Ferralsols are deeply weathered soils, low in weatherable minerals with a CEC in the (ferralic) B-
horizon of less than 16 me/100 g clay. The high degree of weathering is expressed by silt/clay
ratios of 0.2 or less. In classifying the soils of the area silt/clay ratios of 0.3 were permitted,
provided that the CEC-clay was clearly below 16 me/100g. The clay content of the ferralic-B
horizon is over 8%. The Xanthic Ferralsols have a yellow to pale yellow B-horizon (hues of
7.5YR or yellower with a moist value of 4 or more and a moist chroma of 5 or more) caused by
the presence of goethite. The Acri-xanthic Ferralsols are the yellow coloured Ferralsols with an
increase in clay content with depth.
Ferralsols which have plinthite within 125 cm from the surface are the Plinthic Ferralsols. The
Plinthic Ferralsols which show an increase in clay content with depth are classified as Acri-
plinthic Ferralsols. Acrisols are soils with a CEC clay of less than 24 me/100 g and/or with silt-
clay ratios of over 0.2 and with base saturations of less than 50%. Moreover, they show a clay
increase with depth which is more than 8% when the clay content of the topsoil is over 40%
and more than 20% (relative) if the clay content of the topsoil is between 15 and 40%. Ferric
Acrisols have many coarse red mottles or discrete lateritic nodules. Plinthic Acrisols have
plinthite within 125 cm from the surface. Ferrali-ferric Acrisols are Acrisols with a CEC
clay of less than 16 me/100 g. Haplic Acrisols are the normal ones, without additional
Table 5.2 Soils of the TCP research area and their classification according to three international classification systems
Cambisols are soils that have yellowish to reddish subsoils and have more than 8% clay.
Generally these are relatively young soils with little profile differentiation and not
intensively leached. Ferralic Cambisols are soils with CEC-clay of less than 24 me/100 g.
Gleyic Cambisols show signs of water stagnation within 100 cm from the surface.
Fluvisols are young soils usually developed on alluvial sediments. They are stratified or
have an irregular decrease of organic carbon contents with depth and have no diagnostic B
horizon. Dystric Fluvisols have base saturations of less than 50%.
Soil Taxonomy - USDA
The major part of the Nyangong and some of the Ebom soils are classified as Oxisols in the
Soil Taxonomy (Soil Survey Staff, 1992). The typical Nyangong soils classify as Typic
Hapludox. The characteristic Ebom soils classify as Typic Kandiudults. Both Plintudults and
Typic Paleudults represent the typical Ebimimbang soils.
Oxisols are deeply weathered soils with a CEC clay in the (oxic) B-horizon of less than 16
me/100 g and no or only limited clay increase with depth. Clay content of the subsoil is over
8%. The Oxisols in the TCP area are classified at great group level as Hapludox or
Kandiudox. These soils have an udic moisture regime. The udic soil moisture regime is
common to soils of the humid climates. Here, rainfall plus stored soil moisture exceeds the
amount of evapotranspiration in such a way that the soils at about 20 cm depth are not dry
for more than 90 cumulative days per year. The Hapludox are the most typical, normal
Oxisols. The Kandiudox have 40% or more clay in the first 18 cm and a clay increase of over 8%
between topsoil and subsoil (kandic horizon). The Ebimimbang soils and most of the Ebom soils
are classified as Ultisols. Ultisols are soils with a clay increase with depth and a base saturation
of <35% in the subsoils. All the Ultisols in the area have an udic moisture regime as explained
above for the Oxisols. The Hapludults are the normal Udults. Ultisols with a kandic horizon have
less that 40% clay in the top 18 cm and have a CEC clay in the subsoil of less than 16 meq/100 g:
Kandiudults. The Paleudults are deeper than 150 cm and do not have a clay decrease with depth
of more than 20%. Plintudults have 50% or more plinthite within 150 cm.
The young Valley Bottom soils with high groundwater levels are classified as Aquents. The
sandy ones are the Psammaquents and the loamy and clayey ones with irregular decrease in clay
contents with depth are the Fluvaquents. Lithic Endoquents are shallower than 50 cm. The valley
bottom soils which are somewhat better drained are classified as Inceptisols. The Inceptisols of
the tropics are the Tropepts and Dystropepts if they have a base saturation of less than 50% in the
subsoil. The Aquic Dystropepts have reduction and oxidation mottles within 100 cm.
