Organisation of Soils Along the Sides of Interfluves in the Western

Asian Journal of Soil Science and Plant Nutrition

5(1): 1-19, 2019; Article no.AJSSPN.51629 ISSN: 2456-9682

Organisation of Soils along the Sides of Interfluves in the Western Highlands of Cameroon: Case Study of an Andosolic Toposequence on Trachyte in the Upper Part of the Southern Limb of Bambouto Mountains

J. C. Fopoussi Tuebue1*, S. D. Basga2, P. Tematio3 and J. P. Nguetnkam4

1Department of Sciences, Jesus and Mary Secondary High School, P.O.Box 185CS101, Yaounde, Cameroon. 2Institute of Agricultural Research for Development (IRAD), P.O.Box 41, Garoua, Cameroon. 3Department of Earth Science, University of Dschang, P.O.Box 67, Cameroon. 4Department of Earth Sciences, University of Ngaoundéré, P.O.Box 454, Dschang, Cameroon.

Authors’ contributions

This work was carried out by author JCFT under the supervision of authors JPN and PT. Author SDB contributed to the exploitation of results. All authors read and approved the final manuscript.

Article Information

DOI: 10.9734/AJSSPN/2019/v5i130058 Editor(s): (1) Dr. Pankaj Gupta, Dolphin (PG) College of Science & Agriculture, Punjabi University, India. Reviewers: (1) Seweta Srivastava, Lovely Professional University, India. (2) Balthazar Michael Msanya, Sokoine University of Agriculture, Tanzania. Complete Peer review History: https://sdiarticle4.com/review-history/51629

Received 16 July 2019 Accepted 21 September 2019 Original Research Article Published 12 October 2019

ABSTRACT

The acquisition of knowledge on soils tells how to use them sustainably. So, the organization of soils along the sides of interfluves in the western highlands of Cameroon was put into relief in order to understand their genesis, organization, and evolution. Field work and lab analysis helped to reach the focuses. These soils are thin, highly differentiated, with many local specificities. Their organization and their thickness vary from up to downhill. Vertically, they include: An isalteritic horizon; a yellow vague polyhedral horizon; a red distinct polyhedral horizon; a discontinuous breastplate with two facies; a porous vague crumby dark reddish brown horizon; and a strongly dark grey, porous, thixotropic and crumby superficial horizon. The pedon/alterite ratio is about 2/1,

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*Corresponding author: Email: [email protected];

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and the hardened level/alterite ratio is 1/4. The alteritic level represents about 1/3 of the whole soil profile. Microscopically, these soils lack plasmic separations. Plasmas are respectively isotic in the dark reddish brown and strongly dark grey horizons, clayey asepic in the yellow and red polyhedral horizons, cristic in the isalteritic horizon, cristic and locally isotic in the discontinuous breastplate. Gibbsite, halloysite, kaolinite and allophane are the main minerals; goethite, quartz, hematite and rutile are also present. Geochemically, aluminum is the main chemical component in the deepest horizons, while in those at the top of the soil profile, silicon and aluminum has quite similar concentrations. Others specificities include their low bulk density (0.6 to 0.9 gcm-3), the abundance of clayey particles at the bottom of the soil profile, sand and silt at its summit. Ferrallitic and andosolic characteristics coexist in the studied soils. This ambivalence makes them to be simply «andic ferallitic, desaturated, humic and strongly rejuvenated soils».

Keywords: Gravity; breastplate; rejuvenation; ambivalence; gibbsite.

1. INTRODUCTION evolution, and the functioning of Andosols along the sides of interfluves of the Cameroonian Andosols are young soils developed on volcanic Western Highlands, which have first of all the materials, or sometimes on fine crystallized rocks particularity to be highly considered for cropping on which alteration can quickly act to release [7], and secondly to have risen up from massive sufficient poorly crystallized substances that and compact parent rocks of Miocene age [9], protect organic matter against bacterial attacks under an equatorial monsoon and highly rainy [1]. The conditions required for their formation type of climate [10]. The parent rock of the are found in temperate zone, or in rainy tropical presently studied soils is an alkaline trachyte areas (>2000 mm of rain per year) with low [11], outcropping in the middle and upper zones temperatures (<15°C) [2,3,4]. Those soils, of the southern limb of the Bambouto Mountains. previously less studied, constitute today the major focus of many researchers since the 2. MATERIALS AND METHODS beginning of the year 1950. In tropical region in general, Andosols appear in volcanic massifs in 2.1 Materials: Physical Characteristics of different topographic positions [2,3,4], where they the Bambouto Massif are sometimes the only soil type existing [4,5]. Many years ago, researchers made basic The volcanic Bambouto massif is located in the characteristics of Andosols their main focus, Cameroonian Western Highlands, between 5°25’ without any interest in their local particularities. It and 5°45’ of North latitude, and between 10°00’ is the case in the Bambouto Mountains. Here, [5] and 10°15’ of East longitude. It is a huge showed that Andosols are present in areas over volcanic shield, reaching 2740 m high at the 2000 m of altitude; in their study, they focused on summit of Mélétan Mount. Geomorphologicaly, the general characteristics of the most superficial the present massif has three main zones with horizons, known as diagnostic horizons particular environmental conditions; these according to the USDA system of classification include: the upper zone, with altitudes higher [6]. The consequence of this is the mystery that than 2000 m, the mid zone, with altitudes ranging remains in the detail about these soils in their between 1600 and 2000 m high, and the lowest different ecosystems. Thus, the major aspects zone, with altitudes ranging between 1400 and studied up to today include: morphological and 1600 m [12,13]. The upper zone shows an analytic characteristics, influence of the aggressive relief. Its climate is foggy and cold, environmental factors, notably the parent rock, with temperatures values ranging between 10 stocktaking, spatial localization, and the and 13°C. The rains are orographic type, with highlighting of the fertility parameters in relation annual average pluviometry of 2600 mm [10,14]. to the farming techniques applied [7,8]. On the The flora is natural and anthropic. The natural other hand, the organization of these soils along part is essentially made of graze, locally the sides of interfluves remains incompletely disturbed by gallery forests along water course known. Considering the importance of these soils [5]; the anthropic part is made of different crops for humans in the ecosystems where they [15,11]. The hydrographic network is radial and appear, they must therefore be better known. dense. Soils are mostly TypicDystrandepts [5]. Thus the present study aims to bring under light Many signs of anthropic activities are present [8]. more knowledge concerning the organization, the The mid zone has a subequatorial type of

