Soils and their distribution on Bambouto volcanic mountain, West Cameroon highland, Central Africa

Soils and their distribution on Bambouto volcanic mountain, West Cameroon highland, Central Africa

Journal of African Earth Sciences 39 (2004) 447–457 www.elsevier.com/locate/jafrearsci Soils and their distribution on Bambouto volcanic mountain, We...

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Journal of African Earth Sciences 39 (2004) 447–457 www.elsevier.com/locate/jafrearsci

Soils and their distribution on Bambouto volcanic mountain, West Cameroon highland, Central Africa P. Tematio

a,*

, L. Kengni a, D. Bitom b, M. Hodson c, J.C. Fopoussi b, O. Leumbe d, H.G. Mpakam b, D. Tsozue´ b

a b

Department of Earth Sciences, University of Dschang, P.O. Box 67, Dschang, Cameroon, Africa Department of Earth Sciences, University of Yaounde´ I, P.O. Box 812, Yaounde´, Cameroon, Africa c University of Reading, P.O. Box 217, Reading, Berkshire, UK d Institut National de Cartographic (INC), Cameroon, Africa Available online 1 October 2004

Abstract Morphological, physical and chemical studies were carried out on soils of Mount Bambouto, a volcanic mountain of the West Cameroon highland. These studies show that the soils of this region can be divided into seven groups according to Soils Taxonomy USA [Soil taxonomy: a basic system of soil classification for making and interpreting soils surveys: USDA Agriculture Handbook 436: Washington, DC, US Government Pronting Office, 1975, 754]: lithic dystrandept soils, typical dystrandept soils, oxic dystrandept soils, typical haplohumox soils, typical kandiudox soils, tropopsamment soils and umbriaquox soils. A soils map of this region at scale 1:50,000 has been drawn up, using the seven soils groups above as soil cartography units. These soils are organised into of three main categories: soils with andic characteristics in the upper region of the mountain (lithic dystrandept soils, typical dystrandept soils and oxic dystrandept soils); ferrallitic soils in the lower part of the mountain (typical haplohumox soils and typical kandiudox soils) and imperfectly developed soils (tropopsamment soils and umbraquox soils). Ó 2004 Elsevier Ltd. All rights reserved. Keywords: West Cameroon; Tropical Mountain; Andic soils; Ferrallitic soils; Soils map

1. Introduction Soils are very useful for human communities. Among others, they are used for agriculture/farming, forests, ore deposits, building materials, civil engineering activities, spreading of industrial or agricultural wastes, etc. (Ruellan and Dosso, 1993). In Cameroon, the first pedological studies were based on soil survey at the small scale (Jacques-Fe´lix, 1950; Claisse and Laplante, 1953; Laplante et al., 1955; Segalen et al., 1958; Bachelier, 1966; Segalen, 1967; Champaud, 1971; Muller and Gavaud, 1979; Valet, 1985); since the 1980Õs, they have been oriented principally to soil genesis and soil evolu*

Corresponding author. E-mail address: [email protected] (P. Tematio).

0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.020

tion (Yongue, 1986; Ildefonse, 1987; Kamga Kabeyene Beyala, 1987, 1998; Muller, 1987; Angue Abane, 1988; Bilong, 1988; Bitom, 1988; Doube´, 1989; Nyeck, 1989; Seiny Boukar, 1990; Braun, 1991; Bindzi Tsala, 1992; Lamotte, 1993; Onguene Mala, 1993; Bekoa, 1994; Etame, 1994; Nguetnkam, 1994; Tematio, 1994; Boli Baboule, 1996; Temgoua, 2001). Only a few studies have been directed to soil cartography at the large scale (Brabant, 1978). With the increasing population in Mount Bambouto region (Zambou, 1976; Dongmo, 1986), the high cattle breeding and great need for arable land resources, increasing pressure is being exerted on the land (Dongmo, 1981, 1986; Tematio and Olson, 1997). Additionally, because of the increasing demand, marginal lands like steep slopes—previously under pasture—and bottoms of valleys are being brought into

