Soil development and fertility characteristics of a volcanic slope in Mindanao, the Philippines

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, whil e those in the mountains were Ultisols and Inceptisols. Presence of "amorph ous components", such as allophane and imogolite, in all the pedons studied was indicated by a >9.4 soil pH in NaF. Halloysite, gibbsite, goethite, he matite, and cristoballite were more common minerals in the clay fraction. S urface layers of all the pedons were slightly acidic and pH increased by de pth. Phosphate sorption maxima ranged from 6944 to 14208 mu g P g(-1), and it was closely associated with oxalate-extractable Al (Al-o) and clay conte nt. Inceptisols had higher phosphate sorption maxima than Oxisols. Soil sam ples representing the mountains shelved the lowest level of both the availa ble K and the potential buffering capacity for K (PBCK), while the upper fo otslopes had the highest level of available K. The PBCK values were lower f or Inceptisols than for Oxisols, and they were found to be positively corre lated with soil pH. There was a large difference between the cation-exchang e capacity (CEC) and the effective cation-exchange capacity (ECEC), an indi cation of a large pH-dependent charge. Mountain soils showed lower base sat uration than soils representing the upper footslopes, the lower footslopes, and the alluvial terraces.