https://orcid.org/0000-0003-3709-3047
Figures
Abstract
The main pillar of agriculture is the soil because it is both the support and the reservoir of essential nutrients for the plant. The food function it provides for humanity can only be fully fulfilled if it is balanced. Therefore, the development of sustainable agriculture in the world necessarily requires knowledge of the soil. The evaluation of agronomic suitability consists in determining its intrinsic capacity to sustain agricultural productivity over time. Many studies have been conducted in the domain of agricultural land assessment, but the applicability of the methods used in the tropical context requires adaptations that are not always obvious. The new methodological approach we propose for agronomic suitability assessment(AA) was designed and tested in Cameroon (Central Africa) in two pilot sites chosen in two different agro-ecological zones. The sites were the bimodal forest in Bokito which developed on yellow ferralitic soils, and the highland in the Bambouto volcanic massif which had a great pedological variability ranging from red ferralitic soils to andic ferralitic soils and andosols. The approach is inexpensive and based on the combination of four intrinsic soil parameters, namely acidity (pH), useful water reserve (RU), cation exchange capacity (CEC) and erodibility (K); according to the formula: AA = pH × RU × CEC × K. The unit of measurement is the « equivalent hour per mega joule per millimeter (eq.hr. MJ-1mm-1) ». The results showed that the agronomic suitability of the yellow ferralitic soils of Bokito varies from 0.00 to10.53 eq.hr. MJ-1mm-1. On the volcanic massif of Bambouto, the agronomic aptitude varies from 0.00 to 15.70 eq.hr. MJ-1mm-1 on the red ferralitic soils of the lower part of the massif, from 15.70 to 41.84 eq.hr. MJ-1mm-1 on andic ferralitic soils of the middle part of the massif and reaches 108.85 eq.hr. MJ-1mm-1 on the andosols of the summit part of the massif. This work could allow, on the one hand, a better allocation of agricultural land and thus participate in the development of second generation agriculture in sub-Saharan Africa; and on the other hand, contribute to determine more precisely the quality and quantity of fertilizer needed to maintain soil balance. Controlling the use of fertilizers will help to significantly reduce the quantities of chemical elements contained in agricultural products, limit water and soil pollution and thus better preserve human health.
Author summary
Agriculture-related activities provide a livelihood for about 60% of the population in Africa. In Cameroon, the agricultural sector is the main employer with 62% of the active population. However, it is clear that agricultural yields have been stagnating for decades and the trend in recent years has been towards a decline in soil productivity. Cameroon’s ambition is to move towards second-generation agriculture, which requires an accurate estimation and large-scale mapping of the agronomic aptitude of the country’s soils. The new methodological approach we propose is fast, reliable and inexpensive. It takes into account only four intrinsic soil parameters and has the advantage of determining a numerical value for the agronomic aptitude of a given soil. In short, it enables a soil health check to be carried out, and consequently allows a precise identification of its qualities and deficiencies. This function contributes to a significant reduction in the quantity of fertilizer used, thus limiting water and soil pollution and contributing effectively to the fight against the harmful effects of climate change.
Citation: Olivier L, Ouafo MR, Ndjigui P, Bitom D, Mfoumbeng MP (2023) Methodological approach for the evaluation and mapping of the agronomic suitability of soils in tropical zones: Case study of the Bambouto volcanic massif (Western Cameroon) and the Bokito district (Central Cameroon). PLOS Sustain Transform 2(7): e0000067. https://doi.org/10.1371/journal.pstr.0000067
Editor: Semra Benzer, Gazi Universitesi, TURKEY
Received: September 1, 2022; Accepted: May 15, 2023; Published: July 13, 2023
Copyright: © 2023 Olivier et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: Repository name: Mendeley Data Data identification number: Volume 43 Direct URL to data: https://www.sciencedirect.com/journal/data-in-brief/vol/43/suppl/C
Funding: The authors received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
1. Introduction
Nearly 2.37 billion people did not have access to adequate food in 2020 and out of a total of 768 million undernourished people in the world in the same year, 264.2 million were identified in sub-Saharan Africa [1]. This figure is a significant increase of 25.2 million people over one year, as in 2019 undernourishment affected 239 million people in sub-Saharan Africa [1]. To reverse this trend, target 2.4 of the Sustainable Development Goal (SDG 2) on "Zero Hunger" calls for progressive improvement of soil quality. A recent report on the State of the World’s Soil Resources indicated that 40% of African soils are moderately to severely degraded [2]. However, agricultural production in the sub region has increased slightly over the last three decades thanks to the cultivation of poor and marginal lands [3]. Thus, the expansion of agricultural areas is taking place at the expense of natural vegetation, which further accentuates the adverse effects of climate change. It is therefore imperative to determine the value of soils in the sub Saharan region in order to better guide sustainable land management policy, which is essential for food security in Africa.
