Potassium: Content, Sources and Forms | Soil

After reading this article you will learn about:- 1. Content of Potassium in Soil 2. Sources of Potassium in Soil 3. Forms 4. Deficiency.

Content of Potassium in Soil:

A large portion of total K in soils comes from the K-bearing and crystal lattices of silicate minerals. The micas (muscovite and biotite) and the feldspars (orthoclase and microcline) constitute the major K-bearing minerals which on weathering gradually release K to the soil.

Besides, secondary minerals like kaolinite, halloysite etc. release K into the soil whereas the release of K is very difficult from 2: 1 type of clay minerals namely, montmorillonite, illite, vermiculites etc.

The potassium content in mineral soils is generally much higher than that of the phosphorus and nitrogen. Average potassium content in the earth’s crust is about 1.9% while that of phosphorus is only about 0.11%. However, the concentration of K in the soil solution varies from soil to soil depending on the parent rock, mineral matter and other climatic conditions.

The potassium content in Indian soils varies from soil to soil. In terms of total K content, the fixed form is about 6.1 per cent and the water soluble and exchangeable form is 0.98%.

In West Bengal, the exchangeable and non-exchangeable K content ranges from less 14 to 132 or more and 172 to about 3000 mg kg^-1 with a mean values of 52 and 450 mg kg^-1 respectively. For most mineral soils, the potassium content (K₂O) ranges from 0.05 to 3.5%.

Sources of Potassium in Soil:

In addition to potassic fertilizers, the presence of potassium in soils originates from the disintegration and decomposition of potash bearing minerals. Some most important minerals are considered as the source of potassium namely potash feldspars (orthoclase and microcline, KAlSi₃O₈), muscovite [KAlSi₃O₁₀(OH)₂], biotite [K (Mg, Fe]₃ AlSi₃O₁₀ (OH)₂] etc.

However, potassium is found in soils as both primary and secondary minerals. Various secondary clay minerals like illites (hydrous micas), Vermiculites, chlorites and interstratified minerals contribute a greater amount of potassium in soils.

Forms of Potassium in Soil:

Based on the availability of potassium to crops, potassium is being grouped into four forms as follows – Each category or form of potassium is present in soils as dynamic equilibrium with other forms and so on.

These four forms of K in decreasing order of availability with an approximate estimate in each form are: Soil solution (1 to 10 ppm); exchangeable (40-600 ppm); Non-exchangeable (fixed or difficultly available, 50-750 ppm) and mineral or reserve K (> 5000 ppm). The relative importance of the above four forms of K depends on the mineralogical composition of the soil.

Mobility of potassium in soils produces dynamic equilibria among various forms of potassium. Exchangeable and soil solution K equilibrate quickly while difficultly or fixed form of K equilibrates very slowly with its exchangeable and soil solution forms.

Release of potassium from the reserve or mineral forms of K to any other there forms of K is extremely slow and hence it is strictly considered unavailable form of K to plants particularly in a single cropping season.

Fixed or non-exchangeable K occurs within clay minerals such as illite, vermiculite and chlorite. On the contrary, exchangeable K is held on soil colloids (clay and organic matter) by electrostatic forces which facilitate the release of K into the soil solution rendering available to crops. Therefore, there is a continuous slow transfer of K from the mineral source to the exchangeable and slowly available forms.

Heavy applications of potassic fertilizers will cause reversion to the slowly available form. From the extracts of many research investigations, it has been indicated that the unavailable form accounted for 90 to 98 per cent of the total K in soils, while that of slowly and readily available forms as 1 to 10 and 0.1 to 2 per cent respectively.

However, the relationships between different forms of potassium are depicted in Fig. 21.8.

On weathering of K-bearing minerals, the release of K takes place in the soil solution which can be either directly taken up by plant roots or be adsorbed on soil colloids. Equilibrium is thus set up between adsorbed K⁺ and the free K⁺ in soil solution.

The K⁺ content in soil solution depends much on the selectivity of the adsorption sites. The concentration of K⁺ in the soil solution is less if the adsorption sites are specific and greater K⁺ concentration with the less specific adsorption sites.

The K⁺ concentration of the soil solution largely controls the diffusion of K⁺ towards the plant roots and hence its uptake by plants. Besides the soil solution K, the K⁺ buffer capacity of the soil is also considered as an important factor in determining the K⁺ availability in soils.

Deficiency of Potassium:

It is evident that potassium deficiency does not immediately show the visible symptoms. Initially, there is only a reduction in the rate of plant growth (hidden hunger), and only at the severe K deficiency (later stage) show chlorosis and necrosis and such symptoms usually start in the older leaves because of inadequate potassium supply.

However, the expression of K deficiency varies with the plant species. Most of the plant species (cereals and fruits) show chlorosis and necrosis in the margins and tips of the leaves, whereas in some plants (clover) show irregularly distributed (uneven) necrotic spots on the leaves.

Plants suffering from K⁺ deficiency result a decrease in turgor and water stress plants easily become flaccid (weak). Resistance to drought is therefore poor and the affected plants show increased susceptibility to frost damage, fungal attack and soluble salt concentrations.

Besides these, potassium deficient plants very oftenly show an abnormal development of tissues and cell organelles as well as restriction of formation of xylem and phloem tissue in plants. Lignification of the vascular bundles is usually inhibited by potassium deficiency making plants more prone to lodging.

Potassium deficiency also results in a collapse of chloroplasts and mitochondria, and retardation of cuticle development.