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production. The consequences are accelerated degradation of land with reduction of crop yields (Tematio and Olson, 1997). In order to assure sustained production and increased crop yields in this region, a judicious management of soils is needed. This is not possible without a proper understanding of the soils of the region. This implies among others, the study of their physical and chemical properties, which leads to soil classification and soil cartography. These are some of the most important ways to express the reality of soils in a given region (Duchaufour, 1983). Soil classification and soil cartography help to estimate natural soil resources, to plan research projects, and to suggest solutions for soil management and soil conservation (FAO-UNESCO, 1989; AFES, 1992). The obiectives of the present note are: (i) to identify, characterise and classify the main soil groups of Mount Bambouto using Soil Taxonomy USA (Soil Survey Staff, 1975) references; (ii) to define the cartography of the soils-units from the soil groups existing and use these cartographic units to produce a soils map of Mount Bambouto region, at scale 1:50,000. This will help to evaluate soil potential of this region, to establish factors governing soil distribution, and to suggest alternative solutions for sustainable management of land resources.

2. Materials and methods The study was carried out on Bambouto Mountain (Fig. 1). It is one of the major volcanic mountains of the continental part of the Cameroon volcanic line, belonging to the West Cameroon Highlands (Tchoua, 1974; Youmen, 1994; NÕni and Nyobe´, 1995; Marzoli et al., 1999, 2000). Bombouto reaches 2740 m at Mount Me´le´tan, the main slopes are the southern and eastern slopes. On this mountain, fifteen sites was selected to carry out the study: Me´le´tan, Temzem, Feumock, Loung, Mbeng, Ngoua, Balatchi, Bawa, Totap, Medji, Zavion, Camp bororo, signal Nkhi, Messong, Tossessa. At every site, random samplings were made using a hand auger. Afterwards, borings were hand dug in each group of soils identified at the site, the soilÕs morphology was described at the well sites, and soil samples collected for laboratory analyses: bulk density, particle size, organic carbon, acidity, exchangeable cations, cation exchange capacity, exchangeable aluminium, total nitrogen and available phosphorus. Bulk density (da) is given by the ratio between drying mass and volume of a given soil sample. Particle size was determined using the Robinson pipette method. Prior to this, the organic matter present in the sample was destroyed by oxygenated water (H2O2) using sodium hexametaphosphate (NaPO3)6 as dispersal agent. The quantity of total nitrogen was evaluated by titration after mineralisation of organic matter and distillation. The cation exchange

capacity was also evaluated by titration after qualitative desorption by K+ and distillation. Exchangeable cations are shifted by ammonium acetate (CH3COONH4) at pH 7. The proportions of K+ and Na+ were evaluated by flame photometry. Those of Ca2 and Mg2+ were determined by complexometry. The available phosphorus was determined by the Bray 2 method, which combines extraction of phosphorus in acid medium and their complexation with ammonium fluoride (NH4F). The quantity of available phosphorus was obtained by spectrophotometry in the presence of blue molybdenum (MoO3). The proportion of organic carbon was obtained after oxidation in a highly acid medium (H2SO4) with potassium dichromate (K2Cr2O7). Organic matter (OM) was obtained from organic carbon (C) using the Sprengel factor. The bases saturation corresponds to the ratio of the sum of exchangeable cations (S) and cations exchange capacity (CEC). The exchangeable aluminium was extracted in a solution of potassium chloride 1 M, and evaluated by colourimetry with the violet pyrocathecol method (VPC). Aluminic toxicity is defined by the Kamprath (1972): m¼

Al  100: ðAl þ SÞ

3. Results: the main soil groups of Mount Bambouto and their morphological, physical and chemical characteristics Seven soil groups have been identified on Mount Bambouto. They are: lithic dystrandept soils, typical dystrandept soils, oxic dystrandept soils, typical haplohumox soils, typical kandiudox soils, tropopsamment soils and umbraquox soils. 3.1. Lithic dystrandept soils group 3.1.1. Morphology of the soil profile A representative profile of this soil group was observed at 2740 m at the Mount Me´le´tan site. It shows from the surface to the bottom: – a thick-humiferous A horizon (about 60 cm thick) with dark grey colour (2.5YR2.5/2-1) when moist; silt loam; well-developed crumb structure; with many rock fragments at the surface. – an alteritic C horizon with greyish to yellow brown weathered rock fragments. These rock fragments are enveloped in light brown (10YR7-6/6), massive and clayey mixed materials. This profile is of AC type; imperfectly developed, less differentiated and shallow; with a thick A horizon above the alteritic C horizon.