Many studies on soil quality assessment and agricultural productivity improvement in sub-Saharan Africa have been conducted [4–11]. The results should be updated and overlaid to accurately reflect the efforts needed to make the agricultural sector, which employs more than 60% of the active population in Africa [2], a real lever for development. The updating of information on soils is all the more important as their properties, which are of interest to agriculture, such as pH, organic matter content, etc., change progressively over time [12]. The superposition of the results will provide coherent, continuous and large-scale agrarian information on large areas. Unfortunately, the consistency of these results is very difficult because the methods used were mostly developed in different environments and are very varied. The immediate consequence is that the large-scale land suitability maps available for Sub-Saharan Africa are very patchy [7,8,13]; and those that cover large areas are at very small scales.
In spite of this, and for lack of anything better, this unreliable agrarian information is still used as a basis for orienting decision-makers in large-scale agricultural projects in sub-Saharan Africa, and naturally, the countries of the sub region are still experiencing real difficulties in migrating effectively to second-generation agriculture.
It is with the aim of providing an outline of a solution to this crucial problem that we propose in this article a new, simple and inexpensive methodological approach, designed and tested in Cameroon (Central Africa) for agronomic suitability assessment (AA). It takes into account four intrinsic soil parameters. The first is soil acidity (pH) which defines the capacity of a soil to support plant growth. The second parameter is the useful water reserve (RU) which determines the capacity of a soil to provide water for the plant. The third parameter is the cation exchange capacity (CEC) which determines the ability of a soil to provide nutrients in quantity and quality to the plant, and the fourth parameter is the erodibility (K) which determines the ability of a soil to preserve its larder. The formula is established as follows: AA = pH × RU × CEC × K [14]. It is expressed in « equivalent hour per mega joule per millimeter (eq.hr. MJ-1mm-1) ».
This formula also offers the advantage that agronomic suitability can be easily spatialized in a geographic information system, thus ensuring harmonization of soil characteristics. It has been successfully tested in two agro-ecological zones in Cameroon. The first test site is the Bokito district in the "bimodal forest" agro ecological zone [15], and the second test site is the Bambouto volcanic massif in the "highlands" agro ecological zone [15]. This methodological approach could allow for faster, more reliable and more relevant decision making in the agricultural domain in Sub-Saharan Africa. Moreover, in addition to performing a soil health check, it allows, in case of deficiency, to detect with precision the nature of the parameter to be corrected as well as the type of amendment necessary to bring the soil back to balance, thus limiting water and soil pollution and better preserving human health [16].
2. Materials and methods
2.1. Literature review
The study of soils began in Cameroon after the Second World War [17] with the description of soils and the examination of their agricultural potential. Subsequently, soil maps at different scales were produced [18,19]. Most of the studies on land evaluation have been conducted according to the FAO parametric method [20,21]. Criteria taken into account are climate, topography, soil moisture status, soil physical conditions, chemical fertility, salinity… [22–24]. The soil units are therefore defined in such a way that they correspond as closely as possible with the uses envisaged by the evaluation. However, the model uses a very large number of parameters and the specialist often has to use personal judgement to modify and adapt the approach to the circumstances [25].
2.2. Location of experimental sites
Cameroon is located in Central Africa, in the Gulf of Guinea. It is very wide in latitude and open to the Atlantic Ocean. It is bordered to the west by Nigeria, to the north by Chad, to the east by the Central African Republic and to the south by Congo, Gabon and Equatorial Guinea.
