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DIVISION S-5-PEDOLOGY

Soil Development and Fertility Characteristics of a Volcanic Slope in Mindanao, the Philippines

^a Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616 USA
^b Dep. of Crop and Soil Sciences, Miller Plant Sciences Building Room 3111, Univ. of Georgia, Athens, GA 30602 USA

ddpoudel{at}ucdavis.edu

Thirteen pedons representing the mountains, the upper footslopes, the lower footslopes, and the alluvial terraces of a volcanic slope in Mindanao, the Philippines were studied to understand relationships between the degree of soil development and fertility characteristics. Soils in the upper and the lower footslopes were Oxisols as were soils in the alluvial terraces, while those in the mountains were Ultisols and Inceptisols. Presence of "amorphous components", such as allophane and imogolite, in all the pedons studied was indicated by a >9.4 soil pH in NaF. Halloysite, gibbsite, goethite, hematite, and cristoballite were more common minerals in the clay fraction. Surface layers of all the pedons were slightly acidic and pH increased by depth. Phosphate sorption maxima ranged from 6944 to 14208 µg P g^-1, and it was closely associated with oxalate-extractable Al (Al_o) and clay content. Inceptisols had higher phosphate sorption maxima than Oxisols. Soil samples representing the mountains showed the lowest level of both the available K and the potential buffering capacity for K (PBC^K), while the upper footslopes had the highest level of available K. The PBC^K values were lower for Inceptisols than for Oxisols, and they were found to be positively correlated with soil pH. There was a large difference between the cation-exchange capacity (CEC) and the effective cation-exchange capacity (ECEC), an indication of a large pH-dependent charge. Mountain soils showed lower base saturation than soils representing the upper footslopes, the lower footslopes, and the alluvial terraces.

Abbreviations: AR^K_e, degree of K availability • BS, base saturation • CEC, cation-exchange capacity • d [subscripted], citrate-dithionite extractable • ECEC, effect cation-exchange capacity • masl, meters above sea level • o [subscripted], oxalate extractable • PaM, P adsorption maxima • PBC^K, buffering capacity for K • Q/I, quantity/intensity relationship • SANREM-CRSP, Sustainable Agriculture and Natural Resources Management Collaborative Research Support Program • -K°, measures of labile K