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Fig. 1. Soils distribution map of Mount Bambouto.

3.1.2. Soils’ physical and chemical characteristics The physical and chemical characteristics of these soils are presented in Table 1. These soils are highly acid (pH 4.9 in A horizon). They have very low bulk density

at the surface (da: 0.54 g cm3). They are sandy silt in the A horizon (57.6% sand particles) and clayey in the C horizon (45% clay particles). The sum of exchangeable cations is very low (S < 2 meq/100 g of soils). The cation

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Table 1 Physical and chemical characteristics of the lithic dystrandept soils Horizon

A C

da (g/cm3)

0.54 0.83

P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

0.09 1.25

Acidity

Ca2+

Mg2+

K+

Na+

S

0.61 0.08

0.46 0.09

0.27 0.05

0.012 0.019

1.35 0.24

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

42.0 2.95

3.2 8.1

32.5 82.5

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

4.9 5.1

4.3 4.0

0.6 1.1

11.6 0.5

20.0 0.8

0.8 0.03

14 13

13 45

29.4 28.4

57.6 26.6

15.38 3.00

da: bulk density; S: sum of exchangeable cations; CEC: cations exchange capacity; S/CEC: bases saturation; m: aluminic toxicity; C: organic carbon; OM: organic matter; N: total nitrogen; C/N: mineralisation factor; A: clay particles; L: silt particles; S: sand particles; dpH: net charge; CECa: cations exchange capacity for clay particles.

– a thin mineral B horizon (<20 cm thick); dark brown at the summit and light brown downward (5YR4/3 to 7.5–10YR4/4); silty clayey; firm and plastic; average to coarse blocky structure. – an alteritic C horizon with many weathered rock fragments enveloped in light brown (10YR7/6), clayey and massive mixed materials.

exchange capacity is important at surface (CEC: 42 meq/ 100 g of soils) due to the abundance or organic matter (OM: 20%). BaseÕs saturation (S/CEC) remains under 10%. The concentration of exchangeable aluminium is average in the A horizon (m: 32.5%) and very high in the C horizon (m: 82.5%). The cation exchange capacity of clay particles is medium in the A horizon (CECa: 15.38 meq/100 g of clay particles) and very low in the C horizon (CECa: 3.0 meq/100 g of clay particles). The concentration of total nitrogen is very low (N < than 1%). The organic matter is averagely mineralised (C/N: 14). Available phosphorus is present in very low concentrations (P2O5 < 2 ppm).

This profile is of A(B)C type; differentiated; less developed; with a thick A horizon on a less developed B horizon. 3.2.2. Soils’ physical and chemical characteristics The physical and chemical characteristics of these soils are shown in Table 2. These soils are moderately acid (pHwater 5.1 in A horizon) with significant values of net charge (dpH: 0.6 to 0.9). The bulk density is very low (da: 0.5–0.8 g/cm3). Texture is sandy loam in the A horizon and clayey in the B horizon. The sum of exchangeable cations is very small (S < 4 meq/100 g of soils), while the cation exchange capacity is very high in the A horizon (CEC: 34.82 meq/100 g of soils). The basesÕ saturation is less than 5% at the surface (S/ CEC: 4.82%) and high in the C horizon (S/CEC:

3.2. Typical dystrandept soils group 3.2.1. Morphology of the soil profile A profile of these soils is present at 2650 m at the Mount Tossessa site. It shows from top to bottom: – a thick humiferous A horizon (72 cm thick) with very dark colour (5YR2.5/1) when moist; silty loam; very porous; flimsy and crumbly; fine to average crumb structure. Table 2 Physical and chemical characteristics of the typical dystrandept soils Horizon

