Characterization of volcanic ash soils in southwestern Tanzania: Morphology, physicochemical properties, and classification

Abstract

This study examined the characteristics of volcanic ash soils in southwestern Tanzania. Twelve pedons of volcanic origin were studied, and 66 soil samples were analyzed. Soil morphology revealed volcanic ash layers of varying thicknesses. Most pedons had a dark thick humus surface and buried A, AB, and BA horizons with melanic indices of 1.7 or less. Except in two pedons, the NaF pH was 9.4 or more, reflecting an exchange complex dominated by amorphous materials and/or Al–humus complexes. The phospate-retention capacity ranged from 65 to 100%, except in two pedons, and was positively correlated with NaF pH. Both Tanzanian and Japanese volcanic ash soils showed comparable ranges of base saturation (BS) values, but the distribution patterns of BS basic cations, for example, showed some differences. Some Tanzanian volcanic ash soils had higher BS values than their Japanese counterparts. While the Japanese soils were generally more calcic and magnesic, the Tanzanian soils were more potassic and sodic than their counterparts, most likely reflecting lithological differences among parent materials in the two study areas. According to the USDA Soil Taxonomy, nine pedons satisfied the requirements for andic properties and were classified as Andisols at the order level, whereas according to FAO World Reference Base (WRB) soil classification, eight pedons were classified as Andosols at the level of reference soil groups.