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Discoveries in Agriculture and Food Sciences - Vol. 12, No. 1
Publication Date: February 25, 2024
DOI:10.14738/dafs.121.16511.
Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various
Previous Soil Histories in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
Services for Science and Education – United Kingdom
Qualities of Soil Upper Layers Under Rubber Plantation and
Various Previous Soil Histories in Zépréguhé (Daloa, west-central
Côte d'Ivoire)
Lacina Yéo
University Jean Lorougnon Guédé Daloa, UFR Agroforestry,
Agrovalorisation Laboratory, BP 150 Daloa (Côte d’Ivoire)
Mekapeu Grace Tokapieu
University Jean Lorougnon Guédé Daloa, UFR Agroforestry,
Agrovalorisation Laboratory, BP 150 Daloa (Côte d’Ivoire)
Dogniméton Soro et Ambéyin Touré
University Jean Lorougnon Guédé Daloa, UFR Agroforestry,
Agrovalorisation Laboratory, BP 150 Daloa (Côte d’Ivoire)
ABSTRACT
This study focused on the diagnosis of soil fertility in Zépréguhé, a village located 8
km from Daloa in the center-west of Côte d'Ivoire. Four soil pits were planted
according to a toposequence oriented West-East under rubber and under previous
cultivation. All the pits have been described and soil samples in the 0-30 cm horizon
have been analysed at the plant and soil analysis laboratory of the Institute National
Polytechnique Félix Houphouet-Boigny in Yamoussoukro. The particle size, the
organic matter and the characteristics of the adsorbent complex were determined.
It emerges from this description and analysis that all soils in the study area are
ferralsols. The values obtained were compared to the critical thresholds in order to
determine the level of soil fertility. The results show that these soils, which appear
to be rich in organic matter, were really low in organic matter and very low in
acidity over the entire toposequence. Due to the crops in place, the upper slope and
Upper mid-slope positions accumulate badly decomposed organic matter.
Furthermore, the cation exchange capacity (CEC) is normal at the upper slope and
upper mid-slope and very low at the bottom of the slope. The sum of exchangeable
bases is small at the level of all the profiles. The soils of the study site have a low to
moderate level of fertility. Rubber and sorghum crops have better soil fertility and
particularly organic matter.
Keywords: Fertility, ferralsols, crop precedent, toposequence, Daloa, Côte d’Ivoire.
INTRODUCTION
In sub-Saharan Africa, population growth has led to an increase in food demand. Intensive
agriculture and the search for new fertile land result in pressure on the ecosystem, which, in
turn, leads to reduced soil fertility [1-2]. In most countries, soils have low fertility and exported
nutrients are not adequately replaced. Moreover, the practice of long-term fallow is tending to
disappear, giving way to short-term fallow and settled agriculture [3-4]. As a result, yields are
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
relatively low and land productivity decreases ([5-3]. Since soil is therefore the basis of all
agricultural production, knowledge of its physical and chemical potential is important for crop
development [6] For cultivated soils, the distribution of organic matter is likely to vary greatly
[7]. Such heterogeneity is a limiting factor in the proper match between the characteristics of
the soil and the doses of fertilizers to be applied to an agricultural plot. There is therefore an
interest in increasing and enhancing knowledge of soils, particularly for agricultural issues that
are ever increasing [8]. Thus, knowledge and mastery of the physical and chemical properties
of the soil are a prerequisite for good practice However, few people in West Africa and
especially in Côte d'Ivoire determine the soil fertility potential of their farm before its
development. If, according to the farmers, certain indicators make it possible to make a
diagnosis of fertility such as plants and animals that are indicators of soil fertility and the
biophysical state of the plot [10], these prove to be insufficient and often unreliable. to
determine the fertility potential of the soil. It is then necessary to resort to soil analysis in order
to specify the agricultural potential of its soil before its development [6].
The soils of the study area being moderately to strongly desaturated ferralitic soils (ferralsols)
are exposed to a rapid loss of their fertility potential [11]. Thus, the hypothesis underlying this
study is that soils under perennial cultivation would limit the loss of fertility less than those
under annual or seasonal cultivation.
The general objective of this study is to compare the level of soil fertility under perennial and
annual crops in Zépréguhé.
Specifically, it was a question of evaluating the physico-chemical characteristics of the surface
horizons of the soils under rubber and under different cropping precedents and deducing the
fertility of these soils.
