Bipindi akom II lolodorf region, southwest



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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  

 
 
44
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 
characteristics. 
 
Table 5.2 Soils of the TCP research area and their classification according to three international classification systems 
 
 
Soil type 
 
FAO-UNESCO 
 
Soil Taxonomy 
 
CPCS 
 
Nyangong 
 
Xanthic Ferralsol 
Acri-xanthic Ferralsol 
Ferralic Cambisol 
Ferric Acrisol 
 
Typic Hapludox 
Typic Kandiudox 
Oxic Dystropept 
Typic Hapludult 
 
Sols ferrallitiques 
fortement 
désaturés 
Sols ferrallitiques 
moyennement 
désaturés 
rajeunis/remaniés 
 
Ebom 
 
Acri-xanthic Ferralsol 
Xanthic Ferralsol 
Haplic Acrisol 
Plinthic Acrisol 
 
Typic Kandiudult 
Typic Hapludox 
Typic Paleudult 
Plinthudult 
 
Sols ferrallitiques 
fortement ou 
moyennement 
désaturés 
 
Ebimimbang 
 
Acri-plinthic Ferralsol 
Haplic Acrisol 
Acri-xanthic Ferralsol 
Ferrali-ferric Acrisol 
 
Plinthudult 
Typic Paleudult 
Typic Kandiudult 
Typic Kandiudult 
 
Sols ferrallitiques 
moyennement 
désaturés 
 
Valley Bottom 
 
Dystric Fluvisol 
Gleyic Cambisol 
 
Psammaquent/Fluvaque
nt 
Lithic Endoaquent 
Aquic Dystropept 
 
Sols 
hydromorphes peu 
humifères 
 
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  

 
 
45
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. 
 
CPCS  
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. 
 
5.3.1 Texture  
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 

 
 
46
 
Table 5.3 Soil physical characteristics of the Nyangong, Ebom and Ebimimbang soils.  
 
 
Soil type 
 
Depth 
 
 
Textur

(clay%

 
Textur

(silt%) 
 
Textu
re 
(sand
%) 
 
Bulk 
densit

(g/cm
3

 
AWC 
(%) 
 
Nyangon

 



 
 35-70 
 50-80 
 - 
 
 10-20 
 5-15 
 - 
 
 10-40 
 10-40 
 - 
 
1.03 
1.21 
1.26 
 
 10 
 12 
 7 
 
Ebom 
 



 
 20-50 
 35-60 
 - 
 
 5-20 
 5-15 
 - 
 
 40-60 
 30-50 
 - 
 
1.22 
1.37 
1.4 
 
 17 
 16 
 12 
 
Ebimim
bang 
 



 
 0-25 
 20-45 
 - 
 
 5-15 
 5-15 
 - 
 
 60-90 
 40-80 
 - 
 
1.3 
1.55 
1.63 
 
 11 
 15 
n.a. 
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
3
 in the 
topsoils (0-10 cm) and 1.0 to 1.7 g/cm
3
 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
3
 and 1.2 to 1.8 g/cm
3
 as 
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
3
, and below 20 cm the bulk densities increase to 
values of 1.2 to 1.3 g/cm
3
. The topsoils and subsurface horizons of the Ebom soils have bulk 
densities around 1.2 and 1.4 g/cm
3
, respectively. Bulk densities of the Ebimimbang soils are 
the highest of the TCP research area, i.e. 1.3 g/cm
3
 for the topsoils and 1.5 to 1.7 g/cm
3
 for 
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, 
1996).  
 
 

 

 
 
48
The pH
2)
 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 
Ebimimbang soils. 
 
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 
and 5.5. 
 
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
2
 = 0.94). 
 
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.  
                                                 
     
2
 
With pH, we mean pH(H
2
O), unless otherwise indicated. 

 

 
 
50
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 
Ebimimbang soils. 
 
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 
enrichment

 
Table 5.4b Ranges in some chemical characteristics of the four soil types in the TCP research area 
 
Soil type 
 
Depth 
 
 
K
+
 
 
 
Mg
++
 
 
 
Ca
++
 
 
 
TEB 
 
 
Al
+++
 
 
 
ECEC 
 
CEC 
 
 
BS 
(%) 
 
Nyangong 
 


 
0.1-0.5 
0-0.1 
 
0.2-0.8 
0-0.5 
 
1-2 
0.2-0.6 
 
1.5-3.5 
0.5-1.2 
 
4-9 
2-6 
 
6-10 
5-7 
 
10-20 
6-9 
 
10-20 
5-15 
 
Ebom 
 


 
0.1-0.9 
0-0.2 
 
0.4-1.6 
0-0.5 
 
0.5-4 
0-0.5 
 
2-5 
0.3-1 
 
0.5-6 
2-5 
 
4-9 
3-6 
 
12-25 
5-8 
 
10-50 
5-20 
 
Ebimimbang 
 


 
0.1-0.7 
0-0.2 
 
0-1.5 
0-0.5 
 
1-8 
0-2 
 
3-7 
0.2-1.5 
 
0-3 
1-2.5 
 
3-10 
1.5-1 
 
4-12 
2-6 
 
50-
100 
10-70 
 
Valley 
Bottom 
 


 
0-0.5 
0-0.1 
 
0.3-1.5 
0-0.2 
 
0.5-4 
0-0.5 
 
4.5-6 
0.5-0.8 
 
0-3 
0-1.5 
 
5-7.5 
0.5-2  
 
7-17 
1-8 
 
15-50 
20-40 
 
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 
soils. 
 
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, 
 
 

 

 
 
52
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) 



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