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Physicochemical characteristics, degradation rate and vulnerability potential

of Mount Bambouto soils in Western Highlands of Cameroon

Article · December 2016

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SYLLABUS

REVIEW Science Series

RESEARCH ARTICLE - GEOLOGY

Physicochemical characteristics, degradation rate and vulnerability potential of Mount Bambouto soils in Western Highlands of Cameroon Désiré Tsozué1* and Primus Tamfuh Azinwi2

1 Department of Earth Sciences, Faculty of Sciences, University of Maroua, P.O. Box 814 Maroua, Cameroon. 2 Department of Soil Sciences, Faculty of Agronomy and agricultural Sciences, University of Dschang P.O. Box 222, Dschang, Cameroon. *Corresponding author: Tsozué Désiré, E-mail : tsozudsir@yahoo.fr Tél. 00237675121489

Received: 30 November 2015 / Revised: 30 March 2016 / Accepted 17 May 2016

Ecole Normale Supérieure, Université de Yaoundé I, Cameroun

______________________________________________________________________________________________________________

Abstract

The objective of this study was to determine the physicochemical properties, soil degradation rate (SDR) and vulnerability potential (Vp) of soils in the upper part of the eastern slope of the Bambouto Mountains. Thus, soil samples were collected in order to determine bulk density, particle size distribution, pH, exchangeable bases, cation exchange capacity, organic carbon, total nitrogen and available phosphorus (P). The SDR or Vp was assessed by assigning parametric values to levels of texture, bulk density, pHH2O, organic carbon, total nitrogen, available P, and base saturation percentage. The main results indicate that bulk density values range from 0.55 to 0.88 g/cm3. The soil texture is dominated by the silt fraction. The soil pH ranges from 4.2 to 5.5 with a mean value of 5.02. Exchangeable bases, cation exchange capacity and base saturation values are low, ranging respectively between 0.16 and 8.03 cmol.kg-1, 12.11 and 43.34 cmol.kg-1 and 1.14 and 15.09%. The organic carbon ranges from 2.52 to 15.80%, with an average value of 9.26% for the entire area. Total nitrogen and available phosphorus vary respectively from 0.26 to 0.80 % and 0 to 22.50 ppm. The SDR/Vp weighted value for pHH2O and available P was 4/2, showing severe degradation or vulnerability while that of base saturation was 5/1, characteristic of extreme degradation or vulnerability. The problem of soil acidity could be solved by restoring cation balance through fertilization and liming. The supply of Ca2+, Mg2+ and phosphorus fertilizer would enhance the base saturation and increase phosphorus availability in the soil. Keywords: Physicochemical characteristics, degradation rate, vulnerability potential, Bambouto Mountains, Cameroon.

Résumé

L’objectif de la présente étude était de déterminer les propriétés physicochimiques, le taux de dégradation et la vulnérabilité potentielle des sols dans la zone haute du versant est des monts Bambouto. Ainsi, des échantillons de sol ont été collectés pour déterminer la densité apparente, la granulométrie, le pHH2O, le carbone organique, l’azote total, les bases échangeables, la capacité d’échange cationique et le phosphore assimilable. Le taux de dégradation ou la vulnérabilité potentielle des sols est obtenu en assignant des valeurs paramétriques à la texture, densité apparente, pHH2O, carbone organique, azote total, phosphore assimilable et au taux de saturation en bases. Les résultats montrent que la densité apparente varie entre 0,55 et 0,88 g/cm3. La texture du sol est dominée par la fraction limoneuse. Le pH varie entre 4,2 et 5,5, avec une valeur moyenne de 5,02. Les teneurs en bases échangeables, la capacité d’échange cationique et le taux de saturation en bases sont bas, compris respectivement entre 0,16 et 8,03 cmol.kg-1, 12,11 et 43,34 cmol.kg-1 et 1,14 et 15,09 %. La gamme de carbone organique est de 2,52 à 15,80 %, avec une valeur moyenne de 9,26% pour l’ensemble de la zone d’étude. L’azote total et le phosphore assimilable varient respectivement de 0,26 à 0,80 % et de 0 à 22,50 ppm. Les sols sont sujets à une dégradation ou une vulnérabilité potentielle sévère (SDR/Vp = 4/2) due au pH et au phosphore assimilable et extrême (SDR/Vp = 5/1) due au taux de saturation en bases. Le problème d’acidité des sols pourrait se résoudre par la restauration de la balance cationique à travers la fertilisation et le chaulage. L’apport de Ca2+, Mg2+ et des fertilisants phosphorés pourrait améliorer le taux de saturation en bases et augmenter la disponibilité du phosphore dans le sol. Mots clés : caractéristiques physicochimiques, taux de dégradation, vulnérabilité potentielle, monts Bambouto, Cameroun.

