Evaluation of Soil Quality under Cocoa Trees in the Marahoué Region (Central-Western Côte d’Ivoire)

This study, which assesses the quality of soils under cocoa trees in the Marahoué region of Côte d’Ivoire, is being carried out against a backdrop of declining crop yields due to a number of factors, including the loss of soil fertility. The aim was to identify homogeneous groups of soils in terms of fertility and propose sustainable management methods for each group. The study was carried out on twenty-four cocoa orchards belonging to the SCOOP KAPATCHIVA cooperative, spread over eight sectors. A morphological characterisation of the soils was carried out in the field, followed by physico-chemical laboratory analyses of soil samples taken in the field. Soil fertility was analyzed on the basis of these two groups of data. The results showed that soil depths were generally high, except in some high altitude areas, where soils were often enriched in ferromagnesian elements and compacted. The texture of the soils varied between loamy and sandy-loamy, influencing cocoa production. Indeed, the soils of the plots with high production were looser and without ferromagnesian concretions, while those of the plots with low production were more gravelly and compact. Biological activity in the upper soil layer (0 - 30 cm) was notable with an abundance of termites and earthworms. The organic matter and nitrogen content varies considerably between sectors, directly impacting soil fertility. The soils were slightly acidic and showed significant differences in the levels of exchangeable magnesium and potassium, and not at all for phosphorus which appeared at very low doses. Chemical analysis also revealed that nutrient balance was a key factor in orchard productivity, with varying ratios of calcium, magnesium and potassium. Knowledge of these results made it possible to make recommendations for soil management with a view to improving cocoa yields in the study area.

• Soil
• Chemical Balances
• Fertility
• Cocoa Farming
• Marahoué Region

1. Introduction

Cocoa and coffee were at the root of the ivorian economic miracle of the 1970s. Since then, Côte d’Ivoire has remained the world’s leading cocoa producer. Cocoa alone feeds thousands of families and is a strategic product for the ivorian economy [1] . However, cocoa farming faces a number of difficulties that are at the root of yield losses, including ageing orchards, pest pressure due to swollen shoot, global warming and the ongoing loss of soil fertility [1] . However, increasing cocoa production is a necessity and a challenge for the main producing countries, particularly Côte d’Ivoire, the world’s leading producer [2] [3] .

To meet this challenge, Côte d’Ivoire is taking a number of steps, including: organising the industry, guaranteeing a large part of the CIF (Cost, Insurance, Freight) price to producers, extending the area under fallow land and scientific research [4] . Stakeholders in the sector are constantly striving to improve production quality, which, according to [5] , requires regular monitoring of soil quality. To this end, [6] , having noted that most of the current recommendations made to Ivorian producers are based on a single formula of NPK 0-23-19 fertiliser combined with small quantities of calcium and magnesium, proposed a soil diagnosis in order to adapt mineral fertiliser inputs to the chemical quality of the soil under cocoa trees. Their work was carried out in the east of the country.

This research is set in the same context. Its aim is to assess the soil fertility of the cocoa farmers of SCOOP KAPATCHIVA, a cooperative society of farmers based in Bonon, in the Marahoué region of central-western Côte d’Ivoire, one of the country’s largest cocoa-producing areas, which is currently facing yield declines.

2. Materials and Methods

Study Area

The SCOOP KAPATCHIVA intervention zone is located in the Marahoué region in central-western Côte d’Ivoire. It covers the departments of Bonon and Bouaflé ( Figure 1 ) and is subdivided into 8 sectors: Blaizekro, Bozi, Grâce, Gonanfla, Katana, Koffikro, Ndouffoukankro and Solidarité. Each sector covers several villages and therefore several producers’ plantations.

It is a transition zone between dense forest and wooded savannah, two types of plant formation which, depending on the dominance of one over the other, are distributed to the south and south-west for the dense forest, then to the north and north-east for the wooded savannah [7] .

The area straddles the sub-equatorial Atean climate and the humid tropical Baulean climate. There are two main seasons, alternated by two shorter ones. The long rainy season begins in March and ends in June, while the short rainy season runs from mid-August to October. Between the two rainy seasons, there is the short dry season from July to mid-August and the long dry season from November to February. Annual rainfall is between 1800 and 2000 mm, with an average annual temperature of around 25.30˚C and an average humidity of 75 pc. The area is drained by the Bandama rouge or Marahoué and Bandama blanc rivers and their tributaries [8] .

The region’s relatively flat topography consists of low plateaux with a few low-lying areas in the plains and hills with an average altitude of 260 m. The soils are generally Ferralsols (Eutric) with slightly different characteristics, depending on whether they are in forest or savannah. There are also pockets of Acrisols in the north and north-east and Gleysols along the rivers [9] .