A B C

da (g/cm3)

0.55 0.79 0.83

P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

0.38 0.00 0.00

Acidity

Ca2+

Mg2+

K+

Na+

S

1.17 0.48 2.32

0.30 0.30 1.48

0.20 0.06 0.02

0.009 0.009 0.023

1.679 0.849 3.843

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

34.82 13.55 5.81

4.82 6.26 66.14

33.08 2.30 20.81

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

5.1 4.7 4.7

4.2 4.5 4.1

0.9 0.2 0.6

9.37 3.68 0.28

16.15 6.34 0.48

0.586 0.222 0.017

16.0 16.6 16.5

60.1 51.1 38.1

40.0 16.0 22.0

43.9 32.9 39.9

15.65 1.70 12.72

P. Tematio et al. / Journal of African Earth Sciences 39 (2004) 447–457

66.14%). The cation exchange capacity of clay particles is low (CECa < 20 meq/100 g of clay particles). The exchangeable aluminium is important in the A horizon and the C horizon (m: 33.08 and 20.81% respectively), and very low in the B horizon (m: 2.3%). The concentrations of organic matter are important in the A horizon and the B horizon (OM: 16.15% and 6.34% respectively). This organic matter is not very mineralised (C/ N: 16). These soils contain low concentrations of total nitrogen (N < 1%). The available phosphorus is present at the upper part of the profile, but in very small quantities (P2O5: 0.38 ppm).

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is present in the A horizon (m: 14.37%) and totally absent in the B horizon. These soils are averagely rich in organic matter (OM: 5.64% in A horizon). The concentrations of total nitrogen are low (N < 1%). The organic matter is weakly mineralised (C/N > 18). There are small amounts of exchangeable cations (S < 2 meq/100 g of soils). The cation exchange capacity is average in the A horizon (CEC: 12.82 meq/100 g of soils). The basesÕ saturation remains low (S/CEC < 30%). The cation exchange capacity of clay particles is very small (CECa < 3 meq/100 g of clay particles). The available phosphorus is present in very small amounts (P2O5: 0.6 ppm).

3.3. Oxic dystrandept soils group 3.4. Typical haplohumox soil group 3.3.1. Morphology of the soil profile A representative profile is described at 2055 m at the Balatchi site. It presents from the surface to the bottom:

3.4.1. Morphology of the soil profile A representative profile of this group of soils is observed at the Bawa siteat 1500 m. From top to bottom, there is:

– a thick humiferous A horizon (62 cm thick) with very dark colour (2.5YR2.5/2) when moist; silty clay loam; porous; fragile and crumbly; puffy structure and smeary consistence. – a mineral B horizon (92 cm thick) with red to dark red colour (2.5YR4/4–6); clayey loam; plastic; sticky; well-developed coarse blocky structure; with alumino-ferruginous fine gravels. – an alteritic C horizon with many weathered rock fragments and light brown (10YR7/6), clayey and massive mixed materials. This profile is of ABC type; well developed; well differentiated; thick; with thick A horizon above a well differentiated and indurated B horizon.

– a humiferous A horizon (31 cm thick), dark red (2.5YR3/6) when moist; silty clay; well-developed crumb structure. – a thick mineral B horizon (more than 100 cm); red (10R3/6) when moist; very clayey; well-developed coarse blocky structure. This profile is of ABC type, thick, well differentiated, well developed, with a thick and well-differentiated B horizon. 3.4.2. Soils’ physical and chemical characteristics The physical and chemical characteristics of these soils are given in Table 4. These soils are weakly acid (pHwater more than 6). The net charge is significant in the A horizon (dpH: 1.1). These soils have sandy clayey texture in the A horizon and very clayey texture in the B horizon. The exchangeable aluminium is present in very small amounts in the A horizon (m: 5.43%), and is totally absent in the B horizon These soils are averagely rich in organic matter (OM: 6.51% in A horizon).