The three main soil types in the TCP research area can be placed in the `Classe des Sols
Ferrallitiques'. These ‘sols ferrallitiques' are developed in the humid tropics and are characterized
by high contents of the minerals kaolinite, gibbsite, goethite and hematite. The `Classe des Sols
Hydromorphes' is also found in the TCP research area. These soils are characterized by a zone of
alternating oxidation-reduction conditions and/or a permanent reduction zone (gley).
The Nyangong soils and the majority of the Ebom soils meet the requirements for the `sous-
classe des sols ferrallitiques fortement désaturés en (B)', which are soils with pH lower than 5 and
base saturation lower than 20%. The Ebimimbang soils and some of the Ebom soils meet the
requirements for the `sous-classe des sols ferrallitiques moyennement désaturés en (B)', which
are soils with pH around 5 and base saturation between 20 and 40%. The majority of the
Nyangong, Ebom and Ebimimbang profiles can be placed in the group `groupe typique' and
`sous-groupe jaune' of the system. In places they are in the 'groupe remanié or rajeuni'. They are
all developed on `roches métamorphique' (Segalen, 1957). The Valley Bottom soils belong to the
`sous-classe des sols hydromorphes minéraux ou peu humifères' and the `groupes des sols
hydromorphes peu humifères à gley'. The depth of the gley horizon determines the subgroup.
5.3 SOIL PHYSICAL CHARACTERISTICS
Several physical characteristics were measured to characterize the soils. In Annex III the different
methods used for texture, bulk density and water retention (physical) analysis are presented.
Annex IV gives the analytical data of the sampled soil profiles. Table 5.3 presents the ranges of
clay, silt sand contents and the average values of bulk densities and water retentions of the three
main soil types.
In Table 5.3 textures of the topsoils and subsoils of the most common soils are presented. Clear
clusters are present: (i) Nyangong soils with topsoil clay contents of about 45%, (ii) Ebom soils
with intermediate clay and sand contents of about 35% and 45% respectively in the topsoils and
(iii) Ebimimbang soils with topsoil clay contents of only 10%. Clay contents increase with depth
for all three soil types. The deeper subsoils (BC, CB or C horizons) show slightly lower clay
contents compared to the horizons directly above
Table 5.3 Soil physical characteristics of the Nyangong, Ebom and Ebimimbang soils.
Soil depth classes: 1 = 0-20cm; 2 = 20-60 cm; 3 = 60-90cm. Bulk density values are the average of at least 4 observations.
AWC= Available Water Content (one or two observations per soil depth class).
The Valley Bottom soils are dominated by a sandy texture, but in fact all texture classes may
occur in all possible sequences. Stratification is characteristic for these soils. In general, the
Valley Bottom soils have 5-30% clay and 40-90% sand in the topsoil. The subsurface
horizons generally are more sandy.
5.3.2 Bulk density
The ranges in bulk densities of the soils in the TCP research area are 0.8 to 1.4 g/cm
topsoils (0-10 cm) and 1.0 to 1.7 g/cm
in the subsoils (20-90 cm) (Table 5.3). The bulk
densities of all three main soil types increase strongly with depth, as effects of cultivation
and organic matter content decrease. The increase of the bulk densities with depth take place
within 25 cm from the surface. The bulk densities of the Nyangong/Ebom soils and the
Ebimimbang soils are within the normal range of 1.0 to 1.6 g/cm
and 1.2 to 1.8 g/cm
given by Landon (1991) for clayey and sandy soils, respectively.
Average values of bulk densities for the Nyangong, Ebom and Ebimimbang soils at three
depths (0-20 cm, 20-60 cm and 60-90cm) are given in Table 5.3. The Nyangong topsoils
have bulk densities of around 1.0 g/cm
, and below 20 cm the bulk densities increase to
values of 1.2 to 1.3 g/cm
. The topsoils and subsurface horizons of the Ebom soils have bulk
densities around 1.2 and 1.4 g/cm
, respectively. Bulk densities of the Ebimimbang soils are
the highest of the TCP research area, i.e. 1.3 g/cm
for the topsoils and 1.5 to 1.7 g/cm
the subsoils. There is thus an increasing bulk density with a decreasing clay content.