In short, the physical characteristics of the zone give it many agricultural advantages, both for food production and cash crops. As a result, it is one of Côte d’Ivoire’s main production bases for food crops and cocoa.

Figure 1

Figure 1. Map showing the location of the study area.

3. Materials and Methods

The study covered twenty-four cocoa orchards evenly distributed between the eight different sectors of the SCOOP KAPATCHIVA intervention zone. It consisted of a morphological characterization of the soil in the field, followed by a physico-chemical analysis of soil samples in the laboratory. Soil fertility was assessed using data collected both in the field and in the laboratory.

3.1. Morphological Characterization of the Soil

In each plantation, three soil pits (F1, F2 and F3) ( Figure 2 ) were established according to the state of vigour of the cocoa trees and/or the production level of the cocoa trees, to take into account the fact that when the topography of the soil is almost flat (slope ≤ 8 pc), as was the case in almost all the plantations explored, it is the level of crop development (growth, health status, infestation, yield) that better reflects the nature of the underlying soil [10] . Thus, pits F1, F2 and F3 corresponded, in this order, to the soils of high, medium and low production subplots.

Figure 2

Figure 2. Example of an installed soil pit.

The observations at each pit consisted of identifying the different horizons and determining, for each horizon: the thickness, the texture, the general structure of the soil and that of the flow, the compactness and cohesion, the presence of organic matter, porosity and permeability, the natural drainage quality of the soil, the depth of appearance of the hydromorphic roof, and above all, the approximate proportion of the soil volume occupied by coarse elements, estimated visually.

3.2. Physico-Chemical Analysis of Soil in the Laboratory

The physico-chemical properties of the soil measured are listed in Table 1 , together with the methods used. These analyses were carried out at the plant and soil analysis laboratory of the Polytechnic Institute Félix Houphouët-Boigny (Yamoussoukro, Côte d’Ivoire) and involved samples taken from the top 30 cm of pits F1, F2 and F3 in each plot. Most of the lateral roots that provide the cocoa tree with mineral and water nutrition are concentrated in this upper part of the soil, although the plant prefers well-drained soils that are at least 1.5 m deep. Clearly, the layer being evaluated is the main site for the absorption of water and mineral elements by plant roots; it is also the site of biochemical reactions involving the transformation of organic matter by soil microorganisms [11] . Understanding its properties is therefore of vital importance in making recommendations for sustainable soil management.

Table 1

Table 1. Physico-chemical properties of the soil determined.

Types of variables	Variables	Methods
Chemical	pH	pH glass electrode meter [12]
	Organic carbon (C)	Walkley & Black [13]
	Organic matter (OM)	OM = 1724 × C
	Total nitrogen (N)	Kjeldahl modified [12]
	Total phosphorus (P)	Olsen modified [14]
	Exchangeable calcium (Ca2+)	Atomic absorption spectrometry [15]
	Exchangeable magnesium (Mg2+)
	Exchangeable potassium (K+)
	Sum of Exchangeable Bases (SEB)	SEB = Ca2+ + Mg2+ + K+
	Cation exchange capacity (CEC)	Kjeldahl modified [12]
	Base saturation (V)	V = SEB/CEC
Physical	Sand	Robinson pipette [16]
	Silt
	Clay

3.3. Analysis of Soil Quality

Based on the soil variables measured, an analysis of variance was carried out to identify homogeneous groups of soils and to compare the average characteristics of these groups with existing threshold values. A full hierarchical clustering analysis (HCA) was then used to divide the soils into homogeneous subsets of equal fertility. Finally, a principal component analysis (PCA) of the soil variables was carried out to determine the determining variables in the functioning of each soil fertility subset identified, after these variables and the individuals had been projected onto the same factorial plane (1 × 2). These analyses were carried out using XLSTAT software.

4. Results

4.1. Soil Morphology

While the depth of the soil appeared to be very great (>120 m) in most of the plots ( Figure 3 ), it was restrictive in some of the plots explored in the hill zone, mainly in the Bozi and Koffikro sectors. In these areas, at depths of between 50 and 70 cm, the soil was generally enriched with coarse ferromagnesian elements and was often compacted. In the other sectors, only the soils in the low production subplots (F3) showed a similar gravelly state, occupying around 50pc of the soil volume and often associated with compaction. In contrast, the soils in the high production sub-plots (F1) were generally loose and almost devoid of concretions ( Figure 2 ). In the illustration in Figure 3 , the F1 pit was observed in the Grâce sector, while F3 was observed in Koffikro, where the soils appeared to be more clayey on the surface (16.58 pc on average), although this difference does not significantly distinguish the soils from one another. The same is true for sand levels and for cumulative clay and silt levels ( Table 2 ). The silt content, on the other hand, shows differences between the soils, which have either a silty texture or a sandy-silty texture ( Table 2 ).

