4.2 LANDFORM CLASSIFICATION
At reconnaissance scale seven different landforms are distinguished in the TCP research area.
These are characterized by relief intensity, slope steepness, slope length and drainage density.
The landforms are: dissected erosional plains, moderately dissected uplands with rolling relief,
strongly dissected uplands with hilly relief, isolated hills, complex of hills, mountains, and valley
The moderately dissected uplands with rolling relief have slopes of 100 to 200 meters long;
these slopes are moderately steep (10-20%). The number of interfluves per kilometre ranges
from three to four and the relief intensities are between 10 and 40 meters. This landform
covers large surfaces in the central and northern parts of the TCP area, has altitudes between
350 and 500 m asl, and has a general NE-SW direction. It has gradual boundaries with the
dissected erosional plains and the strongly dissected uplands with hilly relief. Sharp
transitions are found with the hills and the mountains. The `rolling uplands' cover about 27%
of the TCP research area.
The strongly dissected uplands with hilly relief have two to three interfluves per kilometre, slopes
of 150-300 meter long which are moderately steep (10-30%), and relief intensities of 30-80
meter. The overall relief is hilly. The `hilly uplands' with an extent of 39% of the survey area are
the most predominant landform in the TCP research area. They are confined to the 120-700 meter
altitude range. They have relatively sharp boundaries with the mountains, hills and complexes of
hills, and gradual boundaries with the rolling uplands.
4.2.3 Hills (h1 and h2)
Hills are natural elevations of the earth's surface. Hills occupy smaller areas, are lower and/or
have lower relief intensities than the mountains. Two types of hills are discerned in the present
reconnaissance survey: isolated hills (h1) and complexes of hills (h2). Both landforms are
characterized by erosion, particularly sheet erosion (creep) and rockfalls.
The isolated hills are characterized by steep (> 30%) long slopes (250-500 m) and relief
intensities of 120-300 meters. Rock outcrops are common and they are frequently encountered at
the convex upper slopes. The isolated hills are steep-sided, isolated, residual and circumde-
nudated like inselbergs. They are scattered throughout the TCP area and are a characteristic
aspect of the landscape, especially in the uplands of the central region. The isolated hills are
conspicuously present in the landscape, even though they cover only 9% of the total TCP area.
The landform h2 is a complex of moderately steep to steep (20-40%) hills which are strongly
dissected. The slopes are relatively short (200-350 m) and the relief intensities (80-200 m) are
relatively low, when compared to the isolated hills and mountains. The complex of hills are
characterized by a large variation in summit and valley bottom levels. Complexes of hills are
mainly situated in the transition zone between the uplands of the central region and the mountains
in the eastern part of the TCP area. They have a general NE-SW direction. Altitudes vary
between 500 and 700 m asl. They are absent in the northwest of the TCP research area. The
complexes of hills cover a surface of about 130 km², which is 9% of the TCP research area.
4.2.4 Mountains (m)
Mountains are complex dissected plateaus or massifs which are isolated and rising above the
surrounding landscape. This landform is characterized by an abrupt rise in altitude to a higher
level. Sheet erosion (creep) and rockfalls also occur in this landform. The outside slopes of the
mountains blocks are very long (> 400 m), very steep (30 to over 60%) with a relief intensity
higher than 250 meters. The inside slopes are moderately long (250-400 m), steep (30-60%) and
have a relief intensity of 120-250 meters. Rock outcrops are common and mainly occur on the
convex upper slopes. The mountains are restricted to the eastern part of the TCP area with
altitudes above 500 m asl where they cover about 100 km², i.e. some 7% of the TCP research
4.2.5 Valley Bottoms (v)
Valley bottoms are poorly to very poorly drained, nearly flat to flat depressions between
interfluves. They occupy extensive areas within the uplands and dissected erosional plains.
Their widths are mainly 50-150 meters and are thus in general too small to be mapped
separately at 1 : 100 000 scale. The slope percentages are between 0 and 2%. Stagnation of
water is also an important characteristic. Soils are developed in unconsolidated, unspecified,
stratified alluvium, which recently has been deposited.
The valley bottoms are found predominantly in the southeast of the TCP research and in the
western lowlands. Large valley bottoms, i.e. wider than 250 m, are separately mapped in the
present reconnaissance survey. These large valley bottoms cover approximately one percent
of the TCP research area. However, this does not reflect the importance of the landform as
the majority of valley bottoms are part of the dissected erosional plains, and uplands.