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Syllabus Review, Sci. Ser. 6 (2016): 46 – 57

ISSN 2409-319X

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1. Introduction

Soil degradation has severe negative impacts on food security, environment quality and living standard (Mulumba, 2004). It remains a serious problem in most developing countries especially in sub-Saharan Africa because of the decrease in soil fertility and the negative nutrient balance exacerbated by soil erosion (Mulumba, 2004). A continued and ever increasing threat to productivity, food security and environmental quality in the tropics, especially in ecologically-sensitive ecoregions characterized by fragile soils still prevails (Lal, 1994; Mulumba, 2004). Soil quality is thus fundamental for sustainable agriculture development (Dumanski and Pieri, 2000; Tematio et al., 2011). Land should be used based on its capacity to meet human needs and ensure the sustainability of ecosystems (Amiri and Shariff, 2012). The land use and management systems strongly influence soil quality expressed by changes in soil chemical and physical properties (organic matter content, CEC, sum of exchangeable cations, acidity, bulk density, aggregates stability, etc.) (Tematio et al., 2011). Thus, improper land use and management systems reduce soil fertility and subsequently food security (Oldeman et al., 1991; Tematio et al., 2011). In the Bambouto Mountains, a volcanic massif of the West Cameroon Highlands, strong human pressure on lands expressed by the tillage and burning management systems have affected the agro-ecosystems over the past 30 years (Tematio and Olson, 1997; Tematio et al. 2011). One of the consequences is the overexploitation of soil resources with subsequent crop yields decrease (Tematio et al., 2011; Tsozué et al., 2015). In the eastern slope in particular, the high population pressure and the exposition of this slope to the harmattan wind which blows from the North East are essential causes of land degradation (Tsozué et al., 2015). This explains the poor agricultural yields, the extension of cultivated areas, the movement of populations towards more fertile soils in the upper part of the mountain, above 2000 m of altitude, formerly devoted to cattle rearing (Tsozué et al., 2015). Ngoufo (1988) reported that in 1968, more than 50% of the eastern slope of the Bambouto Mountain was occupied by grazing land and today only some rare relicts subsist in the upper zone of the massif. The high population pressure and the

intensification of agricultural activities without any conservation measures in the upper part of this eastern slope may enhance degradation of soils in the area. The objective of this study is to evaluate the degradation rate and vulnerability potential of soils, and suggest ameliorative measures for improve soils productivity in the upper part of the eastern slope of the Bambouto Mountains.

2. Material and methods

2.1. Description of the study site

Mounts Bambouto constitute the third largest volcano (800 km2) of the Cameroon Line after Mounts Cameroon and Manengouba (Fig. 1). This massif is situated between longitudes 09°33'E and 10°13'E and latitudes 05°30'N and 05°50'N. Volcanic products of the massif are composed of basalts, basanites, trachytes, phonolites, rhyolites, tephrites, hawaiites, mugearites, tuffs and ignimbrites (Marzoli et al., 1999; Nono et al., 2004; Gountié et al., 2012), largely dominated by basalts and trachytes. Their ages range from 21.12 Ma to 0.50 Ma (Youmen et al., 2005; Nkouathio et al., 2008; Kagou Dongmo et al., 2010). The study area corresponds to the upper part of the eastern slope of this massif, above 1800 m of altitude. The climate, which is pseudotropical with temperate characteristics due to altitude (Morin, 1988), is fresh and humid, characterized by frequent mists and fogs which impede visibility. Temperatures vary from 10-12°C and mean annual rainfall is 2507 mm. Relief is uneven and natural vegetation is a lawn grass of Sporobolus prairies typical of temperate environments. This vegetation is strongly degraded by human activities. The main soil types identified in this part of the Bambouto Mountains are Andosols (Tematio et al., 2004; Tsozué et al., 2009).