Figure 3

Figure 3. Representative profiles of soils from high production plots (F1) free of ferromanganic concretions and low production plots (F3) with horizon B rich in these coarse elements.

Table 2

Table 2. Granulometry of the first 30 cm of soil.

Secteur	Clay (pc)	Silt (pc)	Clay + Silt (pc)	Sand (pc)	Texture
Blaisekro	13.11a	37.16ab	50.28a	48.14a	Silty
Bozi	13.42a	28.30ab	41.72a	56.10a	Silty-sandy
Gonanfla	10.00a	33.95ab	43.95a	45.64a	Silty
Grâce	14.55a	31.10ab	45.65a	52.92a	Silty-sandy
Katana	10.79a	42.04a	52.84a	46.01a	Silty
Koffikro	16.58a	34.40ab	50.98a	48.03a	Silty
Ndouffoukankro	11.25a	32.22ab	43.47a	55.45a	Silty-sandy
Solidarité	14.82a	24.93b	39.76a	58.81a	Silty-sandy
CV	0.172	0.159	0.103	0.098
P	0.466	0.0244	0.289	0.329
Effect	ns	s	ns	ns

Means assigned the same letter following a column are not significantly different at the 5pc level; P: Probability; s: Significant difference; ns: Non-significant difference; CV: Coefficient of variation.

The 0 - 30 cm layer of the soil was generally the site of good biological activity, with an abundance of termites and earthworms. Many other faunal groups apart from earthworms and termites were also observed, including Myriapoda, Coleoptera, Gastropoda, etc. In addition, a thick litter of dead leaves covering the ground was observed at all the sites.

Organic carbon (C) levels in the 0 - 30 cm soil layer ranged in the study area from 0.57 pc at Koffikro to 1.25 pc at Bozi ( Table 3 ). The average equivalent organic matter (OM) content ranged from 0.98 pc in Koffikro to 2.16 pc in Bozi. For total nitrogen (N), the rates observed in the different sectors varied from 0.040 pc in Koffikro to 0.082 pc in Bozi. As a result, the C/N ratio is between 10.93 (Solidarité sector) and 15.02 (Grâce sector).

Table 3

Table 3. Soil organic matter and nitrogen.

Sector	C (pc)	N (pc)	C/N	OM (pc)
Blaisekro	0.912a	0.069a	12.99a	1.56a
Bozi	1.25a	0.082a	13.69a	2.16a
Gonanfla	1.001a	0.075a	12.92a	1.72a
Grâce	0.828a	0.060a	15.02a	1.42a
Katana	0.665a	0.050a	12.92a	1.14a
Koffikro	0.570a	0.043a	12.56a	0.98a
Ndouffoukankro	0.622a	0.048a	13.10a	1.07a
Solidarité	0.826a	0.072a	10.93a	1.42a
CV	0.269	0.224	0.087	0.269
P	0.402	0.323	0.608	0.403
Effect	ns	ns	ns	ns

Means assigned the same letter following a column are not significantly different at the 5pc level; P: Probability; s: Significant difference; ns: Non-significant difference; CV: Coefficient of variation.

Table 4 shows soil acidity by sector. The soils are all slightly acidic, with a pH in the range 4.98 - 5.63. There are no significant differences between the soils in the different sectors.

Table 4

Table 4. Soil acidity.

Sector	pH
Blaisekro	5.63a
Bozi	5.68a
Gonanfla	5.37a
Grâce	5.08a
Katana	4.98a
Koffikro	5.05a
Ndouffoukankro	5.25a
Solidarité	5.22a
CV	0.049
P	0.293
Effet	ns

Means assigned the same letter following a column are not significantly different at the 5pc level; P: Probability; s: Significant difference; ns: Non-significant difference; CV: Coefficient of variation.

The characteristics of the adsorbent complex of the soils studied are reported in Table 5 . Of all the variables determined, only exchangeable magnesium (Mg²⁺) and potassium (K⁺) levels show a significant difference between the soils (P < 0.01). According to the levels:

Table 5

Table 5. Characteristics of soils adsorbent complex.

