Saturday, 15 July 2006
125-8

Characteristics, Genesis and Degradation of Soils from Volcanic Rocks in Leyte, Philippines.

Victor B. Asio, Dept of Agronomy and Soil Science, Leyte State Univ, Baybay, Leyte, 6521-A, Philippines, Reinhold Jahn, Inst of Soil Science and Plant Nutrition, Martin Luther Univ, Halle(Saale)210, D-06108, Germany, and Karl Stahr, Inst of Soil Science and Land Evaluation, Univ of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany.

Very few studies have been conducted on the characteristics, genesis and degradation of soils from volcanic rocks in geologically young tropical islands of the Philippines. Such soils may be distinct from those in other tropical areas because of the unique environmental conditions that influenced their genesis. Likewise, there is a need for detailed data on volcanic soils in the Philippines since they are widespread and thus are important for agriculture. In addition, large degraded areas in the archipelago, which are now the subject of forest rehabilitation, have volcanic soils. We conducted this study at two sites in the island of Leyte, Philippines located at 124o17' and 125o18' east longitude and between 9o55' and 10o48' north latitude. The first site, Ormoc, is located at an elevation of about 700m above sea level (asl) having andesitic volcanic rocks of Quaternary origin (probably Holocene to Upper Pleistocene) while the second site is on the lower mountain slopes at about 100m asl and derived from a relatively older volcanic rock material of Lower Pleistocene to Pliocene origin. The climate is humid tropical and the natural vegetation is rainforest of the Dipterocarp type. In each site, four soil profiles representing major land use systems were examined and sampled to determine the occurrence of soil degradation brought about by land use change. The least disturbed (under forest vegetation) of the four soil profiles from each site was then subjected to detailed physical, chemical and mineralogical examination to evaluate the genesis of the soils. The soil in the Ormoc site is poorly developed as reflected by its Ah-AB-BC-CB horizon sequence. It is silt loam to sandy loam in texture, has low bulk density, very high friability, high organic C content, acidic reaction with tendency for positive charge in the subsoil, very low effective CEC, low base saturation, and a very high P retention capacity. Its clay fraction is composed primarily of imogolite, allophane, gibbsite, goethite, ferrihydrite and some layered clay minerals such as chlorite and vermiculite. Gibbsite occurs as feldspar pseudomorphs apparently in microsites with high leaching rates. The soil is classified as Typic Hapludand (USDA Soil Taxonomy) or Umbric Andosol (FAO). The soil in the Baybay site is much more developed and is characterized by an Ah-AB-Bt-BC-CB-Cw horizon sequence which is more than 4 m deep. It has heavy clay texture, is friable but plastic and sticky when wet. The soil is acidic and has low effective CEC and high P retention capacity. Its clay fraction is dominated by kaolinite, halloysite, goethite and hematite. The kaolinite is probably the product of the aging of halloysite. Halloysite could have formed from preweathered material and not from imogolite. The soil is classified as Typic Hapludult or Haplic Alisol. The soils have generally comparable degree of desilification although the loss of basic elements is slightly lower in the Andisol than in the Ultisol. The presence of some comparable properties in the two soils suggests that the Baybay soil (an Ultisol) probably developed from an Andisol. The chemical composition and age of the parent rock, local topography and climate appear to be the dominant factors that have influenced the genesis of the soils studied. Our results also revealed that land use change and cultivation have caused physical and chemical soil degradation in the secondary land use uses evaluated and the degree of degradation seems to depend on the kind and degree of human disturbance. However, it is apparent that soil degradation was greatly the immediate effect of forest removal and cultivation in the past and not necessarily the effect of the prevailing secondary land uses.

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