3.3.2. Soils’ physical and chemical characteristics The main characteristics of these soils are presented in Table 3. These soils are averagely acid (pHwater 6.1 in B horizon). The net charge is weak (dpH < 0.5). These soils have a clayey loam texture (more than 60% in A horizon and B horizon). They have very low bulk density (da: 0.6 g/cm3). Exchangeable aluminium Table 3 Physical and chemical characteristics of the oxic dystrandept soils Horizon

A B

da (g/cm3)

0.62 0.64

P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

0.66 0.66

Acidity

Ca2+

Mg2+

K+

Na+

S

0.86 1.02

0.16 0.17

0.07 0.07

0.23 0.36

1.32 1.62

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

12.82 5.9

10.2 27.4

14.37 0.00

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

5.2 6.1

4.8 6.0

0.4 0.1

3.27 1.24

5.64 2.13

0.18 0.07

18.4 18.5

60.1 63.5

31.7 16.6

9.8 19.9

2.56 2.57

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Table 4 Physical and chemical characteristics of the typical haplohumox soils Horizon

A B

da (g/cm3)

0.9 1.2

P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

3.57 1.00

Acidity

Ca2+

Mg2+

K+

Na+

S

6.44 1.54

2.16 0.56

0.28 0.09

0.008 0.007

8.9 2.2

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

22.37 6.38

39.70 34.33

5.43 0.00

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

6.2 6.1

5.1 5.8

1.1 0.3

3.83 1.23

6.51 2.12

0.227 0.063

17.1 19.62

37 74

26.6 19.6

36.3 6.3

The amount of total nitrogen is very low (N < 0.5%). The organic matter is weakly mineralised (C/N more than 17). The sum of exchangeable cations is averagely important, particularly in the A horizon (S: 8.9 meq/ 100 g of soils). The cation exchange capacity is high in the A horizon (CEC: 22.37 meq/100 g of soils). The basesÕ saturation is average (S/CEC between 30% and 40%). The available phosphorus is significant in the A horizon (P2O5: 3.57 ppm). The cation exchange capacity of clay particles is very important in the A horizon (CECa: 25.38 meq/100 g of clay) and very small in the B horizon (CECa: 2.96 meq/100 g of clay).

25.38 2.96

This profile is of ABC type; well differentiated and not very thick. 3.5.2. Soils’ physical and chemical characteristics The physical and chemical characteristics of these soils are given in the Table 5. They are strongly acid in the A horizon (pHwater 4.9) and moderately in the B horizon (pHwater 5.5). These soils have clayey sand texture in the A horizon and clayey texture in the B horizon. They have small amounts of organic matter (OM: 2.32% in A horizon). The concentrations of nitrogen are less than 0.1%. The ratio C/N is more than 12, showing that the organic matter is moderately mineralised. The sum of exchangeable cations is very small (S < 1 meq/100 g of soils). The cation exchange capacity (CEC) is less than 6 meq/100 g of soils. The basesÕ saturation is very low in the A horizon (S/CEC: 8.27%) and average in the B horizon (S/CEC: 26.58%). In the A horizon, there is a large amount of exchangeable aluminium (m: 47.34%) and a very small amount in the B horizon (m: 7.43%). The available phosphorus is <1.5 ppm.

3.5. Typical kandiudox soils group 3.5.1. Morphology of the soil profile A representative profile is described from Ngoua, at around 1485 m. From top to bottom, there is: – a humiferous A horizon (13 cm), dark brown (10YR4/ 3) when moist; sandy loam; crumb structure, – a thick mineral B horizon (112 cm); brown red (7.5YR5/6) when moist; clayey sand; coarse blocky structure; with weathered rock fragments and numerous quartz grains, – an alteritic C horizon with many weathered rock fragments enveloped by brown-red, massive and clayey sand mixed materials.