Sector	Ca2+ (cmol∙kg−1)	K+ (cmol∙kg−1)	Mg2+ (cmol∙kg−1)	P (ppm)	CEC (cmol∙kg−1)	SEB (cmol∙kg−1)	V (pc)
Blaisekro	1.26a	0.114b	0.603b	36.42a	4.61a	2.02a	45.88a
Bozi	2.20a	0.115b	1.481a	64.42a	5.92a	3.83a	55.83a
Gonanfla	1.17a	0.108b	0.679b	50.92a	5.63a	2.00a	37.03a
Grâce	2.12a	0.110b	0.493b	89.83a	7.96a	2.78a	38.33a
Katana	1.12a	0.116b	0.685b	48.42a	4.61a	1.98a	49.06a
Koffikro	1.26a	0.110b	0.619b	74.92a	4.10a	2.03a	52.74a
Ndouffoukankro	1.36a	0.103b	0.325b	74.00a	3.88a	1.83a	49.90a
Solidarité	1.75a	0.136a	0.925ab	81.07a	5.17a	2.86a	58.03a
CV	0.283	0.085	0.481	0.281	0.249	0.285	0.157
P	0.388	0.008	0.004	0.53	0.377	0.305	0.291
Effect	ns	s	s	ns	ns	ns	ns

Means assigned the same letter following a column are not significantly different at the 5pc level; P: Probability; s: Significant difference; ns: Non-significant difference; CV: Coefficient of variation.

Table 6 shows the chemical equilibria in the soils. Certain balances, namely Ca²⁺/K⁺, K⁺/CEC, N/P, (Ca²⁺ + Mg²⁺)/K⁺ and (SBE + 6.15)/N do not distinguish the soils significantly (P > 0.05), unlike others, namely Mg²⁺/K⁺ and Ca²⁺/Mg²⁺ (P < 0.05).

For the last two ratios mentioned, starting with the Mg²⁺/K⁺ ratio, the values obtained ranged from 3.21 (Ndouffoukankro sector) to 12.78 (Bozi sector), making it possible to distinguish two homogeneous groups of soils and an intermediate group. The first homogeneous group is made up of the soils of the Bozi sector, where the ratio is 12.78, while the soils of the Katana, Koffikro, Blaisekro, Grâce and Ndouffoukankro sectors form the second homogeneous group, with ratios of between 3.21 and 5.92. The intermediate group is made up of the Gonanfla sector, where the ratio reaches 6.23. As before, the Ca²⁺/Mg²⁺ ratio values identify two homogeneous groups and an intermediate group. The first homogeneous group is made up of soils in the Ndouffoukankro and Grâce sectors, with ratios of 4.15 and 4.14 respectively. The Gonanfla and Katana sectors, which had the lowest ratios (1.66 and 1.65), form the second homogeneous group. Soils in the Solidarité, Bozi, Bla and Koffikro sectors form the intermediate group. Their ratios range from 2.10 to 2.41. In all the soils studied, the N/P, (SEB + 6.15)/N and (Ca²⁺ + Mg²⁺)/K⁺ ratios ranged from 10.23 to 34.26, 92.27 to 163.87 and 15.59 to 31.89 respectively, with no significant difference between soils.

Table 6

Table 6. Chemical balances in soils.

Sector	Ca2+/K+	Ca2+/Mg2+	K+/CEC (pc)	Mg2+/K+	N/P	(Ca2+ + Mg2+)/K+	(SBE + 6.15)/N
Blaisekro	11.03a	2.144ab	2.6a	5.28b	21.13a	16.31a	124.22a
Bozi	19.10a	2.22ab	2.5a	12.78a	23.20a	31.89a	106.14a
Gonanfla	10.55a	1.66b	2.00a	6.23ab	34.26a	16.79a	128.01a
Grâce	19.10a	4.14a	2.1a	4.45b	15.09a	23.55a	152.43a
Katana	9.67a	1.65b	3.00a	5.92b	22.29a	15.59a	134.46a
Koffikro	11.35a	2.10ab	2.9a	5.63b	9.08a	16.98a	163.87a
Ndouffoukankro	13.37a	4.15a	3.00a	3.21b	10.23a	16.58a	154.15a
Solidarité	13.10a	2.41ab	2.7a	7.60ab	12.63a	20.70a	92.27a
CV	0.277	0.395	0.143	0.451	0.454	0.282	0.186
P	0.445	0.00807	0.469	0.04006	0.301	0.426	0.742
Effect	ns	s	ns	s	ns	ns	ns

Means assigned the same letter following a column are not significantly different at the 5pc level; P: Probability; s: Significant difference; ns: Non-significant difference; CV: Coefficient of variation.

Figure 4 shows the hierarchical grouping diagram used to organize the soils studied into a tree structure according to their similarities in terms of fertility. There are three groups of soils:

Figure 4

BLA: Blaisekro; BOZ: Bozi; GON: Gonanfla; GRA: Grâce; KOF: KAT: Katana; Koffikro; NDO: Ndouffoukankro; SOL: Solidarité.Figure 4. Soil fertility groups.

Figure 5 , result of the PCA, links the soil groups with the characteristics that best distinguish them. It can be seen that:

Figure 5

BLA: Blaisekro; BOZ: Bozi; GON: Gonanfla; GRA: Grâce; KOF: KAT: Katana; Koffikro; NDO: Ndouffoukankro; SOL: Solidarité.Figure 5. Soil fertility groups and their characteristics.