2.2. Experimental design and soil sampling

Ten soil samples were collected at 0-15 cm depth in ten locations, globally on basalts and trachytes, with five soil samples on each rock. At each sampling point, information on elevation, position and shape of slope were collected (Fig. 2 and Table 1). Each soil sample was stored in a well labelled polyethylene bag and transported to the laboratory for analysis. In the laboratory, analyses were carried out on the fine fraction, and include bulk density,

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Table 1. Particle size distribution of soils in the studied site.

Sample

Characteristics of the sample area Effective rooting depth

(cm)

Sampling depth (cm)

Bulk density (g/cm3)

Sand (%)

Silt (%)

Clay (%)

Texture

1 Summit of interfluve on trachyte 48 0-15 0.88 32.00 51.00 17.00 Silt loam

2 Midslope on trachyte > 150 0-15 0.76 39.00 40.00 21.00 Loam

3 Footslope on trachyte > 150 0-15 0.80 37.30 41.60 21.00 Loam

4 Summit of interfluve on basalt 50 0-15 0.65 38.34 27.63 34.03 Clay loam

5 Midslope on basalt > 150 0-15 0.61 56.70 25.30 13.00 Sandy loam

6 Footslope on basalt > 150 0-15 0.75 43.34 39.64 17.02 Loam

7 Summit of interfluve on basalt > 150 0-15 0.64 4.00 75.00 21.00 Silt loam

8 Midslope on basalt > 150 0-15 0.55 8.00 73.00 19.00 Silt loam

9 Summit of interfluve on trachyte 105 0-15 0.87 6.00 62.00 32.00 Silty clay loam

10 Midslope on trachyte > 150 0-15 0.79 6.00 64.00 30.00 Silty clay loam

Minimum / / 0.55 4.00 25.30 13.00 /

Maximum / / 0.88 57.70 75.00 34.03 /

Mean / / 0.73 27.068 49.917 22.505 Loam

Coefficient of variation (CV) (%) / / 14.60 67.40 34.00 29.80 /

Standard deviation (SD) / / 0.106 18.244 16.972 6.710 /

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Altitude < 400 m

Altitude between 400 and 800 m

EquatorialGuinea Gabon Congo

Campo

DOUALA MbamMinkom

Ngov

ayan

g

SanagaMt Cameroon

Bamendjingdam

Donga

Katsina

South Cameroon plateau

Mbakaodam

Mt Gotel

Tchabal Mbabo

Lagdodam

Hosséré Vokré

Garoua

Bénoué

Kaélé

Maroua

Mt Mandara

Logone

Chari

Kousséri

Chad

CentrafricaRepublic

Nigeria

N

12

10

8

6

4

2

161412108

0 100 200 km

Nyong

Dja

Boumba

Ngoko

TownCampo

Key

AtlanticOcean

Chad lake

Study site

Sanaga

River

Altitude between 800 and 2000 m

Altitude > 2000 m

0 1200 km

Cameroon

Mt Bambouto

Western

High

land

s

Adamaoua

Africa

Figure 1. Location of the study area.

Basalts

Trachytes

1

2

3

45

6

7

8

9

10

1800

1825

2082

2074

10°05’30” 10°07’00”

10°05’30” 10°07’00”

5°43’44”

5°42’30” 5°42’30”

5°43’44”

Key

River

Toposequence

1800 Contour line

10 Sampling point

2082Spot height

0 0.27 0.54 km

N

Figure 2. Location of sampling points.