Cameroon is distinguished by the presence of five agro-ecological zones with distinct morphological, climatic, phytogeographical and pedological conditions. These zones are the bimodal forest zone covering 165,770 km2 in the eastern part, the monomodal forest zone covering 45,658 km2 in the coastal parts, the highland zone covering 31,192 km2 in the western part, the high savannah zone covering 123,077 km2 in the central part and the sudano-sahelian zone covering 100,353 km2 in the northern part [16]. For the test of the functionality of the equation, pilot sites were chosen in two agro ecological zones, these are the Bokito site and the Bambouto volcanic massif site.
The Bokito site belongs to an agro-ecological zone called "bimodal forest". It is located between latitudes 4° 20’ and 4° 40’ N and longitudes 11° 00’ and 11° 20’ E “Fig 1”.
Covering an area of 1115 km2, the district of Bokito belongs to the department of Mbam and Inoubou, in the contact zone between the forest and the savanna. It is characterized by a subequatorial Guinean climate with four seasons: two dry seasons (from mid-November to mid-March and from mid-June to mid-August) and two rainy seasons (from mid-March to mid-June and from September to mid-November). The district of Bokito is marked by the presence of plains, plateaus and a few wooded hills in its western part, and is bordered to the south by two rivers, the Mbam and the Sanaga [26]. The average annual temperature is 23.4°C, annual rainfall is 1677 mm [27]. Soils are yellow ferrallitics with hydromorphic soils in the valley and colluvial areas [28].
The Bambouto volcanic massif site belongs to an agro-ecological zone called "highlands". It is located between latitudes 5° 25’ and 5° 45’N and longitudes 10° 00’ and 10° 15’E “Fig 2”.
Caption credit: Roose E, Duchaufour H, De Noni G. Lutte antiérosive: Réhabilitation des sols tropicaux et protection contre les pluies exceptionnelles. Marseille: IRD Edition. 2012; http://books.openedition.org/irdedition/12419. ISBN: 9782709922753. DOI:10.4000/.
The Bambouto volcanic massif covers an area of 800 km2. This massif presents four zones with distinct morphological, climatic, phytogeographic and pedological conditions: it is a collapse caldera opening in a horseshoe shape towards the west in the summit part, a high zone with an altitude of between 2000 and 2740 m, a medium zone with an altitude of between 1600 and 2000 m and a low zone below 1600 m.
The caldera is characterized by a very uneven relief, a very cool (10°C on average) and foggy climate [29], a very high rainfall (4000 mm of water per year). The vegetation consists of grasslands, nebeldwald with a shrub layer and a dense humid forest of guttiferous transition [29]. The soils are andosolic [30]. The high zone has the same characteristics with a relatively lower rainfall (2,500 mm of water on average per year) [31]. The middle zone has a hilly relief, a cool and humid climate (about 18°C) and an average rainfall of 1690 mm per year [29]. The soils are ferralitic andic [32]. The low zone has a hilly relief. The climate is hot and humid (23.5°C on average) and the rainfall is 1750 mm per year. The soils are red ferralitic [30].
2.3. Data collection
2.3.1. Base map source and metadata.
The base data was created in ArcGIS 10.2 (ESRI) from data extracted by manual scanning on a mosaic of seventeen sheets of the 1: 200,000 scale topographic base map of Cameroon, and on a total of five sheets of the 1: 50,000 scale topographic map of Cameroon. These topographic maps were acquired from the National Institute of Cartography of Cameroon. They were produced from vertical aerial photographic coverage taken in 1963, published in 1965 and reissued in 1976. The information extracted includes contour lines, administrative boundaries, study site boundaries, road networks, hydrographic networks and locality names.