5.1 LITERATURE REVIEW
The `Atlas du Cameroun' (1 : 2 000 000) and the `Carte Pédologique du Cameroun Oriental' (1 : 1
000 000) provide information on the soils in the TCP research area. The scales of these soil maps
differ greatly from this reconnaissance inventory (1 : 100 000). These maps with their
explanatory notes (Segalen, 1957; Martin & Segalen, 1966), together with the `Atlas Régional,
Sud-ouest 1' by Franqueville (1973) give a good indication of the main soils of Southwest
Cameroon. The three main soil types of Southwest Cameroon, according to the above mentioned
studies, are `les sols ferrallitiques jaunes sur roches acides (gneiss)', `les sols ferrallitiques rouges
sur roches acides' and `les sols alluviaux' (see also section 2.6).
According to the `Atlas du Sud Cameroon, scale 1 : 500 000 (MINREST & ORSTOM, 1995), the
`sols ferralitiques fortement désaturées typiques jaunes ou rajeuni jaunes à ocres' and `les sols peu
évolués ou bruts' are found around the city of Ebolowa east of the TCP study area.
South of the line Ebemvok and Akom II, a detailed soil study of 65 hectares has been carried out
by Roubain who paid special attention to the genesis of soils. He reports that well drained
yellowish brown clay soils dominate on gneisses, whereas the reddish brown soils occur on
basaltic rocks. Laterite banks are exceptional in this region (ORSTOM - Roubain, pers. comm.,
Soil studies in the Mbalmayo area, situated northeast of the TCP research area, were carried out
on inventory scale by Yemefack & Moukam (1995) and on detailed scale by Ndjib (1987) and
Ngeh et al. (1995). The soils in this area have been developed on (mica)schists, gneisses and
granites of Precambrian age. Yemefack & Moukam (1995) classify the dominating well drained
yellowish brown clay soils as Xanthic Ferralsols (low cation exchange capacity, low base
saturation) with or without textural differentiation.
Ndjib (1987) differentiates the strongly weathered soils on the basis of texture, coarse fragment
content and colour. He relates these differences to parent material (soils on schists have more
coarse fragments than the ones on gneiss) and physiographic positions (development of argic
properties on top positions and mid-slopes).
Bilong (1992) reports that around Akongo, which is also east of the TCP research area, well
drained yellowish brown clay soils are formed on all kinds of parent materials. He relates the
yellowish colour to goethite, an iron hydroxide. Goethite, which can be formed under a soil
climate with sufficient moisture in the dry seasons, gives a yellowish brown colour to the soil
According to Touber (1993b) the soils in the TCP research are remarkably uniform
throughout the area. They are well to moderately well drained, very deep, yellowish brown
clay loams to light clays. Variations in depth, internal drainage, texture and gravel content
occur. Some patches of redder soils may occur and an altogether different soil type is found in
The present study confirms that deep, moderately well to well drained yellowish brown
tropical clay soils predominate in the TCP research area. These soils also described as `les
sols ferrallitiques jaunes sur les roches acides (gneiss)' are characterized by Martin &
Segalen (1966) as follows:
topsoil (3-10 cm):
15 to 20% clay, ± 3.5% organic matter (C/N = 15), CEC < 8
meq/100 g, base saturation between 12 and 20% and pH < 4.5;
subsoil (> 10 cm): 35-50% clay, low in organic matter (< 3.5%), CEC between 5 and
7 meq/100 g and base saturation < 10%.
Vertical clay movement is described. The mineralogical composition of the clay is: 50-
60% kaolinite, 35-45% goethite and < 10% gibbsite and the SiO
ratio is between
1.1 and 1.6.
5.2 SOIL TYPES AND CLASSIFICATION
The soils in the Bipindi - Lolodorf - Akom II area have been subdivided into four main soil
types. This subdivision is based on soil drainage and texture in both topsoil and subsoil
Well drained, deep to very deep
, yellowish brown to strong brown clay soils, classified
according to FAO-Unesco (1990) as Xanthic Ferralsols, are dominant in the area. These well
drained soils are further subdivided according to the clay content in topsoil and subsoil into:
– Nyangong soils which are very clayey soils with no or only a gradual increase in clay
content with depth. The clay contents of the subsoils range between 50 and 80%.
– Ebom soils which are clay soils with a gradual to strong differentiation in clay content
between topsoils and subsoils. The clay contents of the subsoils range between 35 and
The moderately well drained soils are:
– Ebimimbang soils which are moderately deep to very deep clay soils with sandy topsoils.
