• Potassium (potash) is one of the major plant nutrients and of great importance both in the soil and plant and crop nutrition;
  • It is concerned with the quality rather than the size of the plant;
  • Potassium is taken up by the young plant early in its growth and held mainly in the stems and leaves. It plays an important part in the build-up of proteins;
  • Potassium increases the rate of photosynthesis in the green leaf of the plant which leads to an increase in the amount of sugar and starch produced. It increases the efficiency of the leaf whereas nitrogen increases the area of the leaf;
      Stamina: the ability to sustain prolonged physical or mental effort.

It affects the movement of carbohydrates around the plant. This is important with potatoes where carbohydrate is moved into the tubers as they grow. Potato tubers have a high carbohydrate and potash content;

  • It increases the stamina of the plant making it more resistant to disease and less liable to attack by insects making recovery after insect damage faster;
  • Potassium delays the maturity of the plant (the formation of seeds). This is important in areas where drought leads to the early maturing of the plant. This can be reduced to some extent by applying potassium and;
  • It improves the quality of fruit, vegetables and cereal grains by increasing the amount of sugar or starch in the seed.

A deficiency of potassium can cause the plant leaf to die off around the edges (generally yellow in colour) and at the tip. In legumes, small white spots appear on the lower leaves. The deficiencies are mainly found on light sandy soils and as potassium is highly soluble it gets leached out of the soil making plants more susceptible to frost damage and some diseases.

Figures 1 and 2: Show deficiencies in potassium of a soya bean plant.

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An excess of potassium in the soil can cause too much potash to be taken up by the plant instead of some other element. This can happen with magnesium causing a magnesium deficiency which in the case of grass is passed onto the grazing animal. This is particularly important in the U.K where magnesium deficiency in milking cows can lead very rapidly to the death of the animal.


  • Potash is held in the soil in the form of potassium ions (K+) which are attracted to and held by the clay particles (the clay colloids). This is known as the exchangeable potassium  because it can be brought into the soil solution and taken up by the plant;
    • The amount of exchangeable potassium in the soil will depend on the texture in the soil and the type of clay.
    • Clay soils are usually rich in potassium;
    • Sandy soils are usually poor in potassium and;
    • Sandy soils that have been weathered from granite containing potassium feldspar (pink feldspar) are fairly high in potassium.
    • The Potassium available to the plant in the form of K+ is held in the topsoil in clay soils. In sandy soils it may be lower down having been washed down (leached) by rain water.
    • This Potassium may be available to the plant roots but does not show in an analysis of the topsoil;
    • Sandy soils become deficient in potassium more quickly than clay soils but less potassium fertiliser should be applied to remove the deficiency because sandy soils cannot hold potassium as well as clay soils;
    • Potassium that is unavailable to the plant is held either in an insoluble form, or locked up inside the soil minerals (micas, feldspars of the clay minerals) and;
    • Potassium fertiliser is always applied to the seed bed and it is never as a top dressing onto the growing crop.


Potassium fertilisers are sold in the form of water soluble potassium oxide (K2O) and the percentage of K or K2O is stated on the fertiliser packet.


This is almost pure potassium chloride (KCI) which is dug out of the ground in parts of France and Germany. It is a pink crystalline substance which is hygroscopic (e.g. it absorbs water out of the air and must be stored in plastic bags.) It contains 60% K2O, and also some salt (NaCI).

Figures 3 and 4: Shows muriate of Potassium (left) and potassium sulphate (right).

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This is manufactured by treating muriate of potassium (KCI) with sulphuric acid (H2SO4). It is a non- flammable substance that is a white crystalline salt. This form of potassium fertiliser provides potassium and sulphur and is soluble in water. The formula is K2SO4.


This can be used as a source of potassium and contains 2 – 5% K2O. There are 3 ways in which wood ash can improve soils. By acting as a soil conditioner which can neutralise acidity, used as a form of pesticide if scattered on the surface of the soil as it repels most crawling insects and as an organic fertiliser.

Figures 4 and 5: Shows wood ash which can be used as a form of potassium fertiliser

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This is very rich in potassium and used as a fertiliser in places like Japan. It can be applied to the soil as mulch or onto a compost heap because it is a very good activator. It contains all soil nutrients including the micro nutrients but has a high salt content.

Table 1: Muriate of potassium is cheaper than sulphate of potassium and is used, except in the following cases:

  Tobacco and potatoesAlways use sulphate of potassium because the quality of the crop is affected by the salt in muriate of potassium.
Brak soilsAgain use sulphate of potassium because of the salt in muriates.
Fruit Trees Vines Sugar Cane  It is better to use sulphate of potassium.