For the extraction of the administrative boundaries of the Central, Western, North-Western and South-Western regions, the topographic base maps at a scale of 1:200,000 were used: NB-32-XXIII (Edéa), NB-32-VII (Linté), NB-33-XIX (Akonolinga), NB-33-XII (Yoko), NA-32-VI (Bafia), NB-32-V (Ndikinimeki), NB-32-XXIV (Yaoundé) and NB-33-I (Nanga-Eboko), Buea-douala (NB-32-VI), Tibati (NB-32-XIX), Deng-deng (NB-33-XIX), Bertoua (NB-33-II), Banyo (NB-32-XVIII), NB-32-XI (Bafoussam), NB-32-X (Manfé), NB-32-XVI (Akwaya) and NB-32-XVII (Nkambé) were used.
The boundaries of the Bokito site, the road network, the hydrographic network and the names of the localities of this site are extracted by manual digitizing on portions of sheets NB-32-VI-3b, NB-32-VI-3a, NB-32-VI-1d, NB-32-VI-1c at a scale of 1:50 000.
The boundaries of the Bambouto volcanic massif site correspond to those of the 1:50,000 scale topographic map named NB-32-XI-3a. Thus, the road network, the hydrographic network and the names of the localities of the Bambouto volcanic massif site are extracted by manual digitizing of this sheet.
The digital terrain models of the two study sites were made after manual digitizing of the contour lines on the corresponding 1:50,000 scale topographic maps.
2.3.2. On the field.
In the field, predefined layons were opened manually with a machete. Boreholes were then drilled with manual spiral Edelman soil augers Ø7cm—mixed soil. Soil pits varying in depth from 1.5 to 2 m were dug manually and the profiles were carefully described along the cooled walls.
The geographic coordinates of the layons, augering points and soil pits were surveyed using Garmin 73 GPS navigation devices. In the soil pits, the thickness of the horizons was measured with a tape measure, the color of the horizons was determined using the Munsell soil color chart [33], and the proportion of coarse elements in situ was determined using the "Visual aid for estimating the proportions of coarse elements [34]. Samples are collected from the walls, between 0 and 30 cm deep, and stored in plastic bags previously labelled.
Thus, in the Bokito site, 50 layons were opened, 681 boreholes drilled, 66 soil pits dug and 66 disturbed samples collected. In the Bambouto volcanic massif site, 45 layons were opened, 321 holes were drilled, 25 soil pits were dug and 25 disturbed samples were collected.
Thus, granulometric and chemical analyses were performed on 25 samples in the Bambouto site. In the Bokito site, granulometric analyses were carried out on 66 samples and chemical analyses on 23 samples. The selection of the 23 samples was made on the basis of the texture map made with ArGIS 10.1 software “Fig 3”.
2.3.3. In the laboratory.
The granulometry was done at the Laboratory of Soil Sciences of the University of Yaoundé 1. Particle size was determined by the Robinson pipette method after dispersion with sodium hexametaphosphate.
The chemical analyses were carried out in the soil laboratory of the International Institute of Tropical Agriculture (IITA) in Yaoundé.
Organic carbon (OC) was determined by the method of Walkey and Black, after mineralization by oxidation with potassium dichromate and concentrated sulfuric acid. Total nitrogen was measured with sulfuric acid after mineralization by oxidation in a digester. The pH water was measured with a pH meter on a soil/water mixture of 1/2.5 (Tables 1 and 2).
2.4. Evaluation of the agronomic suitability of soils
The agronomic suitability was calculated using formula: AA = pH × RU × CEC × K.
The pH is unitless. Useful water reserve (RU) is expressed in millimeter per centimeter of soil (mm.cm-1of soil). Cation exchange capacity (CEC) is expressed in milliequivalent per 100 g of soils (). Soil erodibility factor (K) is expressed in ton hour per mega joule per millimeter (t.hr. MJ-1 mm-1). Agronomic suitability (AA) is expressed in « equivalent hour per mega joule per millimeter (eq.hr. MJ-1 mm-1).
2.4.1. Acidity (pH).
pH map was spatialized using ordinary kriging in the software ArcGIS 10.1 (ESRI).
2.4.2. Useful water reserve (RU).
Useful soil water reserve (RU) was computing using equation [35]: where, RU: useful reserve water express in millimeters; H: thickness expressed in centimeters; TE: textural index determined from the texture class; EG: coarse elements expressed as a percentage. Useful water reserve is expressed in millimeter per centimeter of soil (mm.cm-1 of soil). The operation was carried out under the “raster calculator” in the software ArcGIS 10.1 (ESRI) and the spatialization of this parameter was done by ordinary kriging in the same software.