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particle size distribution, pH, exchangeable bases, cation exchange capacity (CEC) at pH 7, organic carbon, total nitrogen and available phosphorus. Bulk density was determined by the cylinder method (Grossman and Reinsch, 2002). For soil texture analysis, soil organic matter and carbonates were removed with hydrogen peroxide and diluted hydrochloric acid, respectively. Then, soil samples were dispersed with sodium hexametaphosphate and particle size distribution was analysed by the Robinson pipette method. Soil pH was measured potentiometrically in a 1:2.5 soil : solution ratio. Exchangeable bases and CEC were determined using atomic absorption spectrophotometry in a solution of ammonium acetate at pH 7. Total nitrogen was obtained after heat treatment of each sample in a mixture of concentrated sulfuric acid and salicylic acid. The mineralization was accelerated by a catalyst consisting of ironsulfate, selenium and potassium sulfate. The mineralization was followed by distillation via conversion of nitrogen into steam in the form of ammonia after alkalinization of mineralized extract with NaOH. The distillate was fixed in boric acid (H3BO3) and then titrated with sulfuric acid or diluted hydrochloric acid (0.01 N). Organic carbon (OC) was determined by the Walkley–Black method (Walkley and Black, 1934). Soil organic matter (OM) content was obtained by multiplying soil organic carbon content by 1.724 (Walkley and Black, 1934). Available phosphorus was determined by Bray-2 method (Bray and Kurtz, 1945). The structural stability index (SI) was estimated using the following equation (Pieri, 1992):

1.724100;0

( )

OCSI X SI

Silt Clay

where OC (wt.%) is soil organic carbon content and (Silt+Clay) (wt.%) is the soil's combined silt and clay content. An SI > 9% indicates stable structure, 7% < SI ≤ 9% indicates low risk of structural degradation, 5% < SI ≤ 7% indicates high risk of degradation, and SI ≤ 5% indicates structurally degraded soil.

2.3. Soil Degradation Rating (SDR)/Vulnerability Potential (Vp)

The rating scheme for soil degradation developed by Lal (1994) and Akpan-Idiok (2012) for soil physicochemical properties,

namely texture, bulk density, pHH2O, total nitrogen, organic carbon, available P and base saturation were used in this study. The vulnerability potential of these properties was also determined. For the SDR, the weighting sequence was as follows: 1 = none, 2 = slight, 3 = moderate, 4 = severe and 5 = extreme. In this way, good soils have the lowest SDR and poor soils the highest value. For the Vp, the weighting was the reverse as follows: 5 = very low, 4 = low, 3 = medium, 2 = high and 1 = very high (Lal, 1994; Akpan-Idiok, 2012). The cumulative rating index corresponds to the sum of different SDR weighting values (Lal, 1994).

2.4. Statistical Analysis

The data collected were analyzed using descriptive statistics (mean, standard deviation and coefficient of variation) with the help of XLSTAT version 2008.6.03.

3. Results

3.1. Bulk density, particle size distribution and structural stability index

Bulk density values range from 0.55 to 0.88 g/cm3, with a mean value of 0.73 g/cm3 (Table 1). They are slightly variable with a coefficient of variation (CV) of 14.60%. Clay contents are moderately variable (CV = 29.80%) and range from 13 to 34%, with a mean value of 22.50%. Silt contents are also moderately variable (CV = 34%). They vary between 25.30 and 75% with a mean value of 49.91% (Table 1). Sand fractions range from 4 to 57.70%, with a mean value of 27.06%. They are highly variable (CV = 67.40%). Globally, the soil texture is dominated by the silt fraction, giving a dominant loamy textural class (Table 1). Except for one soil sample (sample number 4) whose structural stability index value is 7.04%, all the other values were higher than 9%, ranging between 9.80 and 47.39%, with a mean value of 23.74% and a CV of 48.70% (Table 2).

3.2. Chemical Characteristics

The soil pH ranges from 4.2 to 5.5 with a mean value of 5.02 (Table 2). These values are slightly variable (CV = 7.80%). Exchangeable bases values are very low (Ca: 0.04-5.02 cmol.kg-1; Mg: 0.06-1.23 cmol.kg-1; K: 0.05-1.63 cmol.kg-1; Na: 0.01-0.15 cmol.kg-1).

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Table 2. Chemical properties of soils in the studied site.