3.6. Tropopsamment soils group 3.6.1. Morphology of the soil profile A representative profile of these soils is observed at the Medji site, at around 1500 m. It is made up of a sin-

Table 5 Physical and chemical characteristics of the typical kandiudox soils Horizon

A B

da (g/cm3)

0.80 0.84

P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

1.28 0.82

Acidity

Ca2+

Mg2+

K+

Na+

S

0.16 3.37

0.10 0.22

0.02 0.00

0.18 0.16

0.46 0.75

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

5.51 2.81

8.27 26.58

47.34 7.43

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

4.9 5.5

4.1 4.4

0.8 1.0

1.35 0.58

2.32 1.00

0.098 0.034

13.8 17.1

37.1 46.1

25.6 17.6

37.2 36.2

2.34 1.75

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Table 6 Physical and chemical characteristics of the tropopsamment soils Horizon

A

da (g/cm3)



P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

1.77

Acidity

Ca2+

Mg2+

K+

Na+

S

0.23

0.09

0.20

0.007

0.53

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

4.49

11.80

65.81

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

4.9

4.0

0.9

1.73

2.94

0.088

19.7

33.0

15.6

51.3

4.70

Table 7 Physical and chemical characteristics of the Umbraquox soils Horizon

Ag

da (g/cm3)



P2O5 (ppm)

Exchangeable cations (meq/100 g of soils)

1.44

Acidity

Ca2+

Mg2+

K+

Na+

S

0.60

0.49

0.17

1.26

2.62

Organic matter (%)

CEC (meq/100 g of soils)

S/CEC (%)

m = Al/[S + Al] (%)

18.8

13.93

5.65

Particle size (%)

CECa (meq/100 g of clays)

pHwater

pHKCl

dpH

C

OM

N

C/N

A

L

S

5.7

4.5

1.2

4.68

8.07

3.14

14.9

26.5

26.3

47.2

gle humiferous A horizon with dark brown colour (7.5YR3/3) when moist; 30 cm thick; sandy loam; crumb structure; with many weathered rock fragments. Under this horizon is the bedrock. This profile is of AR type; weakly differentiated; not very thick; imperfectly developed. 3.6.2. Soils’ physical and chemical characteristics These characteristics are given in Table 6. These soils are strongly acid (pHwater 4.9). They have a sandy texture and contain a very small amount of exchangeable cations (S: 0.53 meq/100 g of soils) and have limited cation exchange capacity (CEC: 4.4 meq/100 g of soils). The basesÕ saturation is very low (S/CEC: 11.08%). The available phosphorus is present in very small quantity (P2O5: 1.77 ppm). The cation exchange capacity of clay particles is weak (CECa: 4.7 meq/100 g of clays). These soils are relatively poor in organic matter (OM: 2.9%). The amount of exchangeable aluminium is very important (m: 65.81%). The total nitrogen is very low (N: 0.088%) and the organic matter is weakly mineralised (C/N: 19.7). 3.7. Umbraquox soils group 3.7.1. Morphology of the soils profile A representative profile of these soils is observed at the Ngoua site, at around 1435 m. It is a single moderately organic hydromorphic Ag horizon; dark grey (10YR3/2–1) when moist; silty and massive. This horizon presents a mottled area in the upper part. This profile is strongly influenced by ground water.

10.03

3.7.2. SoilsÕ physical and chemical characteristics These characteristics are given in Table 7. These soils are moderately acid (pHwater 5.7). They have a sandy loam texture. The amount of exchangeable aluminium is very small (m: 5.65%). The available phosphorus is limited (P2O5: 1.44 ppm). These soils have a great amount of organic matter (OM: 8.07%). This organic matter is averagely mineralised (C/N: 14.9). The amount of exchangeable cations is very small (S: 2.62 meq/100 g of soils). The cation exchange capacity is average (CEC: 18.8 meq/100 g of soils). The base saturation is low (S/CEC: 13.9%).

