© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

The Potassium (K) Cycle

  • Supplied to the soil solution mainly by mineral weathering and cation exchange from colloid surfaces.
  • Organic matter mineralization has little effect.
    • Potassium readily leaches out of plant residues and so is not a structural component of soil humus.
  • Certain 2:1 type clays, especially vermiculite, can fix potassium ions in interlayer positions.
    • This K fixation is similar to that of ammonium (NH₄⁺) both become inaccessible to normal cation exchange and to root uptake.
  • In some soils mineral weathering can supply potassium fast enough to maintain a sufficient supply of soluble and exchangeable K.
    • In some soils continued removal of high potassium crops can deplete the available soil supply faster than natural weather can replenish it.
  • Potassium is not lost from soils as gaseous forms.
  • Both leaching and erosion losses can be substantial.

The Potassium (K) Problem in Soil Fertility

  • Easily exchangeable K usually accounts for only a small fraction of the total soil K
  • Much potassium is locked up in the structural framework of soil minerals.
  • Because of the complimentary ion effect, leaching losses can be reduced by liming an acid soil.
  • Certain high K demanding crops (e.g. potato or alfalfa)Although large amounts of potassium can be productively used by
  • Some plants may take up potassium beyond the amount needed for optimum growth
    • Wasteful luxury consumption.

Forms and Availability of Potassium in Soils

Potassium in soils is distributed principally among four forms, the availability of which varies as follows:

  1. Potassium in primary mineral (e.g. micas, feldspars) structures is unavailable.
  2. Potassium in non-exchangeable positions in secondary minerals (e.g., vermiculite or colloidal size mica) is only slowly available.
    • This includes what is termed "fixed K"
  3. Potassium on the cation exchange sites of soil colloids is readily available to plants.
  4. Potassium soluble in water is readily available and the immediate source of ions taken up by plants.

Only 1 to 2% of the total potassium in soils is in readily available forms at any one time, but the potassium continually distributes itself among the four forms listed above.

Factors Affecting Potassium Fixation in Soils

The fixation of potassium in the interlayers of secondary minerals and its release to more available forms is controlled by four factors:

  1. The nature of the soil colloids (especially the content of vermiculite and other 2:1 clays).
  2. Wetting and drying cycles which cause the clay interlayers to become more exposed, both for the fixation and the release of K.
  3. Freezing and thawing cycles, which cause effects similar to those of wetting and drying.
  4. The presence of excess lime which decreases the percent potassium saturation as it increases the soil CEC by activating pH dependent charges, and also decreases the ratio of potassium to other cations (especially Ca and Mg) on the cation exchange complex.

The relationship between a nutrient cation in solution and on the exchange complex is one of equilibrium.

Practical Aspects of Potassium Management

Where crop harvest removes slowly and readily available forms of potassium more rapidly than mineral weathering can replace it, potassium-containing fertilizers may be needed to replenish the soil potassium reserve and to assure sufficient availability of potassium during periods of rapid plant uptake.

Potassium should be applied close to the time of plant need, especially on soils with high potassium fixing tendencies, or very coarse textured soils with high potassium leaching potential.

Complete replacement of all potassium removed in harvest is rarely necessary as advantage should be taken of the considerable inherent potassium-supplying power of many soils. The return of animal manures and plants residues to the full extent possible can greatly reduce the need for fertilizer additions, but in many parts of the world increased use of potassium-supplying fertilizers will be necessary as yields and therefore potassium removals increase.