Sample pHH2O Ca2+ Mg2+ K+ Na+ CEC BS OC OM N C:N

Ratio Avail. P

SI

Ca : Mg Ratio

Mg : K Ratio

(cmol.kg-1) (%) (ppm) (%)

1 5.5 0.22 0.22 0.41 0.084 22.38 4.16 5.62 9.68 0.451 12.5 0.00 14.24 1.00 0.54

2 5.2 2.91 1.23 0.44 0.014 39.95 11.51 10.17 18.26 0.635 16.0 2.95 28.74 2.37 2.80

3 4.7 1.30 0.71 0.12 0.014 41.07 5.21 10.59 18.0 0.651 16.3 0.47 29.17 1.83 5.92

4 4.8 0.29 0.10 0.10 0.010 12.11 4.13 2.52 4.34 0.265 9.5 0.94 7.04 2.9 1.00

5 5.4 2.29 1.10 0.05 0.014 40.04 9.63 10.53 18.15 0.778 13.5 3.63 47.39 2.08 22.00

6 5.4 5.02 1.17 0.33 0.022 43.34 15.09 10.79 18.60 0.806 13.4 1.23 32.82 4.29 3.55

7 5.2 1.06 0.32 1.63 0.15 36.00 8.78 11.30 19.50 0.46 24.4 22.5 20.31 3.31 0.20

8 4.7 0.04 0.06 0.23 0.09 36.80 1.14 15.80 27.24 0.778 20.31 12.60 29.61 0.67 0.26

9 4.2 0.68 0.25 0.75 0.15 33.68 5.46 5.33 9.20 0.36 14.80 12.60 9.80 2.72 0.33

10 5.1 0.18 0.29 0.81 0.11 31.61 4.11 10.00 17.24 0.80 21.5 16.80 18.34 0.62 0.36

Minimum 4.2 0.04 0.06 0.05 0.01 12.11 1.14 2.52 4.34 0.265 9.50 0.00 7.04 0.62 0.20

Maximum 5.5 5.02 1.23 1.63 0.15 43.34 15.09 15.80 27.24 0.806 24.40 22.50 47.39 4.29 22.00

Mean 5.020 1.399 0.545 0.487 0.066 33.698 6.922 9.265 16.021 0.598 16.221 7.372 23.746 2.179 3.696

CV (%) 7.80 107.90 80.70 93.10 83.40 27.20 57.90 38.60 38.60 31.80 26.70 103.90 48.70 51.90 172.10

SD 0.392 1.510 0.439 0.453 0.055 9.166 4.008 3.576 6.184 0.190 4.331 7.659 11.564 1.131 6.361

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Table 3. Pearson correlation matrix for linear relationships between selected soil parameters in the studied site.

Variables pHH2O Ca2+ Mg2+ K+ Na+ CEC BS OC OM N C:N Avail. P SI Ca : Mg Mg : K

pHH2O 1

Ca2+ 0.436 1

Mg2+ 0.465 0.890* 1

K+ 0.017 -0.160 -0.265 1

Na+ -0.310 -0.499 -0.613 0.794* 1

CEC 0.096 0.608 0.666* 0.075 -0.056 1

BS 0.510 0.945* 0.859* 0.086 -0.351 0.533 1

OC 0.150 0.223 0.256 0.087 0.047 0.757* 0.114 1

OM 0.158 0.234 0.271 0.090 0.041 0.761* 0.129 0.999* 1

N 0.363 0.430 0.505 -0.225 -0.245 0.714* 0.274 0.795* 0.794* 1

C:N -0.043 -0.227 -0.208 0.719* 0.622 0.409 -0.137 0.669* 0.668* 0.330 1

Avail. P -0.233 -0.374 -0.436 0.828* 0.840* 0.131 -0.217 0.344 0.341 0.018 0.861* 1

SI 0.427 0.576 0.729* -0.337 -0.467 0.754* 0.475 0.689* 0.690* 0.789* 0.088 -0.194 1 Ca : Mg 0.069 0.682* 0.425 0.188 -0.170 0.167 0.752* -0.212 -0.208 -0.246 -0.247 -0.111 0.024 1

Mg : K 0.326 0.346 0.584 -0.435 -0.494 0.356 0.331 0.158 0.154 0.389 -0.271 -0.325 0.777* 0.046 1