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climate, largely influenced by the altitude [16]; it mineralization by oxidation with potassium is cold and wet, with nine month of rains, from bichromate and concentrated sulfuric acid; the march to November, and a short dry season, amount of organic matter (O.M.) was then from December to February; the average annual deduced using the [19] formula, stipulating that temperature is about 18°C; the annual average %O.M. = (%O.C.)X2 in an area without human pluviometry is 1690 mm. Compared to the upper perturbation. Nitrogen amount was determined zone, the relief here is less aggressive. The using sulfuric acid after mineralization by vegetation is mainly anthropic [15], with some oxidation in a digestor. pHwater was measured local islet of the natural vegetation in the using a pH-meter on a soil-water solution in swamps. The hydrographic network is the1:2.5 ratio; concerning the pHKCl, it was subdendritic. Soils are mostly andic ferallitic [5]. measured on a mixture of soil-KCl at a 1/1 ratio. The lowest zone finally is characterized by a hot Exchangeable basic cations (Ca2+, Mg2+, Na+, and wet climate, with an annual average K+) were extracted from the soil using a neutral temperature of 23,5°C and an average annual ammonium acetate solution. The measurements pluviometry of 1750 mm. The relief is undulating. of each basic cations amount were done with The vegetation is dominantly anthropic [15]. The atomic spectrometry absorption and flame hydrographic network is subdendritic. Soils are emission. Cation exchangeable capacity (CEC) mainly ferallitic with hardpan within [5]. was determined as followed: saturation of Geologically, trachytes are the major rocks in the absorbant complex with NH4+ solution and whole massif [9,17]. There are also few outcrops removal of basic exchangeable cations, + of basalts, phonolites, rhyolites, and elimination of the excess of NH4 solution filling pyroclastites. The substratum is made of granitic the soil pores, quantitative desorption with KCl + and gneissic types of rocks, outcropping in the and NH4 measurements using Kjeldahl lowest zone of the massif [9,17]. distillation. Total exchangeable cation capacity was calculated with the following formula: CECt= 2.2 Methods ECEC + OCEC (effective cation exchangeable capacity + organic cation exchangeable The Bambouto Mountains soils map [5] has capacity). In fact, [20] accesses that 1% of helped to locate Andosols on the field. Their organic matter enriches the soil with 2 meq/100 g description was facilitated by wells and bore of soil of cation exchangeable capacity. holes. Descriptions focused also on trachytes Saturation rate was deducted from exchangeable outcropping, as well as on rocky blocks present basic cations sum and CECt through the at the soil surface. Rock and soil samples were following formula: (S/CEC in %). Available finally collected for lab analysis. In the laboratory, phosphorous was extracted with a MEHLICH-3 four major analyses were carried out, notably solution and measured with colorimetry method petrographic, mineralogical, geochemical, and in the presence of “vert de malachite” as physico-chemical. Petrographic analysis accelerator. All those analysis were made at the consisted in the study of the slides of rock and IITA (International institute of Tropical soil samples under polarizing compound Agriculture), Nkolbisson (Yaoundé). microscope. Soil slides were prepared in the petrography laboratory of IRAD, Nkolbison 3. RESULTS AND DISCUSSION (Yaoundé). Mineralogical and geochemical analyses proceeded respectively by X-ray 3.1 Results diffractometry on a device using cupper anode and by fluorescence, were made on total soils 3.1.1 Petrography, mineralogy and and rock powder in the Mineral Analyses Centre geochemistry of the rock of Lausanne University, Switzerland. The studied soils are located at altitude of 2740 Grain size analysis was made using the method m high. Their parent rock is a dark grey, of Robinson pipette, after dispersion with sodium massive, cracked, hard, dense (Da=2.5) trachyte hexametaphosphate. Real (Dr) and bulk (Da) with saccharoïd aspect, outcropping as large densities were determined respectively by the blocks at the summit and on the limbs of trachytic method of water pycnometer and by the paraffin interfluves. It is microscopically essentially method. Porosity (P) was deducted by the two constituted of phenocrysts of sanidine associated previous values using the following formula P= with pyroxene, beside which are dense minerals. (Dr-Da/Dr) x100. Organic carbon (O.C.) was All these are drowned in a matrix mostly made of measured using the method of [18] after microcyrists of sanidine; micro and phenocrysts