For the soils thickness, the depth of the humus horizon (H) expressed in centimeter (cm) is measured along the profile using a tape.
To determine the soil texture, the clay, sand and silt content of each sample is plotted on the corresponding axis. For each of the percentage values, a straight line parallel to the previous axis is drawn. The texture is read at the intersection of the three lines. To determine the textural index, the texture of each sample is determined by projecting the clay, sand and silt contents into the USDA texture diagram “Fig 4A” and “Fig 4C”.
A. USDA textural diagram of the soils of Bokito. B. Bouma and Van Lanen textural diagram of the soils of Bokito. C. USDA textural diagram of the soils of the Bambouto volcanic massif. D. Bouma and Van Lanen textural diagram of the soils of the Bambouto volcanic massif.
Then, a correspondence is established between the textural class obtained and the textural index (TE) is read from the texture index factor table for the surface horizon (Table 3).
The proportion of coarse elements (EG) expressed in percentage (%) was determined on the walls of the pit by using visual aid for estimating the proportions of coarse elements [34] “Fig 5”. The method consists of a visual assessment of soil stoniness according to estimation grids. These grids can be used mainly for crop profile studies.
Caption credit: Munsell. The Munsell Soil Color Charts with genuine Munsell color chips. Revised edition, Florida. 2009.
2.4.3. Cation exchange capacity (CEC).
The cation exchange capacity (CEC) map was spatialized using ordinary kriging in the software ArcGIS 10.1 (ESRI). CEC is expressed in milliequivalent per 100 g of soils ().
2.4.4. Erodibility (K).
Soil erodibility factor (K) was calculated according to the formula [37]:
1000 × K = 2.8 × 10−4(12 ‒ %MO) × M1.4 + 3.25(S ‒ 2) + 2.5(P ‒ 3) where, MO: organic matter in percentage; M = (%sand + %silt) × (100 ‒ %clay), with S: code on the soil structure; P: infiltration capacity. Soil erodibility factor is expressed in ton hour per mega joule per millimeter (t.hr. MJ-1mm-1). The spatialization of this parameter was done by ordinary kriging in the software ArcGIS 10.1 (ESRI).
To determine the soil structure, the texture of each sample is determined by projecting the clay, sand and silt contents into the textural diagram [38] “Fig 4B” and “Fig 4D”.
Then, a correspondence is established between the textural class obtained and the meaning of code on soil structure (Table 4).
The soil permeability is determining according to the proportion of clay and sand as presenting in the meaning of code on permeability (Table 5).
The methodological approach is summarized in the “Fig 6”.
3. Results
3.1. Statistical analysis of data
The interpretation of the statistical data (Table 6) attests to the consistency and non-scattering of the data both at the Bokito site and in the Bambouto massif.
3.2. Nature of the soils
In Bokito, four soil types are represented. Depleted yellow ferralitic soils in the upstream hills, meduim desaturated yellow ferralitic soils in the middle zone, weakly desaturated yellow ferralitic soils in the lower zone, and hydromorphic soils in the downstream part “Fig 7”.
In volcanic massif of Bambouto, six soil units are represented. Lithic andosols in the caldera and the upper zone on the upstream hill, andosols in the upper zone on gently slopes, andic ferrallitic soils in the middle zone, ferralitic soils and lithosoils in the lower zone, hydromorphic soils in valleys “Fig 8”.
Caption credit: Tematio P, Kengni L, Bitom D, Hodson ME, Fopoussi JC, Leumbe O et al. Soils and their distribution on Bambouto volcanic mountain, West Cameroon Highland, Central Africa. Journal of African Earth Sciences, 2004; 39(3), 447–457. http://10.1016/j.jafrearsci.2004.07.020.