* Significant at p < 0.05

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They are highly variable (CV > 35%). Cation exchange capacity values are low (21 cmol.kg-1 < CEC < 40 cmol.kg-1) to medium (41 cmol.kg-1 < CEC < 60 cmol.kg-1) and sometimes very low (CEC < 20 cmol.kg-1), with a mean value of 33.69 cmol.kg-1. Base saturation values are also very low. They vary from 1.14 to 15.09% (Table 2). The range in organic carbon is from 2.52 to 15.80%, with an average value of 9.26% for the entire area. Total nitrogen varied between 0.26 to 0.80% with a mean value of 0.59%. Available P varies from 0 to 22.50 ppm, with an average value of 7.37 ppm. Globally, the values of bases, base saturation and available P are highly variable (CV > 35%) while those of CEC, organic carbon or organic matter and total nitrogen were moderately variable (15 < CV < 35%) (Table 2). The correlation coefficients of different soil properties are presented in Table 3. There is no significant (p < 0.05) correlation between soil pH or base saturation percentage and other soil parameters (Table 3). There are significant correlations between Ca2+ and Mg2+ (r = 0.89, p < 0.05) and base saturation percentage (r = 0.94, p < 0.05). Mg2+ exhibits a significant positive correlation with CEC (r = 0.66, p < 0.05), base saturation percentage (r = 0.85, p < 0.05) and the structural stability index (r = 0.72, p < 0.05). K+ is significantly correlated with Na+ (r = 0.79, p < 0.05) and available P (r = 0.82, p < 0.05) while Na+ is rather significantly correlated with available P (r = 0.84, p < 0.05). A significant positive correlation is noted between CEC and OC (r = 0.75, p < 0.05), N (r = 0.71, p < 0.05) and SI (r = 0.75, p < 0.05), between OC and N (r = 0.79, p < 0.05) and SI (r = 0.68, p < 0.05) and between N and SI (r = 0.78, p < 0.05).

3.3. Mineralization/Nutrient Availability Ratios

3.3.1. Carbon-Nitrogen Ratio

Values of carbon-nitrogen ratio are low to very high, ranging between 9.50 and 24.4, with a mean value of 16.22 (Table 2). They are moderately variable (15 < CV < 35%) (Table 2). Carbon-nitrogen ratio exhibits a significant positive correlation with available P (r = 0.86, p < 0.05), K+ (r = 0.79, p < 0.05) and OC (r = 0.66, p < 0.05) (Table 3).

Table 4. Rating scheme for soil degradation rates (SDR) and vulnerability potential (Vp) of selected soil quality indicators.

Soil parameters Mean SDR Vp SDR/Vp

Texture Loam 1 5 1/5

Bulk density (g/cm3) 0.73 1 5 1/5

pHH2O 5.02 4 2 4/2

Organic carbon (%) 9.265 1 5 1/5

Total nitrogen (%) 0.598 1 5 1/5

Available P (ppm) 7.372 4 2 4/2

Base saturation (%) 6.922 5 1 5/1

1 = none. 2 = slight. 3 = moderate. 4 = severe. 5 = extreme for SDR. 5 = none. 4 = low. 3 = moderate. 2 = high. 1 = very high for Vp.

3.3.2. Magnesium-Potassium (Mg:K) Ratio

Mg:K ratio is very low (Mg:K < 2) to optimal (2 < Mg:K < 5) except for one soil sample where it is very high (15 < Mg:K < 25), when compared to the critical levels for productive soils. Mg:K ratio varies globally from 0.2 to 22, with a mean value of 3.69. They are highly variable (CV > 35%) (Table 2). There is a significant positive correlation between Mg:K ratio and the structural index stability (r = 0.77, p < 0.05) (Table 3).

3.3.3. Calcium-Magnesium (Ca:Mg) Ratio

The Ca:Mg ratios are low (1.83 to 3.31) to very low (0.62) except for one soil sample where this ratio is moderately high (4.29), when compared with a normal range for productive soils, with a mean value of 2.17 (Table 2). Ca:Mg ratios are highly variable (CV > 35%) (Table 2). There exists a significant positive correlation between Ca:Mg ratio and Ca2+ (r = 0.68, p < 0.05) and base saturation percentage (r = 0.75, p < 0.05) (Table 3).

