Saturday, 15 July 2006

Characteristics, Genesis, and Fertility Status of a Deeply Weathered Soil from Cretaceous Ultrabasic Rock in Samar, Philippines.

Victor B. Asio, Dept of Agronomy and Soil Science, Leyte State Univ, Baybay, Leyte, 6521-A, Philippines, Ian A. Navarrete, Laboratory of Environmental Soil Science, Obihiro Univ of Agriculture and Veterinary Medicine, Inada-cho, 080-8555, Japan, Juvia P. Sueta, Dept of Agronomy and Soil Science, Leyte State Univ, Baybay, Leyte, 6521-A, Philippines, Janice P. Susaya, Inst of Tropical Ecology, Leyte State Univ., Baybay, Leyte, 6521-A, Philippines, and Reinhold Jahn, Inst of Soil Science and Plant Nutrition, Martin Luther Univ, Weidenplan 14, Halle(Saale), D-06108, Germany.

Very little information is available on the nature and genesis of soils from Cretaceous ultrabasic rocks especially under tropical island environmental conditions. No other study has also been done on the subject in the Philippines until now. We conducted this study to evaluate the characteristics, genesis and fertility status of a deeply weathered soil derived from Cretaceous ultrabasic rock (largely peridotite) in the island of Samar, the third largest member of the Philippine archipelago. Geologic records for the area indicate that the above-mentioned rock formation was probably uplifted from the sea during the Tertiary. Representative soil profiles were described and sampled for physical, chemical and mineralogical analyses. In addition, composite surface soil samples were collected from the major land uses in the study site. Results revealed that the red soil is strongly acidic having very high clay content (more than 60%) but low organic carbon, CEC, exchangeable bases, and available nutrient status. It showed high dithionite extractable Fe and very low oxalate/dithionite ratios. The soil mineralogy is dominated by halloysite, kaolinite, goethite, hematite and quartz. The deeply weathered nature of the soil (classified as Acric Ferralsol in the WRB or Rhodic Hapludox according to Soil Taxonomy) appears to be largely the influence of the geochemistry of the parent rock, age of the geomorphic surface, and the perhumid tropical climate. The high exchangeable sodium of the soil could be attributed to salt spray since the site is close to the sea. Moreover, the soil has very high levels of total nickel and chromium which were probably inherited from the ultrabasic parent material. The low nutrient status and the highly plastic and sticky consistence of the soil when wet, appear to be the major constraints to crop production in the area. Application of chicken manure is widespread among farmers and is effective in increasing the yield of cassava and a few other crops

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