4.2. Discussion

In physical terms, the soils in the study area generally have a texture that varies from silty to sandy-loam, which gives them good internal drainage, good air circulation and easy root penetration. For the most part, these soils are deep and therefore suitable for cocoa production, except that in some places this great depth is limited by a large volume of ferromanganic concretions. In fact, the high rate of ferromanganic concretions in soils under cocoa trees is often linked to the different microclimates developed on the plots following the replacement of the initial forest cover by crops and when the soil is exposed to high levels of sunlight. In fact, when the previously mobilized and entrained iron precipitates, it first forms an amorphous ferric hydrate gel (Fe(OH)²⁺ or Fe(OH)²⁺), which is no longer associated with clay, but rather with colloidal silica and fulvic acids, whose insolubilization and polymerization into humic acid it encourages. This form of iron characterizes spodic horizons and rusty patches (pseudogley) in soils with temporary hydromorphy [17] . According to [18] , it is rich in organic matter and forms easily friable flakes or granules. If the proportion of organic matter decreases as a result of an increase in the proportion of ferric oxide, the amorphous iron evolves towards a crystalline form as the soil dries out (high insolation). The amorphous iron then hardens to form concretions.

From a biological point of view, the 0 - 30 cm layer of the soil was generally the site of good biological activity, marked by an abundance of termites and earthworms, followed by many other faunal groups such as Myriapoda, Coleoptera, Gastropoda, etc. [19] associate this massive presence of fauna in the soils of perennial crops such as cocoa with the fact that these crops produce a lot of litter, which acts as a habitat and food source for many species of soil invertebrates. As well as being produced naturally by the leaves and branches that fall to the ground, the litter is also the result of the practice of pruning to maintain the cocoa trees, observed in more than 80 per cent of the plantations visited.

Chemically speaking, the organic status observed throughout the study area is low in relation to the cocoa tree’s need which are estimated at more than 3 pc organic matter in the humus layer of the soil [20] . These low levels are generally the result of a balance resulting from three processes: organic inputs (aerial and root litter, soil improvers), the decomposition of these inputs by soil biological activities and the stabilization of organic matter by the soil mineral matrix (clay particles) [21] [22] . In the present case, it is the third of these processes that seems to be lacking, given the low clay levels observed.

As with organic matter levels, nitrogen levels are low in the soils studied, given that average levels of this element are below 0.2 pc, which is the minimum threshold required for cocoa production [23] . However, the C/N ratio, which indicates the dynamics of organic carbon and nitrogen in the soil, between 10.93 and 15.02, reflects good carbon mineralization due to the action of fauna and soil microbes, particularly for C/N ratio values ranging from 9 to 12 [24] .

Soil acidity (pH ranging from 4.98 - 5.63) is not a constraint for the cocoa tree because, although the optimum pH is 7, the plant can also grow well in soils with an acid pH (pH = 4.5 - 6) or a slightly basic pH (pH 6.7 - 7.5) [25] [26] . However, production is severely limited below a pH of 4.5 or when the pH is above 8 [23] . Under these conditions, the activity of fauna and microbes is affected, leading to a number of constraints, including: inhibition of nitrification reactions, phosphorus deficiency, aluminium and manganese toxicity, high availability of certain minor elements and, finally, limited plant growth.

The adsorbent complex has a satisfactory saturation state (around 50 pc) and a sum of exchangeable bases which, by varying from 1.83 to 3.83 cmol∙kg⁻¹, is in line with the range of 2 to 6 cmol∙kg⁻¹ recommended for cocoa farming [24] . On the other hand, the CEC in these soils, which varies between 3.88 and 7.96 cmol∙kg⁻¹ and is therefore far from the 12 cmol∙kg⁻¹ recommended for cocoa farming [24] , is low and unfavorable, especially as the CEC is a measure of the soil’s ability to fix cations. The higher it is, the more cations are stored by the soil and the more fertile it is. Its level depends on the mineralogical characteristics of the soil (clay content and type) and the level of organic matter, which contributes 4 to 5 times more than clay [27] . The low CEC observed is therefore probably linked to the low levels of organic matter buried in the soil. These low CECs explain the fact that most soils in the area are deficient in Mg²⁺, K⁺, Ca²⁺ and especially P, if we take into account the thresholds set at 2.45 cmol∙kg⁻¹ for Mg²⁺, 0.7 cmol∙kg⁻¹ for K⁺, 5 to 8 cmol∙kg⁻¹ for Ca²⁺ and 150 to 700 ppm for P [24] [28] - [31] .

