AN ASSESSMENT OF COLLOIDAL CONSTITUENTS AND CLAY MINERALOGY OF SOILS DERIVED FROM VOLCANIC MATERIALS ALONG A TOPOSEQUENCE IN RWANDA

Colloidal constituents and clay mineralogical composition of soils der ived from volcanic materials were determined in relation to climate al ong the slopes of an extinct volcano in northwestern Rwanda, Three rep resentative soil pedons were sampled from sites located at 2700 m, 240 0 m, and 2000 m to represent differences in rainfall and temperature r egimes. Allophane was present in all three soils but tended to be high er in subsurface than in surface horizons, The lower allophane content in surface horizons was attributed to high organic C content, Organic C forms strong bonds with Al originating from the weathering of volca nic ash, thus inhibiting the coprecipitation of Al and Si needed for t he formation of amorphous constituents in these horizons. Ferrihydrite was also present in the soils, The 10-Angstrom halloysite was also pr esent in high amounts, particularly in subsurface horizons. All soils contained substantial amounts of mica, feldspar, and iron-based crysta lline clay minerals. In general, percentages of allophane and Al- and Fe-humus complexes were higher in the soil at the 2700-m elevation and decreased with decreasing elevation, Furthermore, the Al/Si ratio of allophane was approximately 2 in this soil and decreased to about 1 in the soil at: the 2000-m elevation, The 10-Angstrom halloysite, howeve r, increased with decreasing elevation, The lower allophane content an d Al/Si ratio in allophane and the higher halloysite content in the so il at the lowest elevation may have resulted from higher Si concentrat ion in the soil solution associated with the lower rainfall regime at this location. Variations in metal-humus complexes, amorphous constitu ents, and the 10-Angstrom halloysite appeared to be a function of the differences in rainfall regimes that existed between sites.