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.