MATERIALS AND METHODS
Location of the Study Site
The study was carried out in Zépreguhé (6°54'13''N and 6°21'51''W) 8 km from Daloa in the
Center-West of Côte d'Ivoire (Figure 1). The department of Daloa is characterized by a tropical
climate, with a very hot and dry period from November-March and a rainy period from April- October. Chromoleana and Imperata are the dominant grasses. The site has low slopes.
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 12, Issue 1, February- 2024
Services for Science and Education – United Kingdom
Figure 1: Study area and plot location
Methods
Morpho-Pedological Characterization of the Soil:
Four cultural pits were opened on a toposequence oriented West – East starting from the
rubber plot, near the village in the lowlands (market gardening site). Each profile was identified
by its topographic position (HV: upper slope; MVinf: lower mid-slope, MVsup: upper mid-slope
and BP: lower slope). These soil pits have been described horizon after horizon. The
classification used is that of [12].
The characterization of the profiles took into account:
• the useful depth of the soil and the thickness of the layers were made by direct
measurement with a tape and by observation;
• the soil colour was determined using the Munsell code;
• the texture was made by observation in the field using the coil and ring method;
• the structure was determined by observing the organization and arrangement of soil
aggregates;
• the porosity was determined by observing the aggregates;
• biological activity was assessed by visual observation of the quality and number of roots
as well as microorganisms;
• the load of coarse elements was evaluated using the Munsell code;
• the organic matter content was assessed by visual observation;
• the humidity was assessed by direct visual observation of the horizons.
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
Physic-Chemical Characterization of the Soil:
Soil samples were taken at the end of each profile description in the horizons encountered in
the first 30 cm of the soil for laboratory analysis. Thus, 8 (two per horizon) soil samples were
taken and analysed in the laboratory. All the analysis were carried out at the plant and soil
analysis laboratory of the National Polytechnic Institute Félix Houphouet-Boigny in
Yamoussoukro. The parameters analysed are:
• The particle size is determined by the densiometric method using the Robinson pipette
[13]. This method consists in separating the soil particles according to their dimensions.
It allows to know in a weight form, the distribution of mineral particles of less than 2
mm in diameter according to different textural classes.
• The water pH of soil samples is measured by direct reading with a pH meter according
to a soil/distilled water ratio of 1:2.5 after shaking the suspension [14-15].
Organic carbon was determined after calcination of soil samples in a muffle furnace according
to the method of [16]. This determination was made by the method of cold sulfochromic attack
after oxidation with potassium dichromate (K2Cr2O7) in a strongly acid medium (H2SO4). The
determination of the carbon rate made it possible to calculate the organic matter (OM) content:
OM = C × 1.724 ............................................................... (1)
Where,
• OM: Organic matter content (%)
• C: Organic carbon content (%).
Nitrogen is measured using the Kjeldahl method [17]. Mineralization consists of the
transformation of organic nitrogen into a mineral form (ammonium sulphate) in a concentrated
medium [sulfuric acid (H2SO4)], in the presence of a catalyst. Distillation allows the
transformation of (NH4)2SO4 into NH4OH in the presence of an excess of sodium hydroxide
which alkalinizes the reaction medium. The solution obtained is distilled, then the ammonium
is recovered in a solution of boric acid which has been titrated using a solution of sulfuric acid.
The assimilable phosphorus was determined according to the modified Olsen method [18] and
the total phosphorus by colorimetry after extraction with perchloric acid [19]. The
exchangeable bases (Na+, K+, Ca2+, Mg2+) were extracted with 1 M ammonium acetate buffered
at pH=7. Calcium and magnesium were quantified by atomic absorption spectrophotometry. As
for potassium, it was measured through flame spectrophotometry [20]. The cation exchange
capacity (CEC) corresponds to the sum of the bases of the exchange complex.
Data Processing:
The data collected in the field were entered and coded using the Microsoft Excel 2013
spreadsheet. These data were processed with the R software. The normality of the distributions
of the samples was verified by the Shapiro-Wilk test and the homogeneity of variances verified
by Levene's test. If the variable from which the sample comes follows a normal law and there is
homogeneity of the variances, an ANOVA is carried out, otherwise the non-parametric Kruskal- Wallis’s test was applied. The analysis of variance was completed by Tukey's test which made
it possible to classify the means at the probability threshold of 5%.