The Ebimimbang soils have less stable aggregates than the Ebom and Nyangong soils
(Waterloo et al., 1997), resulting in vertical clay movement and lower permeability of the
Ebimimbang soils. The higher bulk densities of the Ebimimbang soils are explained by the
lower aggregate stability and higher sand contents of these soils.
Topsoils under agriculture have decreased organic matter contents and less roots resulting in
higher bulk densities compared to identical soils under forest (pers. comm. Yemefack,
of the Nyangong topsoils and subsoils is in the range of 3.5 to 4.5 (extremely acid).
Khanna & Ulrich (1984) define the pH range 3.8-4.2 as the aluminum buffer range, which
means that aluminum is exchanged against hydrogen with decreasing pH.
The pH of the Ebom soils is 3.5-5 in the topsoils and 4-5 in the subsurface horizons (very
strongly to extremely acid). The pH values of the Ebom soils are only slightly higher than
those of the Nyangong soils, and they are intermediate between the Nyangong and
The pH values of the Ebimimbang topsoils are 1-1.5 units higher than those of the
Nyangong and Ebom soils. The topsoil pH is 5-6 (medium to strongly acidic), and the
subsurface soil pH is 4.5-5.5 (strongly to very strongly acid). This marked gradient in pH
with depth is characteristic for soils in the cation exchange capacity buffer (4.2-5). Cations
are exchanged against hydrogen to buffer changes in pH. Soils with pH values within this
range are characterized by sharp differences in chemical characteristics (Khanna & Ulrich,
1984), which is confirmed by the base saturation data of Ebimimbang topsoils and
subsurface horizons. The Valley Bottom soil has pH values that resemble those of the
Ebimimbang soil. The topsoil pH is between 5 and 6 and the subsurface pH is between 4.5
All subsoils have pH values between 3.5 and 5.5. In these pH ranges manganese (Mn),
aluminum (Al), iron (Fe) and trace elements as copper (Cu), zinc (Zn) and borium (B) are
available for plant growth (Euroconsult, 1989; Landon, 1991). In general the aluminum
concentrations of the soils in the TCP research area are high and exchangeable potassium
(K), magnesium (Mg) and calcium (Ca) concentrations are very low in the soil solution (see
Table 5.4b.). Crops grown on these soils may experience nutrient deficiencies. High
aluminum concentrations in the soil solution are toxic to many crops. Low pH values as
found in the Nyangong soils, facilitate the formation of iron and phosphate aluminium
compounds. This phosphate is not available to plants. The somewhat higher topsoil pH
values might be explained by recent agricultural activities which include the burning of
vegetation by which alkaline ashes are formed.
5.4.2 Organic carbon and total nitrogen
The organic carbon and related total nitrogen
contents of the topsoils differ significantly between the four soil types. The observed organic
carbon range within the survey area is 2%-9% and the total nitrogen contents range from
0.15%-0.5%. The topsoils have significantly higher organic carbon and total nitrogen
contents than the subsoils. Figure 5.2. shows the relation between organic carbon and total
nitrogen concentrations for all samples. The C/N value calculated with regression analysis
on all data is 13 (R
The Nyangong topsoils have organic carbon contents between 4% and 9% and total nitrogen
contents between 0.25% and 0.4%. The C/N values of the topsoils are between 12 and 18.
The subsurface horizons have organic carbon contents between 1% and 3% and total
nitrogen contents between 0.1% and 0.15% (Table 5.4a.). In the subsoils the organic carbon
content drops rapidly to below 0.5%. The C/N values of the subsurface horizons are in the
range of 10 to 14.
With pH, we mean pH(H
O), unless otherwise indicated.
The Valley Bottom soils have organic carbon contents of 2.5-6.5% in the topsoils. The total
nitrogen contents of the topsoil are comparable with the other soils. The subsurface horizons
show organic carbon and total nitrogen contents similar to those of the Ebom and
5.4.3 Available and total phosphorous
The available phosphorous content in the topsoils of the Nyangong soils ranges between 4
and 11 ppm (low), in the Ebom and Ebimimbang soils between 10 and 26 ppm (moderate)
and in the Valley Bottom soils between 30 and 60 ppm (moderate to high). With depth,
available phosphorous contents drop rapidly to values below 5 ppm and are invariably
approaching 0 in all subsoils.
