E. Nizeyimana et al., AN ASSESSMENT OF COLLOIDAL CONSTITUENTS AND CLAY MINERALOGY OF SOILS DERIVED FROM VOLCANIC MATERIALS ALONG A TOPOSEQUENCE IN RWANDA, Soil science, 162(5), 1997, pp. 361-371
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.