Potassium Cycle

Potassium is a macronutrient taken up by plants in large quantities. Unlike N, P and S, K is present in the soil solution only as a positively charged cation (K⁺). Its behaviour in the soil is impacted by soil cation exchange and mineral weathering, rather than microbial activity. K, similarly to P, does not form gases that could be lost to the atmosphere, and it causes no off-site environmental problems when it leaves soil system.

Potassium is necessary to many plant functions, including carbohydrate metabolism, enzyme activation, osmotic regulation, and protein synthesis. Even though K plays various roles in plant and animal nutrition, it is not actually incorporated into the structures of organic compounds. Instead, potassium remains in the ionic form in solution in the cells or acts as an activator for cellular enzymes. Potassium is essential for photosynthesis, for N-fixation in legumes, starch formation, and translocation of sugars. As a result of several of these functions, a good supply of K promotes production of plump grains and large tubers.

Potassium regulates the opening and closing of plant stomata by a K ion pump. Since stomata are important in water regulation, K reduces water loss from the leaves and increases drought tolerance. K is also important in helping plants adapt to winter hardiness, fungal diseases and insect pests.

Sources

Primary Minerals

The original sources of potassium are the primary minerals, such as:

Feldspars - Orthoclase and microcline - KAlSi₃O₈

Mica - Biotite - KAl(AlSi₃O₁₀)(OH)₂

Mica - Muscovite - K(Mg, Fe)(AlSiO₁₀)(OH)₂

Potassium in the crystalline structure of primary minerals is not available to plants and it accounts for 90-98% of the total soil K.

See pictures of primary minerals in Soil Components.

Potassium Fertilizers

Practically all of the potassium fertilizers are water soluble (i.e., they dissolve in water). They consist essentially of potassium in combination with chloride, sulfate, nitrate or polyphosphate, and include the following:

Potassium chloride (KCl)

Potassium sulfate (K₂SO₄)

Potassium nitrate (KNO₃)

Potassium phosphates (KPO₃, K₄P₂O₇, KH₂PO₄, K₂HPO₄)

Animals

Animals consume plants and in turn add various residues (excrement and remains) to the soil that then serve as a source of soil organic matter or humus.

Humans

Humans consume plants and in turn add various residues to the soil that then serve as a source of soil organic matter or humus.

Losses

Erosion

Erosion is one of the main pathways through which K is lost from the soil.

Runoff

Runoff is one of the main pathways through which K is lost from the soil.

Leaching

Annual leaching loss of K from the soils in a humid region under agricultural production (receiving only a moderate rate of K fertilizer) is usually about 25 to 50 kg K/ha.

Harvesting

Plants take up very large amounts of K. When most or all of aboveground biomass is removed in a harvest, the soil loss of K can be substantial. Annual losses of K through harvesting can be as great as 400 kg K/ha and are common if the plant is a legume and is cut several times for hay.

Forms of K

Non-exchangeable Potassium

Non-exchangeable K refers to K⁺ ions adsorbed in the interlayer spacing of clay minerals, such as illite, vermiculite, and chlorite. This form of K accounts for 1-10% of total soil potassium.

Exchangeable Potassium

Exchangeable K includes those K⁺ ions adsorbed (by electrostatic forces) and released on clay and organic colloids. The exchangeable K accounts for 1-2% of total soil K.

Potassium in Soil Solution

Potassium cations (K⁺) dissolved in the soil solution account for 0.1-0.2% of total soil potassium. Plant roots take up K as the K⁺ ion from the soil solution. Potassium is taken up by plants in large quantities.