4. Discussion and conclusions 4.1. Classification of Mount Bambouto soils All complex natural materials must be classified if they are to be understood (Olson, 1983). Soil classification is a technique by which soils can be divided into categories that are useful for understanding genesis, properties and behaviour (Soil Survey Staff, 1975). The basic data for soil taxonomy consist of soil profile descriptions, and physical and chemical analyses. Soil taxonomy is based on a reference classification system. The classification system used in this paper is Soil Taxonomy USA (Soil Survey Staff, 1975). It is a natural comprehensive system of soil classification, which classifies soil properties for many uses. It is also a tool designed to serve soil cartography, so that characteristics

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of soil can be translated onto maps showing their landscape distribution (De Coninck et al., 1986). It permits us to classify the Mount Bambouto soil groups as follow. Lithic dystrandept soils group. This group has a humiferous A horizon with mollic epipedon and umbric epipedon characteristics (dark colour, high amount of organic matter, base saturation less than 50%, value and chroma below 3.5). This horizon has also very low bulk density (da < 0.6 g/cm3), cation exchange capacity above 30 meq/100 g of soils and a puffy structure. Soils of this group are very desaturated, and have a great amount of organic matter averagely mineralised. The high values of dpH in the A horizon suggest the presence of Al-humus amorphous constituents. In the alteritic C horizon, it may be amorphous aluminium constituents like allophanes (Lulli et al., 1983). This profile is more than 35 cm thick. The lithic contact is clear and shallow. It is a profile of AC type. All these characteristics help to classify this group of soils as follow: order: inceptisols; suborder: andept; great group: dystrandept; subgroup: lithic dystrandept. Typical dystrandept soils group. In this group, the humiferous A horizon has also umbric epipedon and mollic epipedon characteristics. It has a puffy structure and very low bulk density. Al-humus amorphous constituents are present in the A horizon. In the alteritic C horizon, the high concentration of dpH suggests the presence of amorphous aluminium constituents (allophanes). These soils are very desaturated. The profile is more than 35 cm thick. This group is classified as: order: inceptisols; suborder: andept; great group: dystrandept; subgroup: typical dystrandept. Oxic dystrandept soils group. As in the typical dystrandept soils group above, the humiferous A horizon has umbric epipedon and mollic epipedon characteristics. This A horizon has also a bulk density less than 0.6 g/cm3 and a puffy structure. The large amount of exchangeable aluminium in this horizon indicates the presence of Al-humus amorphous constituents (Colmet-Daage et al., 1972; Rosello, 1984; Quantin, 1992). Under the A horizon, is a mineral B horizon containing alumino-ferruginous indurated materials. The absence of exchangeable aluminium here suggests the existence of aluminium hydroxides like gibbsites (Boulange, 1984). The red colour of this horizon is strongly influenced by large amount of iron oxyhydroxydes, probably hematite and goethite (Temgoua et al., 2002). The mineral B horizon of this profile has the characteristics of an oxic horizon. Definitely, this group of soils presents at once the characteristics of andic soils (A horizon) and those of ferralitic soils (B horizon). It can be classified as follows: order: inceptisol; suborder: andept; great group: dystrandept; subgroup: oxic dystrandept. Typical haplohumox soils group. These soils are characterised by complete weathering of primary minerals.

The cation exchange capacity for clay particles is less than 16 meq/100 g, of clays in the B horizon. The sum of exchangeable cations is lower than 100 meq/100 g of soils, and the bases saturation is between 20% and 40%. Exchangeable aluminium is totally absent, suggesting the presence of aluminium hydroxydes (gibbsites). These soils have thus an oxic B horizon. This soil group is classified as: order: oxisol; suborder: humox; great group: haplohumox; subgroup: typical haplohumox. Typical kandiudox soils group. As in typical haplohumox soils, the B horizon has the characteristics of an oxic horizon. These soils are strongly weathered. Quartz is the only residual mineral. All this allows us to classify this group as: order: oxisol; suborder: udox; great group: kandiudox; subgroup: typical kandiudox. Tropopsamment soils group. In this group, there is incomplete weathering of primary minerals. The less developed and less differentiated humiferous A horizon is lying directly on the bedrock. This horizon has a very small amount of organic matter. The profile is developed on steep slopes. It means that these soils are regularly rejuvenated. This group can be classified as: order: entisol; suborder: psamment; great group: tropopsamment. Umbraqox soils group. This group of soils is strongly influenced by the presence of ground water. There is a mottled area in the profile, indicating the action of oxydo-reduction. The soil moisture regime is aquic. This group is classified as: order: oxisol; suborder: aquox; great group: umbraquox. 4.2. Soil cartography units and their distribution on Mount Bambouto Soil cartography involves a suite of processes playing a part in the construction of soil maps (Bornand et al., 1989). It concerns among others the demarcation on a map of zones of homogenous soil groups called cartography units. A cartography unit is then a volume of soils that can be represented on a map with clearly defined limits and properties (Girard, 1983). On Mount Bambouto, we use genetic units, which correspond to the limits of our reference classification system (Soil Survey Staff, 1975). Thus, seven cartographic units have been defined in this area according to the seven major soil groups observed in the region. These are: Lithic dystrandept soils cartographic unit. It is present at the upper zone of the mountain, above 1700 m height. These soils are developed on volcanic rocks (trachyte, basalt, phonolite, tuffs, ignimbrite, etc.). Above 2300 m high, climate is misty, fresh and humid, with low temperature (12–13 °C). This local climate corresponds to a temperate climate of mountainous regions. An AfroAlpine prairie covers this upper part of Mount Bambouto. This soils cartographic unit occupies steep slopes and summits of interfluves, with many rock fragments and blocks at the soilÕs surface.