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of sanidine are oriented in a preferential horizon with crumby structure, and a dark grey direction, conferring to the rock a porphyritic flow horizon with a well-developed crumby structure. like type of texture (Fig. 1); these observations Moreover, in the cracks of the isalteritic blocks, are in accordance with those made previously by pedoturbated matters are present; some are [17]. yellow and others are red. The yellow one is more abundant than the red one. The X-ray diffraction confirmed the presence of particularity of the red one is its high humidity pyroxene; furthermore, it revealed the real nature and its well-developed polyhedral structure. In of the dense mineral observed under polarizing the yellow horizon above, the amount of the red compound microscope in rock slides, notably spots of pedoturbated matters increases magnetite and ilmenite. Chemically, silicon is the upwards. Concerning the red horizon, some of its most abundant element (58.00%SIO2) (Table 1), portions are dark red, with a high degree of followed by aluminum (18.40%AL2O3) and iron humidity; the well-developed polyhedral structure (5.63%FE2O3) (Table 1); alkali are also present is maintained. In these dark red spots, in great quantity (6.56%NA2O for the sodium and translucent volumes with vitreous aspect are 5.23%K2O for the potassium), dominating largely present; they are arranged in the continuity of the earth alkali elements (2.10%Ca for the calcium diaclasis from the mother rock. Towards the and 0.41%MGO for the magnesium magnesium) bottom of the isalteritic horizons, the walls of (Table 1). Other chemical elements found are diaclasis are covered by a film of a hardened and titanium (0.44%Tio2), manganese (0.30%mno), translucent matter with vitreous aspect, with and the phosphorus (0.14%P2O5) (Table 1); thicknesses decreasing from the bottom (about 2 these chemical compositions are similar to those cm) to the top of the present horizon (about 1 obtained by [17]. According to the binary diagram mm). Between the two films, a tanned edging is (Na2o+K2o)/SIO2 of [21], this rock is an alkaline easily observed. The portions of the isalteritic trachyte. blocks closer to the diaclasis are more hardened than those outlying; they have hardened and 3.1.2 Soils petrographic and physico- vitreous centimetric and millimetric domains chemical characteristics within. Some isalteritic blocks completely 3.1.2.1 Soils petrographic characteristics weathering are quite completely hardened. In the middle part of the toposequence, some particular Along the toposequence of Andosols developed masses are observed in the isalteritic horizon; on trachyte in the Western Highlands of they are in fact made of fine, joined and dense Cameroon, the number of soil horizons varies particules, isolating many holes. So, from the top with the topography (Fig. 2). Thus, in the upper to the bottom of the toposequence, one can part of the toposéquence (P1 and P2, seen on notice the disappearance of the horizon with the Fig. 2), the pedological cover shows six hardened centimetric materials along-side with different horizons, including from bottom to top: a the appearance of a light reddish brown horizon. light grey isalteritic horizon, a yellowish horizon, The middle part of the toposequence reveals the a reddish horizon, a horizon made of the presence of a less extended second horizon with juxtaposition of centimetric hardened materials, a hardened materials. All these horizons have as dark reddish brown horizon, and a dark grey parent rock the dark grey alkaline trachyte with a horizon with a well-developed crumby structure. fluid like aphanitic texture below. From this, two The middle part of the toposéquence (P3 on Fig. main groups of soils appear: soils with hardened 3) on its own is made of eight horizons organized matters from the middle of the toposequence as follow: a light grey isalteritic horizon, a towards uphill, and soils without hardened yellowish horizon, a reddish horizon, a horizon matters at the foot of the toposequence. The made of the juxtaposition of centimetric hardened group of soils with hardened matters can also be materials, a light reddish brown horizon, a subdivided into two subgroups: soils with one horizon made of the juxtaposition of centimetric level of hardened matters located uphill and soils hardened materials larger than the previous, a with two levels of hardened matters located in dark reddish brown horizon with crumby the middle part of the toposequence (Fig. 2). structure, and a dark grey horizon with a well- Further, the toposequence thickens from uphill to developed crumby structure. In the downhill downhill where the existent horizons reach their portion of the toposequence (P4 on Fig. 2), the highest thickness. five following horizons are present from bottom to top: a yellowish horizon, a reddish horizon, a light Studies were made deeper in the profile P1 (Fig. reddish brown horizon, a dark reddish brown 2), located at the summit of the toposequence,

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since its top position prevents it from external fairly abundant, resemble those noticed above. supply with matters. The studied profile is fresh, Skeleton grains and plasma are organized in an wet and shallow (about 2 m). It has six horizons agglomeroplasmic and locally porphyroskeletic organized as follow from top to bottom (Fig. 2): manner (Fig. 4). The boundary with the discontinuous hard pan below is abrupt, 0-20 cm: Strongly dark grey horizon (5YR2.5/1), materialized by the appearance of harden silty. Clearly crumby structure, with subspherical, particles. friable, fragile, highly porous, less coherent, light, and small (2 to 8 mm of diameter) aggregates. 40–55 cm: Shallow horizon (15 cm of thickness), Presence of many rootlet, constituting a real root constituted of discontinuous harden material. carpet. Two facies are present: the first one is flatty, hardened, with a rough surface, and the second Under optic microscope, the matrix background one is smooth, hardened, and roundish. The shows out a well organization, with an abundant, flatty, hardened, with a rough surface facies dark and isotic plasma, a skeleton made of shows out a size ranging from 7 to 10 cm in the subangular quartz grain, pile and tubular holes. longest axis. Their matrix is hardened, light grey Skeleton and plasma define an (10YR8/2) and sometimes dark pale yellow agglomeroplasmic and locally intertextic type of (2.5Y8/4), with white (10YR8/1) or purple red assembling (Fig. 3). The boundary with the (7.5YR6/6) millimetric domains, as well as dark horizon below is gradual and regular. brownish (7.5YR3/2) lines. The white domains (10YR8/2) are subspherical with clear contours, 20-40 cm: Dark reddish brown horizon (5YR3/4), meanwhile the purple red domains (7.5YR6/6) silty clayey. Vague crumby structure, with also show out vague shape and boundaries with the subspherical, harder, less porous, more light grey (10YR8/2) to dark pale yellow matrix coherent, heavier, denser, and larger (5 to 20 (2.5Y8/4). Locally, one can observe greyish mm of diameter) than the crumbs of the horizon (10YR8/2) stick like phantoms of feldspath, with above. There are few gravels (2% of the whole clear orientation, recalling those noticed in the horizon), characterized by an irregular shape, mother rock, in the light grey (10YR8/2) and brownish to dark grey, with average diameter sometimes dark pale yellow (2.5Y8/4) indurated ranging from 2 to 6 mm. Rootlet are fewer, matrix. Few empty holes, with irregular contours compared to that noticed in the previous horizon. and few holes filed with yellow (2.5Y8/4) or purple red (7.5YR6/6), silty materials, are also The microscopic analysis of the present horizon present. Concerning the indurated, roundish and reveals a well aggregated matrix background. smooth facies, it is made of larger volumes (≥ 14 The plasma, dark reddish brown, is abundant, cm in diameter), surrounded by the volumes of dense and isotic. The skeleton shows out in the previous facies. In the details, they are equal proportion quartz grain similar to those reddish (7.5YR6/6) and locally vitreous, massif, observed in the above horizon, and shadow of compact and more indurated. All over their sanidine pseudomorphosed by gibbsite. Holes, surface, some roundish vacuoles can be noticed.

Fig. 1. Microscopic organization of the trachyte of Meletan

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Legend:

Fig. 2. The toposequence of Andosols from Western Highlands of Cameroon

The central part of this vitreous matter is locally Some whitish and locally tanned (7.5YR3/2) and tanned (7.5YR3/2) and sometimes reddened sometimes reddened (7.5YR6/6) spots are (7.5YR6/6) in frame manner. The reddish noticeable. It is also the case with some frameworks (7.5YR6/6) are more abundant and yellowish (10YR7/8) spots that can be found dense compare to the brownish (7.5YR3/2) ones. disseminated in the whole matrix.

