[en] Landscape Ecological Survey of the Bipindi-Akom II ... - ITTO

5.5.2 Soil genesis
The following soil forming processes have been contributing to soil formation in the TCP
research area: formation A-horizon, hydrolysis, ferralitization, kaolinitization, plinthite and
laterite formation, eluvation and illuvation of clay and oxidation/reduction processes.
Accumulation of litter which is decomposed by soil flora and fauna in the mineral topsoils
results in the formation of an A-horizon. Mineralization of organic matter releases nutrients
which can be taken up by the surrounding vegetation. The process of decomposition,
mineralization and uptake by the vegetation is relatively quick, therefore A-horizons are thin.
Low pH levels, however, will retard the decomposition and organic matter may accumulate
(Mohr et al., 1972). The chemical fertility of the tropical soil is strongly related to the presence
of organic matter in the topsoil because of its storage and release capacity of nutrients. In the
TCP area there are significant differences in organic carbon contents between the different
topsoils (section 5.4.2.).
The main soil forming process in the TCP area has been hydrolysis (cations in the primary
silicate structures of minerals are exchanged against H+-ions). The hydrogen ion weakens the
mineral structure, facilitating the dissolution of Si and Al from the clay lattices. Ferralitization
or desilication is hydrolysis in an advanced stage. A combination of slow release and subsequent
leaching of cations and silica keeps the concentration in the soil solution low. If the soil
temperature is high and percolation is intense, ultimately all weatherable primary minerals will
be removed from the soil mass. Less soluble compounds such as iron and aluminum oxides and
hydroxides, as well as coarse quartz grains, remain behind (Driessen and Dudal, 1989; Mohr et
al., 1972). A low pH, low concentrations of dissolved weathering products in the soil solution
(low EC - values) and geomorphic stability over prolonged periods of time are conditions which
accelerate the process of ferralitization (Driessen and Dudal, 1989). All these conditions are
present in the TCP area. Due to the presence of gneiss (acid rock) with few easily weatherable
minerals and much quartz, ferralitization proceeds much slower. Although much silica
disappears through leaching (desilication), silica contents remain higher than in soils formed on
basic material. This silica combines with aluminum to the 1: 1 clay mineral kaolinite, which is
called the kaolinitization process. Gibbsite is normally absent. It is however, formed under
freely drained conditions and from richer rocks. The dominant minerals in the soils in the TCP
research area are kaolinite, goethite (FEO(OH» and gibbsite (AI(OH)3)' The colour of the soils,
orange to yellowish brown, is determined mainly by the presence of goethite (section 5.4.5.).
Hematite (FeP3)' (Fe20 3), which gives the soil a bright-red colour, is not observed in the TCP research
area.
Plinthite is an iron-rich, humus-poor mixture of clay and quartz. It is formed by the (relative
and/or absolute) accumulation of sesquioxides (i.e. removal of silica and bases by ferralitization
and/or enrichment from outside) and the segregation of iron mottles (alternating reduction and
oxidation). In the TCP research area plinthite is regularly found on the lower slopes between 40
and 500 meters altitude (uplands and dissected erosional plains). Within the Ebimimbang soils,
a subtype can be distinguished with plinthite in the upper 125 cm. Laterite formation is the
hardening of plinthite to laterite. The main processes are the crystallization of amorphous iron
compounds to aggregates of iron oxide minerals and the dehydration of of goethite to hematite hematite and
of gibbsite to boehmite (Aleva, 1994; Driessen and Dudal, 1989). Laterite gravels are present
in limited amounts in the TCP research area. The laterite gravels are remnants of old eroded
surfaces. Ferruginated rock fragments are more common.
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