3.3. Evaluation and mapping of the agronomic suitability of soils
At the Bokito site, soils are acidic to moderately acidic (5.0<pH<6.91) “Fig 9A”. The useful water reserve is low overall (<0.23 mm/cm) “Fig 9B”. Cation exchange capacity is also low on the weakly desaturated yellow ferralitic soils and on depleted yellow ferralitic soils, to medium on meduim desaturated yellow ferralitic soils () “Fig 9C”. Erobibility is low overall (<0.24 t.hr. MJ-1mm-1) “Fig 9D”. Agronomic suitability is relatively good to very good on the moderately desaturated yellow ferrallitic soils “Fig 9E”.
A. pH map of the Bokito study site. B. Useful water reserve map of the Bokito study site. C. Cation exchange capacity map of the Bokito study site. D. Erodibility map of the Bokito study site. E. Agronomic suitability map of the Bokito study site.
In the Bambouto volcanic massif, the pH is acidic on the ferralitic soils of the lower zone (pH<5.5) and moderately acidic on the andosols (5.8<pH<6.2) “Fig 10A”. The useful water reserve is average on the andosolic soils of the top part (0.55<RU<0.7 mm/cm) and optimal on the ferralitic soils of the bottom zone (0.7<RU<1.46 mm/cm) “Fig 10B”. The cation exchange capacity is low to medium on the ferralitic soils of the lower zone () and rose to
on the andosols of the high zone “Fig 10C”. Erodibility is low on the ferralitic soils of the lower zone of the massif (K<0.20
) and raised on the andosols of the high part where it reaches 0.60 t.hr. MJ-1mm-1“Fig 10D”. The agronomic suitability is good to very good on the andosolic soils of the middle and high zones, and average to poor on the ferralitic soils of the low zone “Fig 10E”.
A. pH map of the Bambouto volcanic massif site. B. Useful water reserve map of the Bambouto volcanic massif site. C. Cation exchange capacity map of the Bambouto volcanic massif site. D. Erodibility map of the Bambouto volcanic massif site. E. Agronomic suitability map of the Bambouto volcanic massif site.
An analysis of the properties of the soils studied shows that the soils with andosolic characteristics of the middle and upper parts of the Bambouto volcanic massif have better intrinsic properties overall than the red ferralitic soils of the lower zone of the Bambouto massif and the yellow ferralitic soils of the Bokito site. The ferralitic soils of the Bambouto volcanic massif are slightly more acidic than the ferralitic soils of the Bokito site. Conversely, the useful water reserve of the red ferralitic soils of Bambouto is much higher than that of the yellow ferralitic soils of Bokito.
Thus, the agronomic aptitude of the soils of the former Bokito site varies from 0.00 to 10.53 eq.hr. MJ-1mm-1 while on the Bambouto massif, it varies from 0.00 to 108.85 eq.hr. MJ-1mm-1.
In the Bokito site, a mapping of the agronomic aptitude shows that the moderated desaturated yellow ferralitic soils of Bokito have a relatively very good (AA = ]5.9, 10.53] eq.hr. MJ-1mm-1) to good (AA = ]2.1, 5.9] eq.hr. MJ-1mm-1) agronomic suitability. The depleted yellow ferralitic soils have a relatively average agronomic suitability (AA = ]0.4, 2.1] eq.hr. MJ-1mm-1), and weakly desaturation yellow ferralitic soils have a relatively low agronomic suitability (AA = ]0, 0.4] eq.hr. MJ-1mm-1).
In the Bambouto volcanic mountain, la spatialisation montre que andosols have relatively very good (AA = ]74.37 , 108.85] eq.hr. MJ-1mm-1) to good (AA = ]41.84, 74.37] eq.hr. MJ-1mm-1) agronomic suitability. The andic ferralitic soils have a relatively average agronomic suitability (AA = ]15.7, 41.84] eq.hr. MJ-1mm-1) and ferrallitic soils have relatively low agronomic suitability. (AA = ]0, 15.7] eq.hr. MJ-1mm-1).