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Pile hole Isotic plasma

Quartz grain (skeleton) Tubular hole

0 2mm

Fig. 3. Microscopic organization of the puffy dark grey horizon

Sanidine phantom pseudomorphosed by a strongly birefringent microcristalline gibbsite (secondary skeleton) Pile hole

Dark reddish brown i soticisotic plasma plasma

Quartz grain (primary skeleton) Tubular hole

0 2mm

Fig. 4. Microscopic organization of the vaguely puffy dark brownish red horizon

Under light microscope, the indurated volumes mm in the long axis), angular, resistant and fairly show out three main domains: the largest one is porous aggregates. The pedoturbated matter is grey, with a strongly birefringent, abundant sticky and plastic. There are some few roots. (80%), and cristic plasma (80); the two other Towards the bottom of the present horizon, some ones, dark and red respectively, in equal areas are dark red (7.5YR7/3), wet, and less proportions, are both isotic. Particularly, volumes resistant compare to the rest of the pedoturbated from the rough facies show out many phantoms material of the same horizon, equally polyhedral, of sanidine and pyroxene, completely sticky and plastic. Some rare (3%), silty, whitish pseudomorphosed respectively by a highly (10YR8/2), flatty lithorelictual volumes are birefringent gibbsitic plasma for he sanidine, and disseminated in this horizon. The transition by isotic iron oxides for the pyroxene. Holes, between those lithorelictuel matters and the mostly elongated, are present, with relative pedoturbated one is brutal. abundance from a facies to another one (Fig. 5). Microscopically, the matrix background of the The transition with the horizon below is brutal, present horizon is largely constituted of a dense shown out by the disappearance of the indurated and weakly birefringent plasma, with a clayey volumes. aspic structure. The skeleton is made of relics of dense minerals, to which are associated 55–80 cm: Light red (7.5YR7/6) horizon, clayey. phantoms of sanidine pseudomorphosed by a Fine clear polyhedral structure, with fine (2 to 8 microcrystalline and highly birefringent gibbsitic

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plasma. The skeleton is drowned in the plasma background occupies 15 to 20% of the whole and define with it a porphyroskeletic like slide; it is composed of a weakly birefringent sail assembling (Fig. 6). The passing to the horizon like plasma. The skeleton is mainly sanidine below is progressive and undulated. minerals, partially pseudomorposed by a whitish grey clayey asepic plasma, and by dense 80-140 cm: Yellow (10YR7/8) horizon, silty relictual minerals. The whitish grey matrix clayey. Vague polyhedral structure. A little bit background is the most abundant amongst all porous. Presence of isalteritic volumes with size (70%). Its plasma is sail like, weakly birefringent increasing with the depth. and whitish. The skeleton appears here in the form of partially weathered sanidine and Microscopically, this horizon shows a matrix pyroxene partially pseudomorphosed by reddish background constituted largely by a dense, brown iron oxide. The purple red matrix weakly birefringent plasma, with clayey asepic background is the least (about 10% of the slide). structure. There is two group of skeleton. The It appears in forms of few red and translucent primary group is made of relics of dense minerals granulated ferruginous domains with vague and the secondary one of phantoms of sanidine contours, locally weakly birefringent and isotic. pseudomorphed by gibbsite. Few crack-like The skeleton here is made of pyroxene holes are present. Skeleton and plasma defines completely pseudomorphosed par iron oxides, a porphyroskeletic like structure (Fig. 7). The forming a partitioning network, within which some boundary with the horizon below is progressive, few holes can be observed (Fig. 8). materialized by the increasing in quantity and size of the isalteritic volumes. 3.1.2.2 Soils physico-chemical characteristics

140–200 cm: Whitish grey isalteritic horizon Granulometry: (10YR8/2), showing many isalteritic volumes, decimetric, joined, silty and soft, with The grain size characteristics of the studied soils sacchoroidal aspect inherited from the mother varies strongly all along the profile (Table 2). In rock, locally separated by a yellow, silty clayey the upper part of the profile, clays and silts show pedoturbated matter. Many reddened and out an antagonistic behavior. The behavior of sometimes tanned cracks travel all over their sands recalls that of silts. In the lower part, clays surface. Phantoms of feldspaths, tanned, and and silts have the same behavior, in the inverse sometimes pale yellow or pinkish, millimetric (2 manner when considering that of the sands. to 5 mm), stick like and preferentially orientated, Hence, in the vague polyhedral yellow horizon, as same as yellow (10YR7/8) silty clayey we have 40.80% of clays, 29.30% of silts, and pedoturbated zones with vague contours are 30% of sands. In the fine polyhedral red horizon, observed in the largely developed grey matrix. we have 43.20% of clays, 25.50% of silts, and 33.20% of sands. In the vague puffy dark reddish Microscopic observation of the slide from the brown horizon, we have 37.80% of clays, 20.40% present horizon revealed three main matrix of silts, and 41.90% of sands. In the clear puffy background: A yellowish brown, a light grey, and dark grey horizon, we have 17.80% of clays, a purple red one. The yellowish brown matrix 36.40% of silts, and 45.90% of sands.

Pyroxene phantom pseudomorphosed by iron oxide

Red isotic plasma Cristic grey strongly Grey stronglybirefringent birefrengent plasma cristic plasma

Dark isotic plasma Sanidine phantom pseudomorphosed Hole by a cristic plasma strongly birefringent 0 2mm

Fig. 5. Microscopic organization of the hardened matters

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Table 1. Chemical composition of the andosolic cover at the summit of meletan mount

SiO2 Al2O3 Fe2O3 TiO2 K2O MgO CaO Na2O MnO P2O5 H2O Total Humifere crumby strongly dark grey horizon 21.50 20.30 7.05 0.97 0.48 0.30 0.23 0.15 0.05 0.35 48.10 99.48 (0-20 cm) Dark reddish brown crumby horizon 23,80 34.00 8.75 1.08 0.60 0.29 0.08 0.20 0.30 0.32 30.20 99.62 (20–40 cm) Discontinuous Hardpan 2.88 55.88 11.68 0.74 0.06 0.15 0.01 0.00 0.27 0.53 26.82 99.02 (40–55 cm) Red Horizon 23.1 39.6 9.01 0.735 0.11 0.14 0.005 0.00 0.15 0.3 27.5 100.74 (55–80 cm) Yellow polyhedral horizon 23.50 39.10 8.94 0.73 0.12 0.13 0.00 0.00 0.14 0.29 28.00 100.95 (80–140 cm) Whitish isalteritic horizon 22.70 41.10 6.63 0.44 0.08 0.09 0.01 0.00 0.34 0.25 26.20 97.84 (140–200 cm) Parent rock 58.00 18.40 5.63 0.44 5.23 0.41 2.10 6.56 0.30 0.14 2.01 99.22

