Potash for Organic Growers

Sustainable nutrient management

All systems of production should maintain an adequate supply of potassium available to the plant. Nutrient management must balance inputs with outputs and losses.

Aims common to all systems are:

to maintain good soil structure;

to maximise soil volume available to the plant roots;

to conserve organic matter;

to promote biological activity in the soil.

Additions of potash in fertilisers or manures must be given to replace that removed by cropping. Failure to do so will affect crop performance and is not sustainable management. Sandy soils with low clay content will be most rapidly affected. If soil analysis shows exchangeable K levels to be sufficiently high, fertilisers and manures should be reduced or omitted (see PDA Leaflet 8: 'Principles of Potash Use').

On some heavy soils the release of potassium from less readily available reserves is sufficient to provide the needs of combinable crops without other additions, but on most soils, and where high demand crops such as roots and forage crops are grown, additional potash will need to be applied in order to maintain soil K reserves and to replace removal in harvested crops.

Penalties of low potash

There is increasing evidence that an adequate potash supply will help reduce crop stress caused by drought, chilling, high light intensity, heat and deficiencies of other nutrients. These stresses can result in oxidative damage to the plant from 'reactive oxygen species' (ROS) free radicals, and production of these damaging ROS can be greatly reduced by a satisfactory potassium status in the plant. Potassium plays a crucial role in maintaining the general health of the plant.

If potash is limiting, response to nitrogen will be reduced, N-fixing bacteria will be less active in legumes and crop health, vigour, and resistance to stress will suffer. Such aspects are of particular importance in organic production where natural resistance through balanced nutrition is an integral aspect of overall husbandry in the absence of agro-chemical protection. Potassium is very involved with the water relations in the plant and a deficiency will be especially serious under dry conditions.

Sources of fertiliser potash

The main reserves of potash in the world are in the clay minerals of the soils and rocks, in the water of the oceans and in the rock salt deposits containing the crystallised minerals from long dried up seas. Potassium salts, principally chloride, sulphate and nitrate derived from these evaporite rocks (and from salt pans which are in current use in certain parts of the world), are the most common forms of fertiliser potash - all of these being naturally water soluble. Perhaps the oldest form of potash fertiliser is wood ash but supplies of this material are obviously no longer practical nor sustainable.

Various process wastes containing potassium (such as lime kiln dusts) have been considered as sources of K but these vary in K availability, can contain undesirable contamination and suffer from irregularity of supply.

Finely ground primary soil minerals, for example feldspars, are offered as 'rock potash' fertiliser but the plant-availability of the potassium in such materials depends on the origin and the mineralogy of the parent material. It is possible, as with matrix K in soils, for a material to contain K but not to release it except over a geological timescale.

Green wastes, composts and other waste materials are increasingly becoming available as alternative potash sources. Animal feeds and bedding bought onto farm also represent a significant supply of the nutrient.

Whenever you are planning to use a restricted material you should consult your Certification Body. You may need prior permission and this will also ensure you do not apply a material that is not allowed.

Organic standards for potash materials

The current classification of potash materials for organic production is based on consideration of a number of characteristics which relate to the objectives listed previously:

Solubility.

Natural or manufactured product.

Sustainability.

Chloride content.

Solubility. Most potash sources - manures and fertiliser forms - are soluble and rapidly add K to soil solution. However despite its natural solubility, potassium applied at appropriate rates is not leached from soils as some other nutrients such as nitrogen or sulphur can be.

Chloride content is included because it has been suggested to be undesirable on the basis that some plant species are more sensitive to high Cl^- levels than others. It has also been asserted that excess chloride may be deleterious to micro-organisms and earthworms, but no experimental evidence has been found to support this, at agronomic rates of application.

Chloride (Cl^-), which occurs in rainfall, fertilisers and manures, is an essential plant nutrient and must not be confused with chlorine gas, hypochlorite used as a sterilant, or other forms which do not occur in soils or plants. The quantity of chloride applied in fertilisers will usually be less than that deposited by rain and salt spray in coastal areas.