Available K in highly weathered soils in Leyte and Samar, Philippines

by Ivy L. Capilitan
Department of Agronomy and Soil Science
Visayas State University, Leyte

Soil potassium (K) may be divided into four pools: water-soluble, exchangeable, non-exchangeable, and matrix K (Darunsontaya et al. 2012). With K depletion by plants, water-soluble K and exchangeable K are lowered to minimum levels characteristic of soils. Brady (1974) reported that approximately 90 to 98% of the total soil K is in the non-exchangeable form (although some become available very slowly due to weathering) and is part of the internal structure of clay mineral particles and parent rock material. This form is not available for plant uptake. Feldspar is a general term for a group of aluminosilicate minerals containing Na, Ca or K. Along with mica, feldspar contains most of the K. The plants cannot use the K in this crystalline insoluble form but over long periods of time, these minerals weather (breakdown) and K is released. This process however is too slow to supply the full K needs of field crops yet as soon as these minerals weather, some K moves to the slowly available pool.  

Approximately 1 to 2% K is in the exchangeable form and is lightly bound or held (adsorbed) on the surface of clay particles and organic matter. This form becomes available rapidly and easily to plants when it exchanges with other cations and moves back into the soil solution. K availability also appears to be diminished by allophane through K fixation (van Reeuwijk and Devilliers, 1968 as cited by Poudel and West, 1999). Hopkins and Huner (2009) noted that positively charged K is naturally attracted to a region in the soil with the dominance of negative charges. Both the soil solution and exchangeable K are measured in a soil test as available K. But according to IPI (1977), the concentration of K in the soil solution is a better indicator of K availability than the amount of exchangeable K.

Relationship of exchangeable K and soil solution K with plant uptake of K (IPI, 1977)

Little information is available about K most specifically on its content and availability in highly weathered soils in the Philippines. Thus, we conducted a study that compared the K concentrations and availability in selected highly weathered soils in Leyte and Samar Islands and evaluated the factors affecting them. The soils we used have developed from basalt (Baybay and Biliran site), shale (Matalom site), and ultrabasic rocks (Salcedo site). In terms of the relative degree of weathering, the following sequence was observed: Salcedo soil > Matalom soil > Baybay soil > Biliran soil. The soils are deep, clayey with color ranging from brown to yellowish-brown and red to yellowish-red. They have angular to sub-angular blocky structure with friable and very friable to very firm consistency when moist but generally sticky and plastic when wet.  In terms of chemical characteristics, the soils have acidic pH values ranging from 5.0 to 5.4 (very strong acid). They have a low to medium amounts of organic matter of 2 and 6 %. The total N contents are high in the upper horizon which decreases with depth. All the soils are dominated by 1:1 type of clay minerals particularly kaolinite and halloysite as well as high amounts of iron and aluminum oxides which reflect the highly weathered nature of the soils. 

All the four soils have low exchangeable K and low soil solution K. In general, soil solution K was significantly correlated with soil pH. On the other hand, exchangeable K was generally not correlated with soil pH. The low levels of "available" soil K appear to be related to the highly leached and highly weathered nature of the soils. The study found no clear evidence of the connection between K availability and the type of parent rock of the soils studied. 

References
Darunsontaya T., Suddhaprikarn A., Kheoruenromne I, Prakongkep and Gilkes R. J.. 2012. Geoderma 170: 11-24.
Brady N.C.. 1974. The Nature and Properties of Soils. Macmillan Publishing Co. New York.
Hopkins W.G. and Huner N.P.A.. 2009. Introduction to Plant Physiology. John Wiley and Sons, Inc., New York.
International Potash Institute (IPI). 1977. Potassium dynamics in the soil. CH-3048 Worblaufen-Bern/Switzerland.
Poudel D.D. and West L.T.. 1999.  Soil Sci. Soc. Am. Journal 63:1258–1273.

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.