CrackCrak like like hole hole Relic of dense mineral

Sanidine phantom pseudomorphosed Clayey asepic dense and red plasma by gibbsite

0 2mm

Fig. 6. Microscopic organization of the fine polyhedral red horizon

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These specified a strongly clayey soils at the horizon). From the top to the bottom of the soil bottom of the chosen profile, and sandy at their profile, this amount decreases highly, reaching summit, where the highest concentrations of silts 3.04% in the vaguely puffy dark reddish brown are noticed. X-ray diffractions coupled with horizon, 0.82% in the fine polyhedral red horizon, microscopic characterizations revealed that and 0.54% in the vague polyhedral yellow those sandy particles are made of sanidine horizon. fragments and quartz grains. Nitrogen is quite absent in the lower part of the Bulk density (Da), real density (Dr), porosity: soil profile (0.04% in the fine polyhedral red horizon and 0.03% in the vaguely polyhedral The real density is high and constant all along yellow horizon), deficient in the vaguely puffy the profile (Table 2). It ranges in fact between 2.6 dark grey horizon (0.13%), and abundant in the at the bottom of the profile and 2.4 at the summit; puffy dark grey horizon (0.64%). this justifies the high porosity, with values ranging from 56.06% at the bottom to 73.64% at C/N ratio reaches 9 at the bottom of the soil summit. profile, and 11.75 at its summit (Table 2).

The bulk density of the mother rock is 2.5. It drops to 1.67 in the isalteritic horizon, 1.1 in the Acidity: vaguely polyhedral yellow horizon, 1.07 in the fine polyhedral red horizon, 1.0 in the vaguely The studied soils are highly acid at their surface puffy dark reddish horizon, and 0.64 in the puffy (pHwater=4.7 in the puffy dark grey horizon) but dark grey horizon where its lowest value is a little bit less towards their bottom (pHwater= de reached. 5.1 in the vaguely dark reddish brown horizon, 5.19 in the fine polyhedral red horizon, and 5.2 in Organic matter, nitrogen and C/N ratio: the vaguely polyhedral yellow horizon). ∆pH reaches 1 only in the vaguely puffy dark reddish The studied soils are rich in organic matter in brown horizon. This testimonies the absence of their upper part (15.04% in the puffy dark grey an acid stock [22,1].

Craky like hole

Phantom of sanidine Dense mineral pseudomorphosed by gibbsite

Clayey asepic dense yellow plasma

0 2mm

Fig. 7. Microscopic organization of the vague polyhedral yellow horizon

Weakly birefringent Relic of sanidine partially altered whitish grey plasma Weakly birefringent clayey asepic whitish plasma Weakly birefringent yellowish plasma Weakly birefringent red plasma

Pyroxene relics more or less 0 2mm pseudomorphosed by iron oxides

Fig. 8. Microscopic organization of the whitish grey isalteritic horizon

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Table 2. Physico-chemical characteristics of the studied soils

3 Horizons and depth (cm) Porosity Density (g/cm ) Granulometry (%) pH P2O5 (%) Dr Da Clay Silt Sand pHwater pHKCl ∆pH (ppm) Strongly dark grey 73.64 2.40 0.64 17.80 36.40 45.90 4.7 4.00 0.7 0.52 horizon (0-20 cm) Dark reddish brown 62.93 2.60 1.00 37.80 20.40 41.90 5.10 4.10 1.00 0.95 horizon (20-40 cm) Red fine polyhedral 58.85 2.60 1.07 43.2 25.5 33.2 5.19 4.35 0.84 1.44 horizon (55-80 cm) Yellow vague polyhedral 56.06 2.60 1.10 40.80 29.30 30.00 5.20 4.50 0.70 1.53 Horizon (80-140 cm)

Organic matter Exchangeable basic cations (meq/100 g of soil) CEC* S/CEC N (%) O.C. (%) O.M. (%) C/N Ca2+ Mg2+ K+ Na+ S (meq/100g of soil) (%) 0.64 7.52 15.04 11.75 0.30 0.19 0.13 0.31 0.93 31.36 2.96 0.13 1.52 3.04 11.60 0.30 0.16 0.24 1.58 2.28 20.40 11.10 0.04 0.41 0.82 10.25 0.19 0.12 0.08 1.59 1.98 18.5 10.70 0.03 0.27 0.54 9.00 0.20 0.10 0.08 1.59 1.96 17.60 22.14 CEC*: Calculated after the recommendations of [20] as follows CEC*= effective CEC + organic matter CEC

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Exchangeable cations and cation Isalteritic horizon: exchangeable capacity: Gibbsite is the principal mineral of this horizon In the whole profile, exchangeable cations are (Fig. 9 and Table 3). There are few quantities quite absent. Their sum remains lower than 2 of halloysite and trace of ilmenite, goethite, meq/100 g of soil in the whole profile (Table 2). quartz, magnetite, and sanidine. Moreover,

Concerning cation exchangeable capacity, its allophane is present. Compared to the mother value is averagely high in the upper part of the rock, Ilmenite and magnetite have remained soil profile (31.36 meq/100 g of soil), but constant (Table 3). Pyroxene and cristobalite decreases quickly with the depth. have disappeared; the quantity of sanidine has decreased; quartz, goethite, and gibbsite have Available phosphorous: appeared.

The available phosphorous is poorly Yellow horizon: concentrated in the whole profile. But, it tends to increase in quantity towards its bottom (Table 2). The mineralogical composition of the present horizon is quite similar to that of the isalteritic 3.1.2.3 Mineralogical and chemical horizon, with gibbsite as principal mineral. Small characteristics of soils quantities of halloysite, quartz, goethite, Mineralogical characteristics: magnetite, and rutile are detected (Table 3 and Fig. 9). Ilmenite and sanidine have disappeared. The mineralogical characteristics of the studied ATP reveals the presence of allophane here and soils were carried out using X-ray diffractions and confirms gibbsite as the most abundant mineral ATP (thermic and weighted analysis). (Table 3).