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 12, Issue 1, February- 2024
Services for Science and Education – United Kingdom
RESULTS
Morpho-Pedological Characteristics of the Soil
The soil observed under the rubber tree is ferralitic, deep and well differentiated in horizons
(figure 2). The profile at the upper slope (HV) presents four horizons. The A1 type surface
horizon has a sandy-clay texture and a lumpy structure; it is fresh, very porous, dark brown in
colour (7.5YR 2.5/2) and very humus-rich. Its thickness is (0-20/25 cm) and presents a regular
limit. The soil observed under sorghum is ferralitic and deep. In fact, at Zépréguhé, this upper
mid-slope soil (MVsup) has four horizons. The A11-type surface horizon has a sandy-loamy
texture and a lumpy structure. This horizon is dry, very porous, dark brown in colour (7.5 YR
3/2) and humus. Its thickness varies from 0-13 cm and has few roots. The soil observed under
cassava-maize is ferralitic. This lower mid-slope (MVinf) soil has four horizons. The A11-type
surface horizon has a sandy-loamy texture and a lumpy structure. This horizon is dry, very
porous and has few roots. It is dark brown in colour (7.5 YR 3/3), humus-rich and estimated at
27 cm thick. The soil observed under previous market gardening is hydromorphic, deep and
well differentiated in horizons. This soil at the bottom of the slope (Bp) has five horizons. The
A11-type surface horizon has a sandy-silty texture and a lumpy structure. This horizon is cool,
humus, porous, well drained, loose and has abundant roots of millimeter size. It is dark brown
in colour (gley1 3/N).
Figure 2: Soil profiles at different topographic positions corresponding to different land uses.
a: ground under rubber at the upper slope; b: soil under previous sorghum at the upper mid- slope; c: previous cassava-maize soil at the lower mid-slope; d: soil under previous market
gardening at the bottom of the slope
Physico-Chemical Parameters of Soils
Granulometry:
Table 1 summarizes the results relating to the particle size under the different positions on the
toposequence. This reveals differences at various levels of significance between the
topographic positions. Only the clay content was not influenced by the position on the
toposequence. The proportions of Silt, Sand and the sum Clay + Silt varied highly significantly
(P ˂ 0.001) from one position to another. For the clay content, the upper mid-slope position
(MVsup) and the upper slope position (HV) had the highest rates with 12.25 and 6.25%
respectively. These same topographic positions presented the highest rates of Clay + Silt with
60.52% for MV sup and 42.67% for HV. However, the highest levels of silt (L) are observed at
the MVsup and lower mid-slope (MVinf) positions with respectively 48.27 and 38.22%. The
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
bottom of the slope (Bp) and MVinf positions were the sandiest with 69.08 and 58.23% sand
respectively.
Table 1: Soil granulometry
Topographical positions Granulometry
A L A+L S Texture
Upper slope (HV) 6.25a 36.42ab 42.67b 55.25b
Loamy-sand
Upper mid-slope (MVsup) 12.25a 48.27a 60.52a 37.03c
Loam
Lower mid-slope (MVinf) 2.00a 38.22ab 40.22b 58.23ab Loamy-sand
Bottom of the slope (Bp) 3.50a 26.37b 29.87c 69.08a
Sandy-loam
CV 0.75 0.216 0.257 0.213
P 0.101 0.00925 0.00105 0.00175
Effect ns ** ** **
A: clay; L: silt; S: sand; In the same column, the means followed by the same letter are not significantly different
at the threshold of α=0.05; **: highly significant difference P ˂ 0.01; ns: non-significant difference P ˃ 0.05; CV:
coefficient of variation
Soil Acidity and Organic Matter:
No significant difference was observed between the different positions of the toposequence for
soil acidity, carbon, total nitrogen, C/Nt ratio and organic matter content. The variations of
these parameters between the different topographic positions are presented in Table 2. The pH
generally varied between 6.50 in the rubber plantation (upper slope) and 6.90 in the previous
market gardening (bottom of the slope) in a very low acid range. The carbon content of the soil,
generally low, varied little on the plots and remained between 0.60% (BP) and 0.89% (MVinf).
The highest carbon content was obtained on the upper mid-slope (MVsup) with a value of
1.41%. The soils of the area were also low in carbon. The Nt content of the soil, generally
acceptable at all levels of the toposequence, is however low at the bottom of the slope with
0.060%. The soil C/Nt ratio in the plots varied between 8.80 (MVinf) and 16.20 (MVsup). This
ratio was very weak at the lower mid-slope and quite good at the bottom of the slope (10.15)
and a little high for the upper and upper mid-slope positions with 15.12 and 16.20 respectively.
The O.M content of the soil, which varied between 1.04% at the bottom of the slope and 2.44%
at the upper mid-slope, is in low proportions in the soils of the plots studied.
