Potassium availability in soils

Potassium (K) is second to nitrogen in terms of the amount absorbed by higher plants. Optimum K level for plant growth ranges from 2 to 5% of plant dry weight (Marschner, 1995). Unlike P, K is present in relatively large quantities in soils coming from the weathering of primary minerals such as feldspars, mica, and others. But it is commonly deficient in highly weathered or old soils. Total K contents of soils range between 3000 and 100,000 kg/ha in the upper 20 cm of the soil profile (Sparks, 2000). The behavior of K in the soil is influenced primarily by CEC and mineral weathering and not by biological processes.

Interrelationship of various forms of soil K (modified from Sparks, 2000)

K in the soil occurs in 4 forms: solution K, exchangeable K, nonexchangeable K, and mineral K (Sparks, 2000).

a) Solution K. This is the K dissolved in the soil solution. It is the form of K that is readily available to plants and soil microorganisms and also is the form of K most subject to leaching losses. It varies in amount from 2 to 5 mg/liter K but can be dramatically changed by the addition of k fertilizers to the soil.

b) Exchangeable K. This is the form of soil K that is adsorbed on the surfaces of soil colloids. It is readily exchanged with other cations in the soil solution and is also readily available to plants. Some authors combine exchangeable K and solution K into one form called readily available form of K which comprises only 1 to 2 percent of soil K. This is also dependent on the CEC of the soil.

c) Nonexchangeable K. This is the portion of soil K that is fixed or held between adjacent layers of 2:1 clay minerals particularly vermiculite and smectite clay minerals. This is continually released to the exchangeable form when levels of exchangeable and soil solution K drops due to plant uptake and leaching losses.

d) Mineral K. This is the K that is part of the crystal structure of primary minerals such as muscovite, biotite, and feldspars. It is the most abundant and accounts for 90 to 98 percent of soil K. It is unavailable to plants and can only be released to the soil solution upon weathering of the minerals.

Leaching is the major cause of loss of K in the soil. Leaching of soil solution K is greatly dependent on the CEC of the soil and thus is influenced by the amount and type of clay and the SOM content of the soil. Soils with higher CEC like clayey soils have greater ability to hold K and thus have lower leaching losses than sandy soils with low CEC. Excess application of K-fertilizers can also enhance leaching losses especially under conditions of high rainfall.

Another form of leaching loss of K (and other nutrients) which is often overlooked is the one that occurs from the leaves of the plants. This can cause substantial nutrient loss exceeding seven times the amount in the standing crop in the case of K. Nutrients are leached from the leaves in the order K>N>P although this would also depend on the nutrient status and leaf morphology. Anything that reduces the water contact with leaves like smooth cuticle, erect leaves, etc. also reduces leaching losses (Chapin, 1980).

References
Chapin, F.S. III., 1980. The mineral nutrition of wild plants. Ann. Rev. Ecol. Syst. 11:233-260.
Marschner, H. 1995. Mineral Nutrition of Higher Plants. 2nd ed., Academic Press, London.
Sparks, D.L. 2000. Bioavailability of soil potassium. In:  In: Handbook of soil Science (M.E. Sumner, ed.). CRC Press, Boca Raton, pp: D38-D53.