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Typical dystrandept soils cartographic unit. It is present at the upper part of Mount Bambouto, above 2300 m. These soils cover moderate slopes and summits of interfluves with smooth surfaces. They are developed exclusively on volcanic rocks. Oxic dystrandept soils cartographic unit. This soils cartographic unit is present in the middle zone of the mountain, between 1700 and 2300 m height. In this region, the climate is still fresh, but one notes a significant reduction of annual precipitation (1600–1700 mm of rain) (Wouendeu Tchatchui, 2003). These soils are also developed exclusively on volcanic rocks. Typical haplohumox soils cartographic unit. It is observed at the lower part of the mountain, between 1400 m and 1600 m. In this domain, the climate is rainy and hot. It corresponds to a mountainous humid tropical climate, with more than 1800 mm of rain in one year (Wouendeu Tchatchui, 2003) and annual average temperature of about 21 °C (Njiosseu Tanko, 1998). These soils are developed exclusively on volcanic rocks. They are observed on summits and slopes of interfluves. Typical kandiudox soils cartographic unit. This unit is also present at the lower part of the mountain, but it is developed exclusively on granite and gneiss basement rocks. These soils are observed on moderate slopes. Tropopsamment soils cartographic unit. It is noted in the lower part of the mountain, on steep slopes with many rock fragments at the surface. Soils are developed exclusively on granite and gneiss basement rocks. Umbraquox soils cartographic unit. This unit is observed everywhere on the mountain. Soils are present exclusively in the bottoms of valleys, where groundwater is permanent. The above-defined cartographic units were identified and delimited at selected sites. Fifteen sites (see above) were selected according to their morphological, their climatic and geological characteristics. At each defined site, holes were dug along transects using a hand auger. This allows definition, at each site, of the aerial extension of recognised cartographic units. Using each site like a landscape model, the soil cartographic unitÕs distribution on it is then extrapolated to similar sites, using aerial photographs. Step by step, a soils distribution map of Mount Bambouto at scale 1:50,000 has thus been drawn up Fig. 1. In general, there are three major categories of soils on Mount Bambouto: soils with andic characteristics (lithic dystrandept soils, typical dystrandept soils, and oxic dystrandept soils); ferrallitic soils (typical haplohumox soils and typical kandiudox soils) and imperfectly developed soils (tropopsamment soils and umbraquox soils). Soils with andic characteristics and ferrallitic soils cover more than 95% of the area. The first category is present at the upper part of the mountain where temperate climate and the exclusivity of volcanic rocks favours their development (Quantin, 1992; Rosello, 1984). The second

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is observed at the lower part of the mountain where humid and hot tropical climate promotes deep weathering of rocks (Bilong, 1988; Temgoua et al., 2002).

Acknowledgments The authors thank START for supporting the fieldwork activities of this research under their START/ PACOM Doctoral Research Fellowship project. We also thank our colleague, Dr Temgoua Emile of EPFL, Switzerland, for his co-operation.

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