Table 2: Soil acidity and organic matter
Topographical positions acidity and organic matter
pH C (%) Nt (%) C/Nt OM (%)
Upper slope (HV) 6.50a 1.20a 0.12a 15.12a 2.07a
Upper mid-slope (MVsup) 6.65a 1.41a 0.10a 16.20a 2.44a
Lower mid-slope (MVinf) 6.55a 0.89a 0.10a 8.80a 1.54a
Bottom of the slope (Bp) 6.90a 0.60a 0.06a 10.15a 1.04a
CV 0.031 0.477 0.636 0.433 0.470
P 0.345 0.531 0.923 0.613 0.531
Effect ns ns ns ns ns
pH: hydrogen potential; C: carbon; C/Nt: carbon-nitrogen ratio; Nt: total nitrogen; OM: organic matter; In the
same column, the means followed by the same letter are not significantly different at the threshold of α=0.05; ns:
non-significant difference P ˃ 0.05; CV: Coefficient of variation.
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 12, Issue 1, February- 2024
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Chemical Parameters:
The results on the chemical parameters are recorded in Table 3. Only the CEC made it possible
to observe a significant difference (P ˂ 0.01) between the topographic positions. This CEC
varied between 2.22 cmol.kg-1 and 11 cmol.kg-1. The CEC is very low in the previous market
gardening at the bottom of the slope (2.22 cmol.kg-1) and on the previous cassava-maize at
lower mid-slope (4.88 cmol.kg-1). It was average on the upper mid-slope (sorghum) and upper
slope (rubber) with respectively 11 and 10.20 and cmol.kg-1. The sum of the bases between 1.2
cmol.kg-1 on the bottom of the slope (BP) and 3.4 cmol.kg-1 on the upper slope (HV) was very
low in these soils when the saturation rate increased from low (22.4% for the previous
sorghum) to medium (56.3% for the previous market gardening). The overall low assimilable
phosphorus content varied from 19 cmol.kg-1 (bottom of the slope) to 21.75 cmol.kg-1 (lower
mid-slope).
The calcium content (Ca2+) varied within acceptable proportions. Thus, the calcium content
varied between 0.772 cmol.kg-1 on the bottom of the slope (BP) and 2.525 cmol.kg-1 on the
upper slope (HV). The high values were obtained on the upper mid-slope (MVsup) and upper
slope (HV) positions with respectively 1.087 and 2.525 cmol.kg-1. The magnesium content
varied between 0.340 cmol.kg-1 (BP) and 1.052 cmol.kg-1 (MV sup). The high levels were
obtained on the upper mid-slope (MVsup) and upper slope (HV) treatments with respectively
1.052 and 0.668 cmol.kg-1. Overall, these levels are acceptable. The soil at the site was very low
in potassium. Indeed, the highest potassium content was obtained in BP with 0.074 cmol.kg-1
for expected values greater than 0.1 cmol.kg-1.
Table 3: Variation of some chemical parameters according to the position on the
toposequence
Topographical positions Chemical parameters
P.ass CEC Ca2+ Mg2+ K
+ Na+
Sb V
Upper slope (HV) 20,50a 10,20a 2,525a 0,668a 0,066a 0,169a 3,429a 33,180a
Upper mid-slope (MVsup) 19,25a 11,00a 1,087a 1,052a 0,066a 0,161a 2,366a 22,484a
Lower mid-slope (MVinf) 21,75a 4,88ab 1,009a 0,561a 0,059a 0,129a 1,758a 39,019a
Bottom of the slope (Bp) 19,00a 2,22b 0,772a 0,340a 0,074a 0,047a 1,233a 56,357a
CV 0,521 0,548 0,571 0,441 0,114 0,493 0,424 0,364
P 0,997 0,022 0,069 0,075 0,392 0,259 0,094 0,073
effete ns * ns ns ns ns ns ns
P.ass: assimilable phosphorus; CEC: cation exchange capacity; Ca2+: calcium; Mg2+: magnesium; Na+: sodium; K+:
potassium; Sb: sum of bases; V: saturation rate; * significant difference P ˂ 0.05. In the same column, means
followed by the same letter are not significantly different at the threshold of α=0.05; ns: non-significant
difference P ˃ 0.05; CV: Coefficient of variation.
Table 4 presents some equilibrium ratios between Ca2+, Mg2+ and K+ cations in the soils studied.
The K/CEC ratio was low in all the soils compared to the expected norms between 2 and 3.
Indeed, this ratio varied between 0.006 (MVSup) and 0.06 (HV). It translates the extreme
poverty of the soil in potassium. These values reflect a potassium (K) deficiency in these soils.