The clay contents of the subsoils range between 20 and 45%.
The poorly to very poorly drained soils are:
– Valley Bottom soils which are moderately deep to very deep soils, locally stratified and
with variable textures in topsoils and subsoils.
The first three soil types are named after villages in the TCP area in which regions these
corresponding soil types dominate. The Valley Bottom soils are, as the name implies,
restricted to the valleys and occur all over the area.
Soil depth, soil stoniness and the occurrence of plinthite in the subsoils are other criteria for
further subdivision of the soils. During the present reconnaissance survey it has, however,
not been possible to find systematic relationships between these factors and positions in the
landscape. Therefore, these criteria have not been used to further subdivide the four main
soil types mentioned above.
Soil depths classes are as follows: shallow 0-50 cm; moderately deep 50-100 cm; deep 100-150 cm and very deep >150 cm.
Table 5.1 Soil types of the TCP research area and their diagnostic criteria
(% clay topsoil)
(% clay subsoil)
35-70%, very clayey
Moderately well drained
Poorly to very poorly drained
topsoil = 0-10 cm; subsoil = 20-60 cm
5.2.1 Nyangong soils
The Nyangong soils are deep to very deep, well drained, yellowish brown to strong brown, clays
with dark yellowish brown to dark brown clay topsoils. They are developed on fine grained,
pyroxene rich gneisses and granites, or other igneous rocks. Soil depth of the deep soils (100 -
150cm) is limited by the occurrence of ferruginated gravel, stones, boulders or weathering
bedrock. Nyangong soils are characterised by a high clay content which can reach 80% in the
subsoils. The topsoils are none to slightly less clayey than the subsoils whereby the increase in
clay content with depth is gradual.
The topsoils are 10 to 20 cm thick with the first 5 to 10 cm having the darkest colours. They have
dark yellowish brown to dark brown colours and 40 to 70% clay. The subsoils have yellowish
brown to strong brown colours and clay contents of 50 to 80%. In places few reddish mottles
and/or laterite gravel is found in the deeper subsoils.
Locally on summits, upper slopes or very steep mountain slopes, moderately deep to very deep,
gravelly and stony soils occur as inclusions within the Nyangong soils. These have clay contents
in the subsoils of 40 to 50% and are less deeply and less intensively weathered than the typical
Nyangong soils. At the present scale of the survey, these soils could not be mapped separately.
The Nyangong soils are characteristic for altitudes above 500 m asl and are therefore found in the
mountains and hills in the eastern part of the TCP area. The complexes of hills and isolated hills
at altitudes above 500 m are also mainly covered with the Nyangong soils. The soils of the
eastern uplands form complexes of Nyangong and Ebom soils. Full profile descriptions with
analytical data of the Nyangong soils are given in Annex IV.
5.2.2 Ebom soils
The Ebom soils are deep to very deep, well drained brownish yellow to strong brown, clays with
yellowish brown to dark brown sandy loam to sandy clay topsoils. They are developed on
gneisses and migmatites. Soil depth of the deep soils (100-150 cm) is limited by plinthite, laterite
and/or quartz gravel or bedrock. A relationship between the occurrence of plinthite or limited soil
depth within 150 cm and topographical position, could not be detected and therefore not mapped
at the scale on the survey. The Ebom soils are characterized by their clayey subsoils with less
clayey topsoils whereby an increase in clay content with depth of about 10% generally occurs
within 20 to 25 cm of the surface.
The topsoils are 10 to 20 cm thick, have yellowish brown to dark brown colours (with the first 5
cm having the darkest colours), and have 20 to 40% clay (sandy loam to sandy clay). The
subsoils generally have 40 to 50% clay with an observed range of 35 to 60% and have
brownish yellow to strong brown colours, with yellowish brown dominating. In a majority of
the soils reddish mottles occur, starting between 20 and 50 cm depth. Iron concretions occur
in the deeper subsoils, starting between 50 to 100 cm depth.
Locally on summits, steep upper slopes or very steep hill slopes, moderately deep to very
deep, gravelly and stony soils occur as inclusions within the Ebom soils. These have clay
contents in the subsoils of 40 to 50% and are less deeply and less intensively weathered than
the typical Ebom soils. At the present scale of the survey, these soils could not be mapped
The Ebom soils are typically found in the uplands and isolated hills in the central and
northern regions of the TCP area at altitudes between 350 and 500 m asl. Complexes of the
Ebom with Nyangong soils are found in the uplands above 500 m. Full profile descriptions
with analytical data of the Ebom soils are given in Annex IV.
