Sunday, August 11, 2013

Highly weathered soils from Visayas, Philippines

Weathering is the alteration by chemical, mechanical, and biological processes of rocks and minerals at or near the Earth’s surface, in response to environmental conditions.

Highly weathered soils (or strongly weathered soils) are soils that have undergone prolonged and intense weathering under the net leaching environment of the humid tropics. They are commonly found on stable and old geomorphic surfaces underlain by easily weatherable rocks such as ultrabasic and basic rocks as well as by pre-weathered sediments (Beinroth, 1982). These soils are clayey, deep, reddish, acidic, and have low nutrient status. According to Jackson et al. (1948), highly weathered soils are characterized by weathering stages of 10 to 12 wherein the clay fraction is dominated by 1:1 phyllosilicates (kaolinite & halloysite), aluminum oxide (gibbsite), and iron oxides (goethite and hematite). This mineralogical characteristic is also predicted by the “residua hypothesis” of Chesworth (1973) which states that soil composition will with time move towards the residua system composed of SiO2, Al2O3, Fe2O3, and H2O. In the USDA Soil Taxonomy, the highly weathered soils belong to the Ultisols and Oxisols orders. In the World Reference Base, these soils belong to the reference soil groups Alisols, Acrisols and Ferralsols. These soils possess nutritional problems for crop growth and thus are a problem for agriculture.

(Beinroth, F.H. 1982.Geoderma 27(1982)-1-73; Chesworth, W. 1973. J. Soil Science 24: 69-81; Jackson, M.L. et al. 1948. J. Physical and Colloidal Chemistry 52: 1237-1260).  

Below are photos of the important highly weathered soils from Leyte, Negros and Samar islands in the Visayas. 

This is an Oxisol that formed from ultrabasic rock in Salcedo, Eastern Samar

The widespread red soil (Ultisol) in the volcanic area of Central Negros

An Ultisol on pre-weathered sediments from basalt in Silago, Southern Leyte

An Ultisol formed on pre-weathered sediments from basalt in Biliran, Leyte

The widespread soil from basalt on the lower slopes of Mt. Pangasugan, Baybay, Leyte

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