Strongly dark grey horizon

Dark reddish brown horizon

Discontinous hardened materials

Red polyhedral horizon

Yellow polyhedral horizon

Isalteritic whitish horizon

p A

A Mother rock

All?: Supposed allophane; San: sanidine; Cr: cristobalite; Ilm: ilmenite; Mg: magnetite Px: pyroxene;Qz: quartz; Gi: gibbsite; Hall: halloysite; Go: goethite; He: hematite; Ru:rutile Kaol: kaolinite; Ap: apatite

Fig. 9. X-rays diffractions spectra of the studied soil

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Table 3. Mineral proportions of the studied soils (They were calculated using ATP)

Horizons and depth Allophane Gibbsite Kaolinite Halloysite Goethite Others (cm) Strongly dark grey ++++ +++ ++ ? horizon (0-20 cm) Dark reddish brown +++ ++++ + ? horizon (20-40 cm) Red horizon fine ++ +++++ + ? polyhedral (55-80 cm) Yellow vague ++ +++++ + + ? polyhedral horizon (80-140 cm) +++++: Very abundant ++++: Abundant +++: Less abundant ++: The least abundant +: In traces ? Non determined

Red horizon: (41.10%Al2O3), silicon (22.70%SiO2), and iron

(6.63%Fe2O3). Alkali and earth alkali elements The mineralogical composition of this horizon is are absent. Few quantities of titane (0.44%TiO2), very close to that of the yellow horizon below. manganese (0.34%MnO), and phosphorous Gibbsite remains the principal mineral (0.25%P2O5) are detected (Table 2). Compared component, beside which there is significant to the mother rock, silicon decreases abruptly quantity of halloysite, few quantities of quartz, (from 58.00% to 22.70%SiO2), aluminum hematite, magnetite, allophane, and halloysite, increases abruptly (from 18.40 to 41%Al203), iron and very few quantities of rutile (Table 3 and Fig. increases moderately (from 5.63 to 6.63%Fe2O3) 9). (Table 1, Fig. 10).

Horizon with hardened fragments: All this is materialized in the SiO2-Fe2O3-Al2O5 The present horizon constitutes here a stone line triangular diagram by the shifting from the surmounting the red horizon (Fig. 2). They are siliceous pole towards the aluminous pole centimetric, whitish and strongly hardened. (Fig. 11). Mineralogically, their main components are Yellow horizon: gibbsite, goethite, and halloysite. This horizon surmounts the isalteritic horizon Dark reddish brown horizon: (Fig. 2). Here, aluminum remains the most The present horizon is mainly constituted by abundant chemical element (39.10%Al2O3), gibbsite and allophane, with few quantities of followed by silicon (23.50%SiO2), and iron kaolinite and quartz, and fewer quantities of (8.94%Fe2O3) (Table 1). Amongst alkali and magnetite, hematite, rutile and sanidine (Fig. 9 earth alkali elements, sodium and calcium are and Table 3). Compared to the yellow and red absent, there is few quantities of potassium (O. polyhedral horizons below, sanidine reappears, 12%K2O) and magnesium (0.13%MgO). Titan kaolinite appears, and hematite remains. (0.73%TiO2), phosphorus (0.29%P2O5), and manganese (0.14%MnO) are present in small 3.1.2.4 Dark grey horizon quantities. Compared to the horizon below, one can mention a little increasing of silicon (from The present horizon has quite the same 22.70 to 23.50%SiO2) and iron (from 6.63 to mineralogical composition with the dark reddish 8.94%Fe2O3) respectively, and a slight brown horizon (Table 3), with however the high decreasing of aluminum (from 41.10 to increasing of the amount of allophane, but 39.10%Al2O3) (Table 1 and Fig. 10). In the SiO2- moderate of gibbsite. Fe2O3-Al2O3 triangular diagram, isalteritic and vague polyhedral yellow horizons are quite 3.1.2.5 Geochemical characteristics of soils superimposed (Fig. 11).

Isalteritic horizon: Red horizon:

The principal chemical elements of the present The red horizon surmounts the yellow horizon horizon are respectively aluminum (Fig. 2). Aluminum remains the most abundant

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element (39.10%Al2O3), followed by silicon (8.75%Fe2O3). Few quantities of titane (23.50%SiO2) and iron (8.94%Fe2O3) (Table 1). (1.08%TiO2), potassium (0.60%K2O%), Concerning alkali and earth alkali elements, manganese (0.30%MnO), phosphorous sodium and calcium are absent; potassium (0.32%P2O5), magnesium (0.29%MgO), and (O.12%K2O) and magnesium (0.13%MgO) are sodium (0.20%Na2O) can be observed. calcium present in few quantities. There are limited is absent (Table 1). Compared to the yellow and quantities of titane (0.73%TiO2), phosphorous red horizons, great variations of the principal (0.29%P2O5), and manganese (0.14%MnO). element aren’t noticeable (Table 1 and Figs. 10 Compared to the yellow horizon, there is a slight and 11). increasing of silicon (from 22.70 to 23.50%SiO2) and iron (from 6.63 to 8.94%Fe2O3) amounts, Dark grey horizon: and a little decreasing of aluminum proportion (from 41.10 to 39.10%Al2O3) (Table 1). In the Silicon (21.50%SiO2) and aluminum SiO2-Fe2O3-Al2O3 triangular diagram, yellow (20.30%Al 2O3) are the most concentrated and red horizons are quite superimposed elements in this horizon. Iron (7.O5%Fe2O3) is (Fig. 11). present (Table 1). Beside these main elements, there are few quantities of titane (0.97%TiO2), Horizon with hardened fragments: potassium (0.48%K2O), phosphorous (0.35%P2O5), magnesium (0.30%MgO), calcium The present horizon constitutes here a stone line (0.23%CaO), and sodium (0.15%Na2O) surmounting the red horizon (Fig. 2). They are respectively (Table 1). Compared to the dark centimetric, whitish and strongly hardened. reddish brown horizon below, silicon (from 23.80 Chemically, their main element is aluminum to 21.50%SiO2) and iron (from 8.75 to (about 60.00%Al2O3,). Iron is significantly present 7.05%Fe2O3) amounts decrease slightly, (11.68%Fe2O3). The amount of the silicon is while the decreasing of alumina is more very low (2.88%SiO2) (Table 1). Potassium, significant (from 34.00 to 20.30% Al2O3) (Table 1 calcium, and sodium are absent. There are few and Fig. 10). On the SiO2-Fe2O3-Al2o3 quantities of phosphorous (0.53%P2O5) and triangular diagram, this is materialized by a slight magnesium (0.15%MgO) (Table 1). We are then shifting from alumina pole towards iron and in the presence of hardened aluminous silicon poles (Fig. 11). Concerning other fragments. chemical elements, a slight decreasing is detected for titane (from 1.08 to 0.97%TiO2), Dark reddish brown horizon: alongside to an increasing of calcium amount (from 0.08 to 0.23%CaO); magnesium and The chemical analysis of the present horizon phosphorous amounts remain constant, testimonies the dominance of aluminum while manganese has completely disappeared (34.00%Al2O3) on silicon (23.80%SiO2) and iron (Table 1).

