in water soluble form, with a small amount bound into the organic material which is released into the soil solution as the organic matter is mineralised. Potash from manures thus behaves in the potash cycle shown below in a similar manner to fertiliser potash. Potassium is not associated with any environmental or health concerns. None of the forms of these materials produce harmful effects unless they are used incorrectly. As with other nutrients, farmers should use all potash sources with care and responsibility whatever the farming system.

The potash cycle

Potassium in soil can be thought of as existing in four pools according to the availability of the K for uptake by plant roots. It is present dissolved in the soil water, adsorbed onto particles of clay and organic matter and held within the crystal structure of clay particles.

Exchangeable K, which is determined by routine soil analysis, is the K that is most readily available for uptake by roots. It is the K in the soil solution and in the 'readily available' pool. Sources of potash to the plant are indicated in the diagram. They include small amounts from rain, generally less than 5 kg/ha/year. Some K is lost by drainage from soil and studies indicate for most soils this loss is approximately 1 kg K for every 100 mm of through drainage. Potassium loss is higher from dung and urine patches after grazing because of the very high concentrations in these areas. Uneven or large applications of slurry or FYM can also lead to greater loss than from correctly managed fertiliser or manure.

Manures contain useful amounts of potash depending upon the type of livestock and litter/straw (if any) from which they are derived (see PDA Leaflet 'Nutrient Contents of Manures'). Potassium from these sources behaves in the same way in the soil as from fertiliser. Methods of manure storage are also important as K losses can be considerable - up to 50%. Unless imported from another farm, manures only recycle nutrients within the farm and do not replace nutrient which is removed in the products sold.

It is the soil itself which supplies the crop with nutrients and additions of fertiliser or manure are made to replenish these soil reserves. Most soils contain very large quantities of potassium - up to 100 t/ha - but most of this is not available to the plant. Plants take up potassium (as K⁺) from the soil solution which contains only small quantities - less than 20 kg/ha K. As plants take up K⁺ from the soil solution it is replenished by the release of K⁺ held by the clay in the soil (clay minerals are negatively charged and thus attract and hold the positively charged cations - potassium, calcium, magnesium, sodium etc.). Depending upon how strongly it is bonded to the clay, K⁺ may be released rapidly (exchangeable K), slowly (less readily available K) or by weathering over long periods of time (very slowly available K [also known as matrix K]).

Careful cultivation, conservation of organic matter and improvement of biological activity will maximise the total availability of potassium from all these sources according to the individual nature of each specific soil. Even under ideal management however, the natural release of potassium is unlikely to be sufficient to maintain the necessary available nutrient levels in the soil and extra supplementation from the sources listed below will be required.

Soil Analysis

Routine soil analysis measures the potassium in the soil solution plus the exchangeable K. Unfortunately it is difficult to measure or predict the release of less readily available K.

Whilst all the 'pools' are shown separately in the diagram on the previous page, in the living soil there are no sharp divisions and the entire system is dynamic with potassium becoming more or less available according to many varying factors.

Soil analysis provides the best practical guide to the adequacy of reserves of available nutrient for plant growth, and to any need for nutrient supplementation (see PDA Leaflet 24: 'Soil Analysis, Key to Nutrient Management Planning').