In order to validate the results, field visits were made to compare the maps produced with the reality in both sites. Thus, in the Bokito site, the locality of Bonganda, around which the areas with high agronomic suitability are concentrated, is the main agricultural hub of the Mbam and Inoubou department. The main crops grown are plantain, peanuts, macabo, cocoa, pineapple and citrus. In the Bambouto volcanic massif, the middle and upper zones are among the main food and market garden basins of the West Cameroon region. The main crops grown are potatoes, tomatoes, carrots, peppers, onions, garlic and cabbage. In the lower zone, the main crops are taro and macabo, green beans, maize and coffee [39].
4. Discussion
4.1. Justification of the method
Agronomic suitability depends on soil quality which is related to physical properties and soil richness defined by chemical properties [4,12,40]. For the assessment of the suitability and potential of agricultural land, the USDA [41] and FAO [41] have developed methodological approaches that take into account intrinsic and extrinsic soil factors, and the results are qualitative. Soil intrinsic factors change over time and are amenable to correction [16,42,43], while soil extrinsic factors are essentially unchangeable [5,34]. It is also established that validation of results from qualitative studies is more complex than those from quantitative studies [44–47]. The new agronomic suitability assessment method we propose is a multiplicative function, focused on intrinsic soil factors. It integrates only four parameters of proven importance for soil characterization and plant growth [10,11,48–50] and the results are expressed both qualitatively and quantitatively.
4.2. Choice of input parameters
4.2.1. The hydrogen potential (pH).
Soil pH is a determining factor of fertility, it governs microbial activity, nutrient availability and the ability of plants to assimilate them [9,51,52]. It also provides information on toxicity risks [53]. Thus, the optimal soil pH for crops is between 5.5 and 8 [54–57]. The pH varies from 5 to 6.90 on the yellow ferralitic soils of the Bokito site, and from 4.9 to 5.7 on the red ferralitic soils of the lower zone of the Bambouto massif. It reaches 6.2 on the andosols of the summit part of the Bambouto massif.
Thus, the pH of the red ferralitic soils of the Bambouto massif is below the optimal threshold for plant growth.
4.2.2. The useful water reserve (RU).
Water stored in the soil is used as a source for 90% of global agricultural production [40]. Several authors have emphasized the importance of useful water reserve in plant growth [39,49,58,59]. In the Bokito site, the useful water reserve varies from 0.01 to 0.23 mm/cm on the yellow ferralitic soils, while in the Bambouto volcanic massif, it varies from 0.55 on the andosols of the upper zone to 1.45 mm/cm of soil on the red ferralitic soils of the lower zone. These results are in line with those obtained on ferralitic soils in southern Cameroon by [59] and on andosols in the mountain region [60–62].
In sum, the red ferralitic soils of the Bambouto volcanic massif have satisfactory hydric properties because a useful water reserve of 1 to 1.4 mm/cm of soil can significantly compensate for climatic contingencies [60].
4.2.3. The cation exchange capacity (CEC).
Cation exchange capacity expresses the potential fertility of a soil as it the reservoir of nutrients. [63]. The CEC varies from 2.93 to 21.90 on the yellow ferrallitic soils of the Bokito site, and from 10 to 27
on the red ferralitic soils of the lower zone of the Bambouto massif site. However, it reaches 42
on the andosols of the high zone of the Bambouto massif. It is established that CEC increases with the increase in organic matter [39,58,64] and this is verified in the study sites where the organic matter rate is capped at 5.65% in the yellow ferralitic soils of Bokito to 8% in the red ferralitic soils of the Bambouto massif, while it reaches 20.0% in the andosols of the Bambouto massif.
In summary, the yellow ferralitic soils of Bokito and the red ferralitic soils of the Bambouto volcanic massif therefore have a comparable cation exchange capacity. The CEC increases significantly on the andosolic soils of the middle and high zones of the Bambouto massif.
4.2.4. The soil erodibility (K).
Erodibility defines the soil’s susceptibility factor to erosion. In ferralitic soils, erodibility varies from 0.01 to 0.30 MJ-1mm-1hr. [65]. These results are consistent with those obtained on the yellow ferrallitic soils of Bokito, where erodibility varies from 0.01 to 0.17 MJ-1mm-1hr and on the red ferrallitic soils of the lower zone of the Bambouto massif where it varies from 0.05 à 0.28 MJ-1mm-1hr. However, [39] specifies that the erodibility can reach 0.7 on the most fragile soils (silts). However, the erodibility reaches 0.6 MJ-1mm-1hr. on the typical andosols of the high zone of the Bambouto massif [39] which pointed out that erosion can scale up to 0.7 on the most fragile soils. This is once again true here because the erodibility on the andosols of the upper part of the Bambouto massif is 0.6.
