A TOPOSEQUENCE OF SOILS DERIVED FROM VOLCANIC MATERIALS IN RWANDA - MORPHOLOGICAL, CHEMICAL, AND PHYSICAL-PROPERTIES

Soils in the northwestern region of Rwanda developed from volcanic mat erials that originated from eruptions of six volcanoes during the Quat ernary period. This study assessed primarily soil morphological, chemi cal, and physical property differences along a toposequence on the slo pes of these volcanoes. Three representative soil pedons were describe d in the field and sampled for laboratory analyses from sites located at 2700-m, 2400-m, and 2000-m elevations on the slopes of one of the v olcanic peaks, The sampling site locations varied in rainfall and temp erature regimes, All soils were characterized by thick and dark A hori zons and dark brown or grayish brown cambic horizons. The soil structu re was primarily weak fine granular in surface horizons and weak fine subangular blocky in subsurface horizons, All three soils appeared to be similar in morphological properties. Organic C was high in all thre e soils, particularly in the soil at the 2700-m elevation, and decreas ed with decreasing elevation. The Cation Exchange Capacity varied with elevation in the same manner as organic C but appeared to be affected also by amorphous constituents. Phosphate retention, exchangeable aci dity, and exchangeable Al were lower in the soil at the lowest elevati on, Soil bulk density increased whereas water retentions at -33 kPa an d -1500 kPa tension and Water Retention Difference decreased with decr easing elevation. Although soils at the upper two locations fitted the order of Andisols that at the lower location was classified as an Inc eptisol because of the lack of andic soil properties. The variations i n soil properties, and hence in soil taxonomic classes, with elevation were attributed primarily to changes in organic matter, and probably allophane, and the 10-Angstrom halloysite induced by rainfall and temp erature differences that existed between sites. In general, variations in soil properties with elevation found in soils derived from volcani c materials in other parts of the world.