Influence of Altitude on the Petrological Features of a Soil Climosequence in the Humid Tropical Zone of Cameroon
Although lateritic soils are well documented in the humid tropical zone, work on adiabatic effect, which imposes local climatic variations with increasing altitude, has received very little attention in this zone. The aim of this work was to study and specify a soil climosequence in the humid tropical zone of Cameroon and to show the influence of altitude on the geochemical functioning of tropical soils. The work was done in the field and completed by a battery of laboratory analyses. The results enabled the definition of a soil climosequence for the humid tropical zone in Cameroon. This climosequence is marked by soils whose evolution is conditioned by the geochemistry of iron, silicon and aluminium. In the plateau ecosystems (500-800 m altitude), iron is expressed either as hardened materials (e.g. ferruginous nodules or duricrust) or light patches on the soils, while silicon and aluminium are combined as kaolinite. In the high plateaus (800-2000 m altitude), soil evolution is dominated by aluminium geochemistry, where aluminium forms gibbsite (either indurated or loose) often associated with small amounts of goethite and hematite. Finally, in the mountainous massifs (> 2000 m altitude), soil evolution is controlled by silica geochemistry, where silica is combined with amorphous aluminium to form allophane. In this soil, the presence of a dark and thick humiferous andosolic horizon in the upper part of the profile is very remarkable. Overall, this soil climosequence reflects geochemical functioning processes which grade with altitude in the humid tropical zone of Cameroon, similar to that commonly observed under the temperate climate. This general scheme could be extrapolated to other humid tropical regions with similar climatic and relief conditions as those observed in Cameroon. However, this general tendency often includes site-specific modifications caused by human activity and local variations of environmental conditions.
Clay minerals and humus complexes in five Kenyan soils derived from volcanic ash
Abstract Clay minerals and humus complexes and their effects on soil chemical properties were studied for five profiles around the Longonot and Susua volcanoes in Kenya. Morphologically, all profiles have some resemblance to Andepts, but there were clear-cut differences in clay contents and mineral composition as well as pH between profiles 1 through 4 and profile 5, which occur under semi-arid to subhumid and under humid climatic conditions, respectively. Profiles 1 through 4 contained embryonic halloysites with X-rays amorphous iron oxide and silica, whereas profile 5 contained a kaolin mineral, chloritized 2:1 layer silicates, gibbsite and hematite. These mineral associations probably represent early and advanced stages of weathering of volcanic ash under weak and strong leaching conditions, respectively. Embryonic halloysites consisted of more or less poorly ordered halloysite and “1.0- and 0.7-1.0-nm minerals”. The carbon contents of A horizons ranged from 100 to 112 g/kg in profiles 2, 3 and 5. Humus complexes increased in the order; Fehumus ⩽ Al-humus ⩽ Ca-humus in profiles 2 and 3, whereas Al- and Fe-humus were found exclusively in profile 5. The pH (NaF), phosphate retention and acid oxalate extractable Al qualify only profile 5 as an Andept or Andisol.
Embryonic halloysites in a paddy soil derived from volcanic ash.
Embryonic halloysites were found in the Ap horizon of a paddy soil derived from volcanic ash. These halloysites were nearly X-ray amorphous to poorly crystallized. They showed infrared absorption bands, more or less poorly defined but characteristic of halloysite, and appeared as "open" spherical to oval particles with diameters of 15 to 60 nm and as thinly spread "amorphous" materials. About half of halloysites were dissolved incongruently by 0.2 M oxalate-oxalic acid (pH 3.0) and congruently by hot M/3 sodium citrate. Embryonic halloysites were associated with goethite and ferrihydrite containing Al and Si respectively, and abundant diatoms, and considered to form from volcanic ash in a Si-rich environment.