Strongly dark grey horizon Dark reddish brown horizon Discontinuous hardened materials Red polyhedral horizon

Yellow polyhedral horizon

Whitish isalteritic horizon Mother rock Depth(cm)

Fig. 10. Fe-Si-Al vertical evolution in the andosolic cover at the summit of Meletan Mount

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Mother rock Whitish isalteritic horizon

Yellow polyhedral horizon Red polyhedral horizon Discontinuous hardened marterials Dark reddish brown horizon Strongly dark grey horizon

Fig. 11. Localization in the SiO2-Al2O3-Fe2O3 triangular diagram of the principal matters of the andosolic cover at the summit of Meletan Mount

4. DISCUSSION represent their major particularities. This type of disproportion between pedoturbated and alterite From uphill to downhill, the toposequence parts is common to temperate environment, becomes progressively thicker. This could be where the intensity of alteration process is often attributed to the permanent renewing of soils at low; it is then in accordance with the large the uphill position, subtracting matters that move development of proper soil compared to alterite downhill wards with the help of gravity; this as noticed by [27,1,28,29] amongst others. The motion supplies the lower part of the existence in the studied soils of a hardened level toposequence with matters coming upwards is common to hot and humid environments [13]. [23,24,25]; this corroborates the appearance of a This confirms the observations of [30,31,32, second horizon with hardened matters in the 28,33] amongst others. The lowest part of the middle part of the toposequence; it corroborates present soil profile conserves perfectly the also the presence at this position of masses mother rock organization; it is therefore an made of fine, dense and joined particles. In a isalterite [30]; in the upper part of this same practical aspect, this extraction of matters from lowest part of the soil profile, the progressive uphill contributes among other to the reduction of loose of the lithic organization is noticeable: that the thickness of the tillable land [26]. part is then known as an alloterite. Such a vertical succession is common in well drained Morphologically, the studied soils show out four soils from hot and wet environments [30,31,32]. principal levels of unequal thickness: an isalteritic The microscopic observations precise that level, a lower pedoturbated level, a discontinuous sanidine and pyroxene undergo pseudomorphic hardened level serving as a boundary between weathering; this confirms after [34,35,28], the the lower and the upper pedoturbated levels, and isalteritic characteristic of the lowest part of the the upper pedoturbated level. The lower present soil profile; we are then in the presence pedoturbated level consists from the bottom to of pseudomorphic transformations, commonly the top of the yellow (60 cm) and the red (25 cm) noticed at the bottom of ferallitic soils polyhedral horizons. The discontinuous hardened [34,30,32,28]. Those pseudomorphosis are level is the thinnest (15 cm). The upper accompanied by leaching, quite total for earth pedoturbated level is made of two horizons with alkali (Ca and Mg) and alkali (Na et K) elements, 20 cm of thickness each. The pedoturbated but partial for silicon; they are also accompanied level/alterite ratio is 125/60, about 2/1, and the by accumulations, strong for aluminum and weak hardened level/alterite, 15/60, is 1/4. In one for iron, principally in terms of gibbsite and iron hand, the disproportion between pedoturbated oxides (hematite and goethite) respectively. This part and the alterite, and in the second hand, the corresponds to a wonderful aluminum dynamic existence of a hardened level in the studied soils as currently noticed in bauxitic environments

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[36,35]. The granulometry of the yellow and red 56%Al2O3 in the hardened horizon) in this type of horizons reveals a clayey texture. This type of soils [36]. Concerning plasmic separations, which texture is common in well differentiated Andosols are strain cutanes, appearing during phyllites as this was shown by [4]. Moreover, this texture mineral reorientations under humectation- agrees with the clayey asepic structure of the desiccation cycles [39,38], their absence in the plasmas from those horizons [37,38]. pedoturbated level can be related to the non phyllitic nature of the principal plasmic In the upper part of the studied soil profile, the component of the soils, notably gibbsite. The pedoturbated part, bearing 1/5 of the whole soil scarcity of cracks in the yellow and red profile, exhibits its darkish coloration (dark polyhedral horizons, associated to the reddish brown at the bottom and strongly dark abundance of clayey asepic plasmas testimonies grey at the top), its clayey and silty clayey texture the lack of hydric strain at that particular level of in the same order, its high porosity and low the soil profile [31,32]. If the general density, commonly lower than 1 (from 0.7 to 0.8), characteristics of the upper Bambouto Mountains high amount of organic matter (15.05%), fragility, zone soils allow to classify them amongst thixotropy. It shows also its gradually clearly Andosols, one can note in the details, and puffy structure towards the surface. The base of principally in the lowest part, some morphological this darkened pedoturbated part is belted by a and analytic characteristics that bring those soils hardened discontinuous horizon acting as a close to transitional type of Andosols (intergrade) boundary. That hardened belt surmounts at its [1,4]. The occurrence of these transitional turn successively a red polyhedral and a vaguely characteristics within the soils could be related to polyhedral yellow horizons. Hardened, clayey a more accentuated soils differentiation, rarely and polyhedral horizons are commonly observed common in tropical Andosols [2,4], and this, as in ferallitic soils [34,31,32]. In opposite, the low the consequence of the trachytic nature and the bulk density, the darkened coloration, the puffy age (Miocene, from [9]) of the mother rock, more structure, the silty texture, the thixotropy, the resistant to weathering [1]. Such characteristics, fragility and the high amount of organic matter revealing a ferrallitisation [40], are in the mean to (15.05%) are common in Andosols [3,4,29,5]. characterize Bambouto Mountains andosols as «andic ferrallitic, desaturated, humic and strongly Micromorphologically, the lowest part of the rejuvenated soils» after [4]. These transitional studied soil profile lacks plasmic seperations, characteristics, of ferrallitic type, highly present in and contains only clayey asepic plasmas in the the southern limb of Bambouto Mountains, could red and yellow horizons, and cristic gibbsitic in be due to the nature and age of the mother rock, the alteritic horizon. This last point confirms compact and older than usual, and an intense mineralogically the net dominance of gibbsite in alteration in this limb, as a consequence of a the mineralogical paragenesis on iron oxides high pluviometry. Finally, if we focus on the three (hematite and goethite) and clays (kaolinite). essential evolution process of andic soils as Concerning the upper part, it shows isotic suggested by [1], the present study confirms the plasma, assessing the high concentration of restricted clays genesis and the major organic matter at this level of the whole profile; crystallization of gibbsite in these present soils. this part is in terms of consequence highly However, the speed of organic matter concentrated in allophane, followed by gibbsite in mineralization has not been taken into account in the strongly dark grey horizon, but by gibbsite, the case of this present study. followed by allophane in the dark reddish brown horizon. The hardened belt, acting as a boundary 5. CONCLUSION in this soil profile, presents in majority cristic gibbsitic plasma, locally covered by reddish and Soils with andic characteristics were studied in dark plasmas, both isotic. This assesses the case of the present work. They are risen on mineralogically the particular predominance of an alkaline trachyte from Miocene, in a wet gibbsite in mineral paragenesis, followed very far tropical Mountainous zone. These soils are thin, by hematite. highly differentiated, with many local specificities. Their organization and their thickness vary from The dominance of gibbsite in these soils, up to downhill. Signs of matters motions are particularly in the hardened belt, and at its base, recurrent. They are strongly darkened at their bring them very close to gibbsitic ferallitic soils, surface and light at their bottom, with the as characterised previously by [1], implying then presence of a discontinuous hardened level the lack of silicon (less than 3%SiO2 for about between those two parts, less dense (Da from