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
The Ca2+/Mg2+ ratio was optimal for all positions except at MVsup where it is low. This ratio
varied from 1.02 (upper mid-slope) to 4.1 cmol.kg-1 (upper slope). The optimal values of the
Ca2+/Mg2+ ratios are between 2 and 9 according to [21].
The K+/Mg2+ ratio is optimal in all the treatments except at BP where it is high, whereas the
(Ca2++ Mg2+)/K+ ratio is high in all the treatments.
Table 4: Equilibrium ratios between Ca2+, Mg2+ and K+ cations in soils
Ratios Upper slope
(HV)
Upper mid-slope
(MVsup)
Lower mid-slope
(MVinf)
Bottom of the slope
(Bp)
Ca2+/Mg2+ 4,1 1,02 2 2,3
K
+
/Mg2+ 0,09 0,05 0,08 0,20
(Ca2++Mg2+)/K+ 53 35,5 31,2 15,8
K
+
/CEC 0,06 0,006 0,012 0,033
Linking the Parameters Studied:
The physico-chemical parameters of the soil establish between them positive or negative
relationships (Table 5). Sand correlates negatively (r = 0.81 and 0.85; P ˂ 0.05) with clay and
saturation level. The A+L sum established a weak positive correlation (r = 0.79; P ˂ 0.05) with
clay, a strong positive correlation with CEC and magnesium (r=0.84 and 0.90; P ˂ 0.01) and a
strong but negative correlation (r = 0.86; P˂ 0.01) with saturation rate. The sum A+L also
established a very strong positive correlation with silt (r = 0.94; P ˂ 0.001) and negative with
sand (r = 0.99; P ˂ 0.001). Silt correlated positively (r = 0.74; P ˂ 0.05) with magnesium, and
established a very strong negative correlation (r = 0.92; P ˂ 0.001) with sand and saturation
rate.
Carbon established good positive correlations with clay (r = 0.78; P ˂ 0.05) and sum of bases (r
= 0.74; P ˂ 0.05) and a negative correlation (r = 0.79; P ˂ 0.05) with pH. The correlation was
very strongly positive (r = 1; P ˂ 0.001) with organic matter and strongly positive with
magnesium (r = 0.87; P ˂ 0.01) and nitrogen (r = 0.86; P ˂ 0.01). Organic matter content
correlated positively (r = 0.78; P ˂ 0.05) with clay and sum of bases (r = 0.74; P ˂ 0.05) and
negatively (r = 0.79; P ˂ 0.05) with pH. It also establishes a strong positive correlation (r = 0.87
and 0.86; P ˂ 0.01) with magnesium and nitrogen.
Magnesium established a strong positive correlation (r = 0.89; P ˂ 0.01) with clay and a negative
one (r = 0.92; P ˂ 0.01) with sand. Calcium (Ca2+) established a weak negative correlation (r =
0.70; P ˂ 0.05) with pH and a very strong positive correlation (r = 0.94; P ˂ 0.0001) with the
sum of bases. CEC showed a weak positive correlation (r = 0.78; 0.75 and 0.74; P ˂ 0.05) with
silt, magnesium and sum of bases respectively. With sand and saturation rate, CEC establishes
strong negative correlations (r = 0.84; and 0.88 respectively with P ˂ 0.01).
Nitrogen establishes a strong negative correlation (r = 0.83; P ˂ 0.01) with pH, which is weakly
and negatively correlated (r = 0.79; P ˂ 0.05) with the sum of bases.