In summary, the erodibility of the yellow ferralitic soils of Bokito and the red soils of the Bambouto volcanic massif are comparable. It increases significantly on the andosolic soils of the middle and high zones of the Bambouto massif.
4.2.5. The agronomic suitability (AA).
The intertropical zone [66] has been subdivided into plateau landscapes (500 and 800 m altitude) surmounted by high plateaus (800 to 1800–2000 m) and mountainous massifs (>2000 m altitude) [67]. From a pedological point of view, the plateaus (Bokito site) and the high plateaus (Bambouto massif site) are characterized by very thick soils, yellow, red or purple in color [67,68] with the appearance of humus-bearing andosolic horizons above 1600 m [32]. The mountain massifs are the domain of andosols [30,67].
The agronomic suitability varies from 0.00 à 10.53 (eq.hr. MJ-1mm-1) in the site of Bokito, and from 0.00 to 108.85 (eq.hr. MJ-1mm-1) in the Bambouto massif. This difference is related to the nature of the soils, which are yellow ferralitic in the Bokito site, while in the Bambouto massif they are red ferralitic in the lower part, andic ferralitic in the middle part and andosolic in the upper part. In fact, ferralitic soils have a low nutrient retention capacity [68,69], whereas tropical andosols are very fertile [7,11,31,32,60,70].
In addition, the agronomic suitability of the red ferralitic soils of the Bambouto (0.00 to 15,7 eq.hr. MJ-1mm-1) is greater than that of the yellow ferralitic soils of the Bokito site (0.00 à 10.53 eq.hr. MJ-1mm-1). This difference is explained by the fact that these soils are chemically comparable [71], but physically, the red ferralitic soils are significantly better than the yellow ferralitic soils [72,73].
5. Conclusion
With more than 60% of the working population dependent on the agricultural sector for survival, land evaluation in sub-Saharan Africa is a central issue for the development of sustainable agriculture. Numerous methods for assessing the cultural suitability of soils have been developed in the world, but their adaptation to the tropical context is not always obvious. In this study, we propose a simple, inexpensive and relevant methodological approach for evaluating the agronomic suitability of soils that has been designed and tested in two agroecological zones in Cameroon. The sites selected for the tests of the functionality of the equation are the Bokito site in the agroecological zone known as "bimodal forest" and the Bambouto volcanic massif site in the agroecological zone known as "highlands". Four intrinsic soil parameters were taken into account for this evaluation: pH, useful water reserve, cation exchange capacity and erodibility. In the Bokito site, the soils are yellow ferralitic. On the Bambouto massif, the lower part is characterized by the presence of red ferralitic soils, the middle part by the presence of andic ferralitic soils and the upper part is the domain of andosols. Overall, the agronomic suitability of the soils of the Bokito site varies from 0.00 to 10.53 eq.hr. MJ-1mm-1 while on the Bambouto massif, it varies from 0.00 to 108.85 eq.hr. MJ-1mm-1. This is due to the fact that the tropical andosols developed on volcanic materials present in the summit part of the Bambouto massif are very fertile. Indeed, the red ferralitic soils of the lower part of the Bambouto massif have an agronomic aptitude that varies between 0.00 and 15.7 eq.hr. MJ-1mm-1. The acidic pH and the low cation exchange capacity of these red ferralitic soils developed on volcanic materials reduce their agricultural suitability. Nevertheless, the agronomic suitability of the red ferralitic soils of the Bambouto massif is higher than that of the yellow ferralitic soils of Bokito. This difference can be explained by the fact that these two types of soil are comparable chemically, but physically, the red ferralitic soils are clearly better than the yellow ferralitic soils. This study should be continued in the other three agro-ecological zones of Cameroon for a better understanding of the agronomic suitability of tropical soils.
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