5.2.3 Ebimimbang soils
The Ebimimbang soils are moderately deep to very deep, moderately well to well drained,
yellowish brown sandy clay loams to sandy clays with sand to sandy loam topsoils and with
gravelly subsoils. They are developed on coarse grained gneisses and migmatites. They are
characterized by sandy topsoils and a clay increase with depth of 15 to 20% which generally
occurs within 20 cm of the surface. The depth of the moderately deep soils (50-100 cm) is
limited by weathering bedrock, plinthite or high concentrations of gravel (both iron
concretions or quartz gravel).
The topsoils are 5 to 10 cm thick, with colours in the range of dark greyish brown to black
and generally with about 20% clay but clay percentages as low as 9% occur as well. The
subsurface horizons are 5 to 10 cm thick, have yellowish brown to dark brown colours and
have clay percentages of 16 to 25 %, but in places less. As silt contents are low, sand
contents of topsoils and subsurface horizons are high.
Subsoils have yellowish brown colours with a range from light yellowish brown to strong
brown and commonly have about 40% clay. Clay contents ranges between 30 and 64%.
Reddish mottles start to appear between about 15 and 40 cm depth. In places plinthite occurs
with in 150 cm, starting between 75 and 140 cm depth. Iron concretions occur in the deeper
subsoils, starting at irregular depths, appearing between 15 and 140 cm from the surface
downwards. Next to these concretions, quartz gravel and pieces of rotten rock occur in the
deeper subsoils giving them a texture of slightly gravelly to gravelly sandy clay loams to
In the survey area, the Ebimimbang soils typically occur at altitudes below 350 m and are
thus found on the dissected erosional plains, uplands and isolated hills in this altitude zone.
Valley bottom soils occupy up to 15% of these areas but could usually not be mapped
separately as most valley bottoms are narrow (< 250 m). Annex IV gives full profiles
descriptions with analytical data of the Ebimimbang soils.
5.2.4 Valley Bottom soils
The valley bottoms are dominated by moderately deep to very deep, poorly to very poorly
drained soils developed in unconsolidated recent alluvium. The soil profiles are stratified
showing alternations of sand, loam and clay layers. Sand, however, is dominant. Flooding, high
ground water levels and locally greyish colours are characteristics of this soil type. Depth of
groundwater level and soil texture could be used for further subdivision of these soils, although at
reconnaissance scale this is not feasible.
The topsoils are thin (< 5cm) very dark brown loamy sand, to sandy clay loams which locally are
peaty. Where the groundwater level is relatively deep (> 50 cm), subsoils are light yellowish
brown to dark brown, loamy sands to sandy clay loams with oxidation-reduction mottles.
Subsoils have light grey to blue colours. In all valley bottoms in the TCP research area, the soils
described above are found regardless of altitude. The majority of valley bottom soils, however,
could not be mapped individually at scale 1 : 100 000. Valley Bottom soils occupy up to 15% of
the area of the dissected erosional plains and rolling uplands. They occupy up to 10% of the hilly
uplands. The complexes of hills and mountains are only sparsely covered with the Valley Bottom
soils (< 5%). Soil profile descriptions with analytical data of the Valley Bottom soils are given in
5.2.5 Soil classification
All profile descriptions of which complete sets of field and laboratory data are available are
presented in Annex IV. These soils are classified according to three international systems of soil
classification. These classification systems are: the FAO-Unesco system (FAO, 1988; ISRIC
1994), the Soil Taxonomy of the United States Department of Agriculture (Soil Survey Staff,
1992), and the French system of the `Commission de Pédologie et de Cartographie des sols'
Table 5.2 presents the classification of the main soil types occurring in the TCP area according to
three international soil classification systems. The soils which are most typical for a certain main
soil type are heading the list of the soil classification in Table 5.2.
The major part of the Nyangong, Ebom, and Ebimimbang soils classify as Ferralsols in the FAO-
Unesco system (FAO, 1988). The Xanthic Ferralsols are most typical for the Nyangong soils. The
Acri-xanthic Ferralsols are the most characteristic ones for the Ebom soils. The typical
Ebimimbang soils classify as Acri-plinthic Ferralsols and Haplic Acrisols. The poorly drained
Valley bottom soils classify as Dystric Fluvisols, the better drained ones classify as Gleyic