3.4. Soil Degradation Rating (SDR)/Vulnerability Potential (Vp)

Seven physicochemical properties were used as soil quality indicators to assess soil degradation rate (SDR) and vulnerability potential (Vp). Soil quality shows varied potential for degradation and vulnerability potential (Vp) of the soils. The SDR/Vp weighted value for texture, bulk density, organic carbon and available P is 1/5 (Table 4). This value shows very low

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degradation or vulnerability. The SDR/Vp weighted value for pHH2O and available P is 4/2, showing severe degradation or vulnerability (Table 4). The SDR/Vp weighted value for base saturation percentage was 5/1, characteristic of extreme degradation or vulnerability (Table 4). Globally, the sum of different weighting factor gave rise to a cumulative rating index of 17 based on seven soil properties.

4. Discussion

The studied soils are characterized by low bulk density (0.73g/cm3) and a loamy texture. The low bulk density is indicative of the andosolic nature of these soils (Tematio et al., 2004). It might be due to more ground biomass input in the form of leaf (Datta et al., 2015). High fine particles (silt + clay) content might be due to the absence of translocation of finer particles from the surface horizons. The structural stability index is high, indicating a stable structure (Pieri, 1992). All soil samples have pHH2O less than the critical level (6.5-8.5) given by Landon (1991). They have acidic pH, less or equal to 5.5. According to Landon (1991) and Tisdale et al. (1993), most nutrients for field crops are available at pH above 5.5. The lowest values of soil pH in the studied soils could be because of loss of basic cations down the soil profiles even beyond sampling depth through leaching and runoff generated from accelerated erosion (Nigussie and Kissi, 2012). The loss of basic cations enhances the activity of Al3+ and H+ in the soil solution, which reduces soil pH and thereby increases soil acidity (Nigussie and Kissi, 2012). Some cropping systems in the studied area might also have an acidifying effect on the soil in relation to the amount of materials removed at harvest, amount and type of fertilizers normally used and the amount of leaching that occurs (Mulumba, 2004). The low pH coefficient of variation marks the predominance of climatic conditions characterized by high rainfall, which might have accelerated depletion of basic cations. The low CEC values of the soil are probably related to the amount and nature of the clay. Fasil and Charles (2009) reported that the amount of clay and mainly the type of clay mineral are responsible factors for CEC. The clay assemblage of the studied soil is dominated by kaolinite (Jiomeneck et al., 2011; Tematio et al.,

2009; Tsozué et al., 2009), which has the CEC value of 3-15 cmol.kg-1 (WRB, 2006). The presence of high organic matter contents in the studied soils contributes to the slight increase in CEC, as shown by the significant positive correlation between organic carbon and CEC (r = 0.75, p < 0.05). Exchangeable bases contents are low and highly variable. Consequently, base saturation percentage is very low and also highly variable, in line with the classification of these soils into dystrandept order by Tematio et al. (2004). Total nitrogen shows moderate variability (CV = 31.80%) among the soil samples. Their contents in all soils are above the critical level (< 0.3%), in line with high organic matter contents. Available P on the contrary is below the critical level (10-15 ppm) given by Landon (1991) and Sanchez et al., (1982) in most soil samples. The low level of available P in the studied area might be mainly due to the inherently low P levels of the soil or due to its high P fixation capacity (Fisseha and Gebrekidan, 2007; Siéwé et al. (2008). Low acidity also reduces P availability since aluminium and iron can react with soil phosphorus to form phosphorus compounds, and might restrict Ca and Mg uptake by crop plants (Nwaogu, 2014). Except for three soil samples with C:N ratio above 20, the rest has a ratio less or equal to 16, synonymous of a good incorporation of nitrogen during the humification process, which could be linked to biological activity (Caner, 2000). Consequently, this leads to an increase decomposition of organic matter and corresponding release of nutrient elements into the soil solution for plant nourishment (Akpan-Idiok and Ofem, 2014). However, for the other three soil samples, Tisdall and Oades (1982) reported that materials with higher C:N provide persistent binding agents and good fungal development that promote aggregation. The Ca:Mg ratio is an index of the relative proportion of available calcium and magnesium in the soil. It is globally very low (Ca:Mg < 1.5) to moderately low (1.5 < Ca:Mg < 3.5) corresponding to less favourable ratio, except for one soil sample where this ratio is very favourable (3.5 < Ca:Mg < 6) (Euroconsult, 1989; Landon, 1991). The low value indicates low amounts of Ca with considerable amount of Mg in the soil solution. Calcium is absorbed by plants as Ca2+ (Akpan-Idiok and Ofem, 2014). It is essential for root growth and as a constituent of cell wall materials (Epstein, 1972;

