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.
Morphological, geochemical and mineralogical studies of two soil profiles developed on the itabirites of Ntem Complex, southern Cameroon.
Abstract Itabirite-related ferrallitic soils in the humid tropical zone of southern Cameroon have proven economic and scientific interests. However, they have received very little attention. The morphological, mineralogical and geochemical features of two soil profiles (TCN in Nkout and NT in Anyouzok) with significant morphological differences and developed on itabirite in the Ntem Complex of southern Cameroon were studied using macroscopic and microscopic observations, conventional X-ray diffraction (XRD), X-ray fluorescence (XRF) spectrometry and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. The two profiles have similar sets of horizons including a lower weathering unit, a ferruginous and a loose upper units. However, TCN has a saprolite horizon above the saprock and numerous platy ferruginous concretions. The two profiles have similar mineralogical composition with the predominance of hematite, goethite, quartz and magnetite and a low concentration of kaolinite and gibbsite. The geochemical composition is characterized by the dominance of Fe2O3t (58.90–90.76 wt %) and SiO2 (5.63–38.45 wt %) both showing opposite trends across the profiles. These observations suggest that oxidation and dissolution were the dominant geochemical processes during the formation of the profiles. The positive Ce anomalies (Ce/Ce*: 1.12 to 2.27) and negative Eu anomalies (Eu/Eu*: 0.59 to 0.99) are indicators of oxidations coupled with mobile elements leaching in the profiles. The morphological differences between the two profiles are probably due to differences in weathering conditions which led to the development of a thick oxidized iron ore deposit in Nkout compared to Anyouzok.
The Tsetse Problem on the Eastern Cattle Route in the Gold Coast
1. Primary fly-belts round Makongo consisted of three different vegetational communities: a Cynometra-Vitex association along the main length of the Makongo river, on a clay soil; a Ficus congensis association along the river close to the town, on a sandy soil; and Isoberlinia doka woodland on moist sandy soil north-west and south of Makongo. 2. Secondary fly-belts of less dense vegetation occur in the orchard bush savannah round Makongo, and these are not colonised until the commencement of the rainy season. 3. Glossina tachinoides and G. palpalis both occurred in the riverside communities, and G. palpalis alone in the Isoberlinia woodland. 4. Along the river G. palpalis outnumbered G. tachinoides by 3:1 in the dry season. At the commencement of the rains G. tachinoides increased more quickly than G. palpalis , bringing their numbers very nearly equal, but at the end of the rains there was a decrease in G. tachinoides which restored the original proportion. 5. G. morsitans had been found at Makongo formerly but was not met with at the time of this investigation, its absence being due to the disappearance of the larger antelope from the neighbourhood. 6. Smaller game animals, small mammals and reptiles afforded an adequate food supply for tsetse in the fly-belts, and also passing herds of cattle, as well as natives from the village and travellers. 7. The distribution of G. palpalis within the fly-belts was markedly influenced by the proximity of a cattle road along which herds of cattle passed almost daily, and by village water-holes. 8. During the dry season the primary foci were always found to be the places of greatest relative humidity, and the fly does not tend to move out of these until the general atmospheric humidity has reached a certain fairly high degree. 9. In clearing the fly-belts the removal of all primary foci within a quarter of a mile of the town and of thick bush within 100 yards of water-holes was undertaken. The work lasted from the beginning of February to the end of March 1931. 10. The effect of clearing the Isoberlinia doka woodland was to banish G. palpalis completely, and it never returned to these clearings. 11. On clearing the riverside Ficus congensis fly-belt, G. tachinoides was practically eliminated for the first three months, but reappeared in the clearings in June when there was a general increase and up-stream migration of this fly. G. palpalis remained in this clearing in fluctuating numbers up to June, when observations were suspended. 12. 565 marked tsetse were liberated in the fly-belts immediately previous to clearing. Of G. palpalis 12·7% were recaptured in the clearings and 11% outside the clearings. Of G. tachinoides 5·5% were recaptured in the clearings and 6·8% outside.