Despite these deficiencies, most of the soils studied, particularly those in the Katana, Koffikro, Blaisekro, Grâce and Ndouffoukankro sectors, appear to have a good nutritional balance between Mg²⁺ and K⁺. In fact, the optimum range for the Mg²⁺/K⁺ ratio, which is 3 to 4 for many plants, is in line with the results obtained [32] . For other soils, this Mg²⁺/K⁺ ratio is too high and thus indicates a K⁺ deficiency, probably due to the strong fixation of this element on the adsorbent complex expressed by K⁺/CEC ratios evolving in the range of 2 to 3 pc, a sign of a good level of saturation of the K⁺ fixation sites on the adsorbent complex [32] . With regard to the balance between Ca²⁺ and Mg²⁺, only the soils of Ndouffoukankro and Grâce have ratios within the acceptable limit of between 3 and 5 [33] . The soils in the other six sectors may be Ca²⁺ deficient because their Ca²⁺/Mg²⁺ ratios are too low.

On the other hand, all the soils studied showed low availability of P, with N/P ratios ranging from 10.23 to 34.26, well above the target range of 1.5 to 2 [24] . As a reminder, the levels of this element had already appeared to be low, below 150 ppm [31] . These findings are very typical of tropical soils, which make up the majority of the 5.7 billion hectares of soil in the world considered to be problematic in terms of limiting plant phosphate nutrition [31] . During pedogenesis, P content decreases, with the proportion present in organic form increasing to the detriment of inorganic forms [34] . In this context, at the earliest stages of soil development, soils are mainly limited by low nitrogen content and availability, whereas at the most advanced stages of their evolution, they become more limited by P [35] . This is particularly true of tropical soils, which, except in volcanic or sedimentary regions, are often characterized by a high degree of weathering and hence low P content. In addition, in these soils, the abundance of secondary minerals such as iron and aluminium oxides is conducive to a high retention of phosphate ions, limiting their availability [31] . On the contrary, according to their ratios (SEB + 6.15)/N and (Ca²⁺+Mg²⁺)/K⁺ greater than 8.9 and 11.5 respectively, all the soils explored show a good balance between exchangeable bases and nitrogen on the one hand, and between the Ca²⁺-Mg²⁺ binomial and K⁺ on the other [32] .

5. Conclusions and Recommendations

Most of the soils explored are deep and therefore suitable for cocoa farming. Their texture varies from loamy to sandy loam, all of which suggests good internal drainage, good air circulation and easy root penetration. However, their depth is limited in places by a large volume of ferromanganic concretions. To prevent this process from progressing in the area, the cocoa plantation should be shaded by agroforestry. This prevents certain harmful solar rays from reaching the soil. The low clay content in the surface horizons of these soils is also a major chemical constraint, as clay plays a number of functions that contribute to soil fertility. To compensate for this lack of clay, the soil would have to contain more organic matter. Unfortunately, this other major soil constituent is also lacking. Organic matter is rapidly mineralized by soil fauna and microbes. Exchangeable bases are then released regularly into the soil, hence the low overall base total. The low acidity of the soil, which generally slows down the activity of fauna and microbes, has almost no constraining effect on soil organisms. However, this low soil acidity has a negative impact on the retention of bases on the adsorbent complex. As a result, the CEC is low on all the sites surveyed. However, the low CEC of these soils may also be linked to their low clay and organic matter content. Farmers should therefore opt for farming practices that help to extend the residence time of carbon in the soil, in particular: pruning cocoa trees to increase litter, practicing agroforestry to maintain shade on cocoa farms and reducing tillage, which avoids removing the organic matter protection provided by the soil.

Despite the constraints caused by the low levels of clay and organic matter in the soils, the chemical balances are generally acceptable, particularly the ratios of Mg²⁺/K⁺, (SBE + 6.15)/N and (Ca²⁺ + Mg²⁺)/K⁺.

For soils where the Mg²⁺/K⁺ ratio was too high, indicating a potassium deficiency, potassium fertilizer should be applied. Also, for soils where the Ca²⁺/Mg²⁺ ratio was too low, indicating a Ca²⁺ deficiency, calcium fertilizer was recommended. In addition, for all the soils studied, phosphorus fertilizer should be added to overcome the low availability of phosphorus. For all these inputs, growers have a choice of several fertilization methods: mineral, organic or organo-mineral.

Available in liquid or soluble form, chemical fertilizers are directly assimilated by plants. However, these fertilizers are still in the spotlight because of the sharp rise in their prices on world markets, which can increase cocoa production costs. Also, while these fertilizers improve crop yields in the short term, they are detrimental to soil fertility in the long term. We therefore recommend biological practices for renewing soil fertility, which are increasingly diversified and adapted to all farming systems. The cocoa tree is a plant that exports phosphorus and potassium, some of which is found in the leaves from pruning and in the shells of the pods after denting, generating numerous residues with fertility potential. These residues can be recycled by composting. The technique of combined crops should also be encouraged, particularly legumes, which are traditionally grown by cocoa farmers on other plots. If grown under cocoa trees, legumes could provide an effective means of combating soil nitrogen deficiencies.

