Clay mineralogy and surface charge characteristics of basaltic soils from Western Samoa

The clay mineralogy and surface charge characteristics of four basaltic soils from Western Samoa have been studied. The soils contained subordinate to dominant amounts of poorly ordered allophanic material in addition to varying amounts of crystalline free iron oxide minerals. Infrared studies revealed the presence of trace to subordinate amounts of kaolin minerals in all the soils. The surface charge-pH curves followed a constant potential model indicating the presence of substantial amounts of pH-dependent charge. Some negative charge was present, however, at pH values as low as 3.0 and small quantities of positive charge were detected at pH values as high as 9. Values for PZC ranged from 2.2 to 3.9 and these were generally higher than the pHo determined by the ApH method. Western Samoa consists of four islands and a series of islets lying between 13 and 15~ latitude and 171-173~ longitude (Fig. 1). The islands are volcanic in origin and consist mainly of basalt derived from the magmatic material characteristic of the Pacific basin. The two major islands, Upolu and Savai'i are composed of a mass of successive basalt flows and the landscape is the result of subaerial erosion of two major lava domes. The islands commonly consist of rugged land surfaces with only a small proportion of the land being suitable for permanent arable cropping. On steep slopes the soils are often thin and unstable while on more gentle surfaces they are highly leached and lateritized. Thus, most of the soils are of limited fertility (Asghar et al., 1988; Haynes & Naidu, 1990). Based on soil solution composition studies, Naidu et al. (1990b) reported that most of the acidic 1 Present address: CSIRO Division of Land and Water, Private bag No 2, Glen Osmond, SA 5064, Australia. soils from Samoa were deficient in both macroand micro-nutrients. Although the South Pacific Islands support over 1.5 million people and agriculture is the economic backbone of most of these countries, only limited work on the fertility status and nutrient retention characteristics of island soils has been conducted (Haynes & Naidu, 1990). In this paper, surface charge characteristics are discussed in relation to the clay mineralogy for four agriculturally important soils from two major islands of Western Samoa. M A T E R I A L S A N D M E T H O D S