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0.7 to 1), porous, with an alteration level with materials. Experimental recognition of their more or less conserved lithic structure from the behavior. Thesis Doct. Sc., Paris VII. bottom to the top, surmounted by a pedoturbated 1984;194. level showing a high structural differentiation. 4. Quantin P. Soils of the volcanic Pedoturbated level/alterite ratio is about 2/1 and archipelago of the New Hebrides hardened level/alterite ratio is 1/4. (VANUATU). Study of the initial Micromorphologically, these soils have at their pedogenesis in a tropical environment. bottom pseudomorphic transformations, Thesis. Doct. Sc. Strasbourg and Mém. testimonying their isalteritic aspect, meanwhile in ORSTOM. 1992;498. their pedoturbated part, cutans are absent within 5. Tematio P, Kengni L, Bitom D, Hodson M, plasmas, in strict relationship with the cristic Fopoussi JC, Leumbe O, Mpakam HG, gibbsitic nature of the plasmas. The gibbsitic Tsozue D. Soils and their distribution on nature of those plasmas bring them close to bamboo volcanic mountain, West ferallits gibbsitic soil types. Clays, abundant all Cameroon highland, Central Africa. Art., along the studied profile, are associated with Elsevier, J.A.E.S. 2004;447-457. silts, but mostly with sands at the soil surface. 6. Soil Survey Staff - Soil Taxonomy, Second Mineralogically, gibbsite, halloysite and Edition. USDA-NRCS, Agriculture allophane, in this order, are the most abundant Handbook. 1999;436. minerals at the bottom of the soil profile; at the 7. Colmet-Daage F, Kimpe C, Delaune M, top however, it is respectively allophane, Sieffermann G, Gautheyrou JM, Gun G, kaolinite and gibbsite association. Other minerals Koukoui M. Characteristics of some soils such as goethite, quartz, rutile are also present; of Ecuador derived from volcanic ash. Part at the summit, hematite replaces goethite. III: Comparison of the evolution of some Geochemically, aluminum is the most abundant soils of the tropical warm and cold chemical element, followed respectively by temperate climates of altitude. Cah. silicon and iron. Iron is less concentrated. The ORSTOM., Ser. Pedol. 1969;7(4):495-560. presence of andosolic and ferrallitic 8. Tematio P, Tsafack EI, Lucas Kengni. characteristics in these soils represents then Effects of tillage, fallow and burning on their particularity. They are then named «andic selected properties and fertility status of ferallitic, desaturated, humic and strongly andosols in the Bambouto Mounts, West rejuvenated soils». Cameroon. Agricultural Science. Flight. 2011;2(3):334-340. 6. RECOMMANDATIONS 9. Tchoua F. Contribution to the geological and petrographic study of some volcanoes To protect this fragile environment, farmers must of the Cameroon Line (Mount use farming techniques that reduce the rate of Manengouba and Mount Bambouto). soils motion. This implies ridges parallel to the Thesis Doct. State Sc., Univ. Clermont contour lines, alternated crops bands, groves, Ferrant. 1974;347. amongst other. 10. Valet S. Explanatory notes on climate maps, agro-geological landscapes and COMPETING INTERESTS proposals for developing the agro- geological landscapes of western Authors have declared that no competing Cameroon 1/200 000 CIRAT-IRAT. interests exist. Montpellier. 1985;120. 11. Fopoussi Tuebue JC. Petrology of the REFERENCES andosols of the western highlands of Cameroon and fertilization test: Case of a 1. Duchaufour P. Pedology: 1-pedogenesis toposequence developed on trachyte of and classification. Doc. 2nd Ed. Masson, the upper zone of the southern slope of the Paris. 1983;491. Bambouto Mountains. Doctoral Thesis. 2. Sieffermann G. Soils of some volcanic Department of Earth Sciences, Faculty of regions of Cameroon. Pedological and Science, University of Ngaoundere. mineralogical variations of the tropical 2018;233. environment. Thesis Doct. Sc. Strasbourg, 12. Morin S. The fundamental dissymmetries and Mém. ORSTOM. 1973;66:183. of the highlands of Western Cameroon and 3. Rosello V. Mineralogical and micro- their consequences on human occupation. structural characterization of andosolic Example of Bambouto Mountains. Man

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