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.

Ulrich’s soil acidification hypothesis on forest decline

In 1979, Bernhard Ulrich was the first researcher to discover the connection between air pollution and the forest decline or dieback (Waldsterben) in Germany. He hypothesized that acid rain results in soil acidification which in turn causes the forest dieback phenomenon. According to his soil acidification hypothesis, as soil becomes more acidic there is a release of aluminum that damages the roots of the trees. This leads to the following effects: reduction in uptake and transport of some cations, reduction in root respiration, damage to fine feeder roots and root morphology, and reduction in elasticity of the cell walls. The discovery was first published by Ulrich and co-workers in "Deposition von Luftverunreinigungen und ihre Auswirkungen in Waldökosystemen im Solling." Schriften Forstl. Fak. Univ. Goettigen 58, Sauerländer Verlag, Frankfurt a.M., 291pp. 

Photo source: www.museumplatkow.de

In 1986, Ulrich put forward his 8 theses on soil acidification which appeared in the Journal of Plant Nutrition and Soil Science 149: 702-717 (1986) as follows:

1. Rocks contain only bases and no acid precursors. Therefore, with the exception of sulfide containing rocks, soils cannot acidify as a result of atmospheric rock weathering.

2. A consumption of protons in rocks and soils results in a decrease of their acid neutralizing capacity and can result in the buildup of a base neutralizing capacity.

3. Weak acids (carbonic acid) lead in geological times to the depletion of bases without a larger accumulation of labile cation acids. Strong acids (HNO3, organic acids, H2SO4) can lead within a few decades to soil acidification.

4. The acid input caused by the natural emission of SO2 and NOx can be buffered by silicate weathering even in soils low in silicates.

5. The cause of soil impoverishment and soil acidification is a decoupling of the ion cycle in the ecosystem.

6. Acid deposition in forest ecosystems which persists over decades leads to acidification.

7. Formation and deposition of strong acids with conservative anions (SO4, NO3) shifts soil chemistry into the Al or Al/Fe buffer range up to a great soil depth.

8. In the long run, soil acidification by acid deposition results in the retraction of the root system of acid tolerant tree species from the mineral soil, and in water acidification.

Bernhard Ulrich was professor of forest soil science and forest nutrition at the University of Goettingen, Germany, from 1965 until his retirement in 1991. He obtained his PhD in agricultural science from the University of Hohenheim, Stuttgart, in 1953 based on a dissertation on the rapid determination of soil cation sorption capacity. He was widely recognized as the leading expert of soil acidification and forest ecosystem research.