Total phosphorous contents of the topsoils are high in the Nyangong soils and range between
400 and 600 ppm. With depth, these values go down to 50-250 ppm. The Ebom and
Ebimimbang soils have 150-400 ppm total phosphorous in the topsoils and 50-250 ppm in
their subsoils. Total phosphorous contents of the Valley Bottom soils are not available.
Higher available phosphorous contents coincide with higher pH values as at higher pH
values phosphorous is less fixed in aluminium and iron complexes than at lower pH ranges.
The higher available phosphorous contents of the Valley Bottoms are explained by colluvial
Table 5.4b Ranges in some chemical characteristics of the four soil types in the TCP research area
Soil depth: 1 = topsoil, generally 0-10 cm; 2 = subsurface horizon of about 20-60 cm depth. TEB = Total Exchangeable Bases.
ECEC = Effective Cation Exchange Capacity. CEC = Cation Exchange Capacity. BS = Base Saturation. All values, except
BS, in meq per 100 g.
5.4.4 Cation exchange capacity and exchangeable bases
The cation exchange capacities (CEC) of the topsoils in the study area are low to moderate with
CEC values of 4-20 me/100 gr soil. With depth the CECs drop rapidly to low levels of 1 to 9
me/100 gr in the subsurface horizons to even lower levels in the subsoils. The relatively higher
CEC values occur in the topsoils of the Nyangong and Ebom soils and are between 10 and 25
me/100 g soil. These higher values are directly related to the higher organic matter levels of these
The exchange complex is dominated by aluminium (Al) and Al saturation percentages of 30 to
80% are common. The highest saturation values occur in the soils with the lowest pH values,
Next to kaolinite, the aluminum hydroxide gibbsite, is moderately abundant in the Nyangong
and Ebom soils. Gibbsite forms in richer parent material (Driessen & Dudal, 1989; Mohr et
al., 1972; van Kekem et al., 1997). The drainage conditions of the Ebimimbang soils, are
less good when compared to the two other soil types. This might be the explanation for the
difference in gibbsite contents of the different soil types.
Vermiculite, chlorite and smectite are clay minerals which are absent or only present in
relatively small amounts in the TCP research area. Micas (illite) are moderately abundant in
some soil samples, but no relation is found with the different soil types. Feldspars and quartz
are absent or only present in small amounts the clay fractions.
The iron hydroxide goethite is moderately abundant in all soils, whereas the iron oxide
hematite is absent in the three main soil types of the TCP area. Goethite is formed under a
soil climate with sufficient moisture in the dry season even if the soils are well drained. It
gives the soil a yellowish brown colour (Bilong, 1992). Driessen & Dudal (1989) state that
goethite is formed when the iron concentration is low, the organic matter content is high, the
temperature is low and/or soil pH is lower than 4.0.
5.4.6 Nutrient contents
Combining bulk density data and chemical data makes it possible to calculate the total
amounts of nutrients potentially available for plant growth. The data give an indication of
the fertility of the different soils. The calculated nutrient contents for the three main soil
types are averages of four to six profiles. Table 5.5. shows the nutrient contents in kg/ha for
a soil column of 1 meter deep. The majority of these nutrients, however, are concentrated in
the upper 20 cm of the soil profile.
The Nyangong soil has a relatively high nitrogen content (12.5 ton/ha), whereas available
phosphorous and potassium are present in relatively small amounts of 8 and 360 kg
respectively (Table 5.5.). Magnesium and calcium amounts are 195 and 1065 kg/ha,
respectively. The Ebom soil has less total nitrogen (8900 kg/ha), but a moderate amount of
phosphorous (28 kg/ha). The Ebimimbang soil also has a moderate amount of available
phosphorous (24 kg/ha) but the total nitrogen amount is low (7000 kg/ha). The potentially
available K, Mg and Ca amounts are in the same order of magnitude for all three soils.
Table 5.5 Average nutrient contents of the Nyangong, Ebom and Ebimimbang soils in kg/ha (soil column of 1m)