References

1. Tano, A.M. (2012) Crise cacaoyère et stratégies des producteurs de la sous-préfécture de Meadji au sud-ouest ivoirien. Master’s Thesis, Université de Toulouse le Mirail.

2. Jagoret, P., Deheuvels, O. and Bastide, P. (2014) S’inspirer de l’agroforesterie. Production durable de cacao. Centre de Coopération Internationale de Recherche Agrono-mique pour le Développement. Perspective intensification écologique, 27, 4.

3. ICCO (2018) ICCO Quarterly Bulletin of Cocoa Statistics, XLIV, 2, Cocoa Year 2017/ 2018.

4. Bockel, L., Ouedraogo, S.A., Auguste, K.A. and Gopal, P. (2021) Analyse prospective de la filière cacao en Côte d’Ivoire 2020-2030—Vers une politique commune de marché de cacao en Afrique de l’Ouest. FAO. >https://doi.org/10.4060/cb6508fr

5. Current, D. and Scherr, S. (1995) Farmer Costs and Benefits from Agroforestry and Farm Forestry Projects in Central America and the Caribbean: Implications for Policy. Agroforestry Systems, 30, 87-103. >https://doi.org/10.1007/BF00708915

6. Kotaix, A.J.A., Kouadio, K.H., Angui, K.T.P., Kassin, K.E., Gbeuli, T.A., Assi, M.E., Kouamé, N.N., Coulibaly, K., Koko L.A. and Bacayoko, S. (2021) Les engrais minéraux issus du diagnostic sol améliorent la fertilité chimique et la production cacaoyère à l’Est de la Côte d’Ivoire. Journal of Animal&Plant Sciences, 47, 8387-8399.

7. Douka, A.M. (2012) Les cultures vivrières de l’autoconsommation à l’économie de marché en pays Gouro (Centre-Ouest de la Côte d’Ivoire). Revue de Géographie Tropicale et d’Environnement, 2, 25-26.

8. N’guessan, B.V.H., Saley, M., Oga, Y., Yapi, A. and Biémi J. (2017) Caractéristiques de la sécheresse hydrologique dans la région de la Marahoué (Centre-Ouest de la Côte d’Ivoire). International Journal of Engineering Science Invention, 6, 19-27.

9. Douka, A.M. (1981) Le développement des cultures vivrières spéculatives et leur impact sur l’organisation de l’espace. L’exemple du département de Bouaflé. Ph.D. Thesis, Université de Paris I.

10. Freschet, G.T., Violle, C., Roumet, C. and Garnier, E. (2018) Interactions entre le sol et la végétation: Structure des communautés de plantes et fonctionnement du sol. In: Les sols au cœur de la zone critique-écologie, ISTE éditions, 83-99.

11. Baize, D. (2022) L’importance des horizons profonds des sols. Planet Vie. >https://planet-vie.ens.fr/

12. Diack, M. and Loum, M. (2014) Caractérisation par approche géostatistique de la variabilité des propriétés du sol de la ferme agropastorale de l’Université Gaston Berger (UGB) de Saint Louis, dans le bas delta du fleuve Sénégal. Revue de géographie du laboratoire Leïdi, 12, 1-25.

13. Walkley, A. and Black, I.A. (1934) An Examination of the Degtjareff Method for Determining Soil Organic Matter and a Proposed Modification of the Chromic Acid Titration Method. Soil Science, 34, 29-38. >https://doi.org/10.1097/00010694-193401000-00003

14. Hilhorst, M.A. and Balendonck, J. (1999) A Pore Water Conductivity Sensor to Facilitate Non-Invasive Soil Water Content Measurements. Soil Science Society of América Journal, 64, 211-222. >https://doi.org/10.2136/sssaj2000.6461922x

15. Pansu, M. and Gautheyrou, J. (2003) L’analyse du sol minéralogique, organique et minérale. Springer, 1012.

16. Douzals, J.P. (2000) Mesures physiques de la variabilité des sols en agriculture de précision. Editions Ingénieries-E AT, IRSTEA, 45-52.

17. Assa, A.D. (2005) Précis de pédologie. A l’usage des étudiants du premier cycle des études universitaires. Editions Sciences, EDUCI.

18. Schwertmann, U. (1965) Sur la formation de goethite et hématite à partir d’hydroxyde ferrique amorphe. Bulletin bibliographie ORSTOM, XIII, 47.

19. Lavelle, P., Villenave, C., Rouland, C. and Derouard, L. (2000) Dynamique des peoplements de macro-invertébrés du sol aux diverses étapes de la jachère en Afrique tropicale. La jachère en Afrique tropicale—Ch. Floret, R. Pontanier John Libbey Eurotext, 236-241.

