An assessment of colloidal constituents and clay mineralogy of soils derived from volcanic materials along a toposequence in rwanda
1997
Nizeyimana, Egide | Bicki, T. J. | Agbu, P. A.
Colloidal constituents and clay mineralogical composition of soils derived from volcanic materials were determined in relation to climate along the slopes of an extinct volcano in northwestern Rwanda. Three representative soil pedons were sampled from sites located at 2700 m, 2400 m, and 2000 m to represent differences in rainfall and temperature regimes. Allophane was present in all three soils but tended to be higher 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 volcanic ash, thus inhibiting the coprecipitation of Al and Si needed for the formation of amorphous constituents in these horizons. Ferrihydrite was also present in the soils. The 10-Å ; halloysite was also present in high amounts, particularly in subsurface horizons. All soils contained substantial amounts of mica, feldspar, and iron-based crystalline 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-Å ; halloysite, however, increased with decreasing elevation. The lower allophane content and Al/Si ratio in allophane and the higher halloysite content in the soil at the lowest elevation may have resulted from higher Si concentration in the soil solution associated with the lower rainfall regime at this location. Variations in metal-humus complexes, amorphous constituents, and the 10-Å ; halloysite appeared to be a function of the differences in rainfall regimes that existed between sites.
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