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Akpan-Idiok, 2012). However, the studied soils are depleted in bases and consequently show low exchangeable bases contents, in line with the significant positive correlation between Ca:Mg and Ca2+ (r = 0.68, p < 0.05) and base saturation percentage (r = 0.75, p < 0.05). According to Mg:K ratios, six soil samples are deficient in Mg (Mg:K < 2) and the rest has optimal (2 < Mg:K < 5) to very high (Mg:K > 5) ratios (Euroconsult, 1989). Mg in the form of Mg2+ is more likely to be available to crop plants in the soil relative to K (Akpan-Idiok and Ofem, 2014). It is a vital element in the formation of chlorophyll, starch translocation within plants, and is essential for the formation of lipids (Akpan-Idiok and Ofem, 2014). Potassium is absorbed by plants as K+ (Akpan-Idiok and Ofem, 2014). It improves plant’s ability to resist to disease and cold, aids in the production of carbohydrates and proteins and is an active component of plant enzyme systems (Epstein, 1972; Akpan-Idiok, 2012). The significant positive correlation between Mg:K and structural stability index (r = 0.77, p < 0.05) implies that Mg and K could play a main role in the structural stability of the studied soils. Similar findings were reported by Dontsova and Norton (2001), mentioning the mere role of Mg2+, but also Ca2+ in clay flocculation and aggregate stability of soils during rainfall. For K+, its dispersive effects on clay were reported by Rengasamy and Marchuk (2011). The eastern Bambouto Mountain soils in the Cameroon Western Highlands above 1800 m of altitude are subjected to severe degradation or vulnerability due soil pHH2O and available phosphorus (SDR/Vp = 4/2). They are also subjected to extreme degradation or vulnerability due to base saturation percentage (SDR/Vp = 5/1). The problem of soil acidity could be solved by restoration of the cation balance through fertilization and liming. This solution has already been proposed by Verdoodt and Van Ranst (2003) and Akpan-Idiok and Ofem (2014). Liming supplies Ca2+ and Mg2+, and eliminates Al3+ in soil solution, which is the reason for the high exchangeable acidity in the soils (Akpan-Idiok and Ofem, 2014). The supply of Ca2+ and Mg2+ would enhance the base saturation percentage and thus, solve the problem of degradation or vulnerability due to this parameter. In addition, soil pHH2O increase might increase phosphorus availability, enhancing therefore the problem of soil degradation due to this

element. However, the supply of phosphorus in the form of fertilizer is necessary due to the high phosphorus fixation capacity of andosols.

5. Conclusion

The aim of this study was to determine the physicochemical properties, degradation rate and vulnerability potential of soils in the upper part of the eastern slope of the Bambouto Mountains and also suggest ameliorative measures to improve soil productivity. The studied soils show low bulk density, exchange bases, base saturation percentage, CEC, pH, available phosphorus, high organic matter and total nitrogen contents, high structural stability index and a loamy texture. Among these soil parameters, the most limiting soil properties affecting soil quality were soil pH, available phosphorus and base saturation percentage. Except for the above stated parameters to which the studied soils are subjected to severe to extreme degradation or vulnerability, the soils are generally of good quality. This could justify the movement of population towards this upper part of the Bambouto Mountains. The problem of soil acidity could be solved by restoration of the cation balance through fertilization and liming. The supply of Ca2+ and Mg2+, as phosphorus fertilizer would enhance the base saturation percentage and increase phosphorus availability in the soil.

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