20. Somarriba, E., Cerda, R., Orozco, L., Cifuentes, M., Dávila, H., Espin, T., Mavisoy, H., Ávila, G., Alvarado, E., Poveda, V., Astorga, C., Say, E. and Deheuvels, O. (2013) Carbon Stocks and Cacao Yields in Agroforestry Systems of Central America. Agriculture, Ecosystems and Environment, 173, 46-57. >https://doi.org/10.1016/j.agee.2013.04.013

21. Derrien, D., Dignac, M.F., Basile-Doelsch, I., Barot, S., Cécillon, L., Chenu, C., Chevallier, T., Freschet, G.T., Garnier, P., Guenet, B., Hedde, M., Klumpp, K., Lashermes, G., Maron P.A., Nunan, N., Roumet, C. and Barré, P. (2016) Stocker du carbone dans les sols: quels mécanismes, quelles pratiques agricoles, quels indicateurs? Étude et Gestion des Sols, 23, 193-223.

22. Chevallier, T., Razafimbelo, T.M., Chapuis-Lardy, L. and Brossard, M. (2020) Carbone des sols en Afrique. Impacts des usages des sols et des pratiques agricoles. Rome/Marseille. FAO/IRD. >https://doi.org/10.4000/books.irdeditions.34867

23. Aubert, G. and Moullnler, H. (1954) Observations sur quelques caractères des sols de cacaoyères en Côte d’Ivoire. L’Agronomie Tropicale, 9, 428-437.

24. Snoeck, D., Koko, L., Joffre, J., Bastide, P. and Jagoret, P. (2016) Cocoa Nutrition and Fertilization. Sustainable Agriculture Reviews, 19, 155-202. >https://doi.org/10.1007/978-3-319-26777-7_4

25. Appiah, M.R., Ofori-Frimpong, K., Afrifa, A.A., Abekoe, M.K. and Snoeck, D. (2006) Improvementof Soil Fertility Management in Cocoa Plantations in Ghana. Regional Cocoa Scientific and Technical Final Report, Cocoa Research Institute of Ghana, Ghana.

26. Tossah, B.K., Koudjega, T. and Snoeck, D. (2006) Amélioration de la gestion de la fertilité des sols dans les plantations de cacaoyers au Togo. Rapport final scientifique et technique du FSP Régional Cacao. ITRA/CRAF Togo.

27. Anonyme (2015) La capacité d’échange cationique. Chambre d’agriculture de la Drôme, France, Fiche technique.

28. Akanbi, S.O., Ojeniyi, S.O., Famaye, A.O., Ipinmoroti, R.R., Oloyede, A.A., Oyewumi, I.K., Ayegbonyin, K., Adejobi, K.B. and Idrisu, M. (2014) Soil Nutrients and Cocoa Seedling Performance as Influenced by Plant Residue Ash and NPK Fertilizer Addition on a Depleted Soil in Ibadan, South Western, Nigeria. International Research Journal of Agricultural Science and Soil Science, 4, 1-4.

29. Koko, L. (2014) Teractiv Cacao as a New Fertilizer Based Reactive Phosphate Rock for Cocoa Productivity in Côte d’Ivoire: A Participatory Approach to Update Fertilization Recommendation. Procedia Engineering, 83, 348-353. >https://doi.org/10.1016/j.proeng.2014.09.027

30. Aikpokpodion, P.E. (2010) Nutrient Dynamics in Cocoa Soils, Leaf and Beans in Ondo State Nigeria. Journal of Agricultural Science, 1, 1-9. >https://doi.org/10.1080/09766898.2010.11884647

31. Hinsinger, P. (2001) Bioavailability of Soil Inorganic P in the Rhizosphere as Affected by Root-Induced Chemical Changes: A Review. Plant and Soil, 237, 173-195. >https://doi.org/10.1023/A:1013351617532

32. Jadin, P. and Snoeck, J. (1985) La méthode du diagnostic sol pour calculer les besoins en engrais du cacaoyer. Café Cacao Thé, 29, 255-266.

33. Hornus, P. and Snoeck, D. (2010) Le diagnostic sol et la fertilisation du cacaoyer. Document technique. Centre de Coopération Internationale en Recherche Agrono-mique Pour le Développement.

34. Chadwick, O.A., Derry, L.A., Vitousek, P.M., Huebert, B.J. and Hedin, L.O. (1999) Changing Sources of Nutrients during Four Million Years of Ecosystem Development. Nature, 397, 491-497. >https://doi.org/10.1038/17276

35. Vitousek, P.M. and Farrington, H. (1997) Nutrient Limitation and Soil Development: Experimental Test of a Biogeochemical Theory. Biogeochemistry, 37, 63-75. >https://doi.org/10.1023/A:1005757218475