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Discoveries in Agriculture and Food Sciences (DAFS) Vol 12, Issue 1, February- 2024
Services for Science and Education – United Kingdom
Table 5: Correlation matrix of study parameters
V
-0,48
-0,86**
-0,48
-0,55
-0,26
-0,88**
0,28
-0,92**
-0,48
-0,70
-0,38
-0,14
-0,09
0,39
-0,85**
-0,45
1
Sb
0,55
0,43
0,74*
-0,0002
0,94***
0,74*
-0,36
0,29
0,74*
0,59
0,57
0,69
0,11
-0,79*
-0,47
1
S
-0,81*
-0,99***
-0,66
-0,34
-0,18
-0,84**
0,33
-0,92***
-0,66
-0,92**
-0,53
-0,29
0,01
0,40
1
pH
-0,37
-0,36
-0,79*
0,34
-0,70*
-0,50
0,58
-0,30
-0,79*
-0,59
-0,39
-0,83**
-0,33
1
P.ass
0,01
-0,05
0,45
-0,20
0,05
-0,07
0,12
-0,08
0,45
0,23
-0,18
0,39
1
Nt
0,49
0,24
0,86**
-0,63
0,62
0,32
-0,54
0,05
0,86**
0,56
0,17
1
Na+
0,41
0,53
0,36
0,19
0,42
0,51
-0,43
0,50
0,36
0,49
1
Mg2+
0,89**
0,90**
0,87**
0,09
0,30
0,75*
-0,29
0,74*
0,87**
1
MO
0,78*
0,62
1,00***
-0,24
0,55
0,61
-0,38
0,42
1
L
0,54
0,94***
0,42
0,48
0,04
0,78*
-0,38
1
K+
-0,11
-0,32
-0,38
0,45
-0,30
-0,27
1
CEC
0,67
0,84**
0,61
0,47
0,58
1
Ca2+
0,30
0,15
0,55
-0,05
1
C/Nt
0,08
0,38
-0,24
1
C
0,78*
0,62
1
A+L
0,79*
1
A
1 A
A+L
C
C/Nt
Ca2+
CEC
K+
L
MO
Mg2+
Na+
Nt
P.ass
pH
S
Sb
V
A: clay; L: silt; S: sand; pH: hydrogen potential; C: carbon; C/Nt: carbon to nitrogen ratio; Nt: total nitrogen; OM:
organic matter; P.ass: assimilable phosphorus; CEC: cation exchange capacity; Ca2+: calcium; Mg2+: magnesium;
Na+: sodium; K+: potassium; Sb: sum of bases; V: saturation rate; * low correlation; **: strong correlation; *** very
strong correlation
Page 10 of 15
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
Projection on factorial plane 1-2 (Figure 6.A), associates positively, CEC, A+L, A, C/Nt, M.O, Mg2+,
C, L, Sb, Ca2+, Nt and negatively V, S to axis 1. It associates positively with axis 2, pH and K+ but
negatively with P.ass. The factor map (figure 6.B) thus defines three fertility groups:
Group 1 consists only of the soil at the bottom of the slope occupied by the previous market
garden. It is characterized by pH, presence of sand (S) and saturation level (V). These soils are
the least rich in organic matter and minerals. The lower mid-slope forms group 2, which was
characterized by better availability of assimilable phosphorus (P.ass).
Group 3 includes the soils of the upper slope (rubber tree) and the upper mid-slope (previous
sorghum). It is characterized by the highest availability of clay (A), silt (L), organic matter (MO)
and total nitrogen (Nt), with a higher sum of bases (Sb) and cation exchange capacity (CEC).
These two levels represent the most fertile soils in the toposequence.
Figure 3: PCA associating soil parameters
A: clay; L: silt; S: sand; pHeau: hydrogen potential; C: carbon; C. Nt: carbon to nitrogen ratio; Nt: total nitrogen;
M.O: organic matter; P.ass: assimilable phosphorus; CEC: cation exchange capacity; Ca2.: calcium; Mg2.:
magnesium; Na: sodium; K.: potassium; Sb: sum of bases; V: saturation rate; BP: Bottom of the slope; HV: upper
slope; MVinf: lower mid-slope; MVsup: upper mid-slope; G1: group1; G2: group2; G3: group3.
DISCUSSION
The physical description of the profiles showed that the soil of the study area presents few
physical constraints for agricultural development in its lower half. Indeed, with the exception
of the mid-slope with plinthic characteristics which presented an induration at less than 50 cm,
all the other positions have deep soils that are more or less loose on the surface. The surface
horizons of the soils of the area, with a loamy-sandy texture, are certainly poor in clay but seem
to have a good content of organic matter (mostly gray to dark gray) especially on the upper part
of the toposequence under rubber and sorghum mulch. These characteristics show that the soil
of the site has good cultivation potential. These results corroborate those of [22] who
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demonstrated that a lumpy structure associated with a loamy-sandy texture offers physical
properties conducive to plant development. The realization of the textural triangles made it
possible to locate the soils in the category of soils with a balanced texture for water
management, with the exception of the soil at the bottom of the slope, which filters too much.
The abundance of sand in the lower positions of the toposequence reveals a greater ease of
displacement of this coarse fraction compared to the finer elements more solidly in links.
Moreover, the good porosity of the soils can be explained by the abundance of roots and coarse
elements in the upper horizons. This porosity is likely to promote good root growth and good
plant development [23]. However, the low clay content could have consequences on the quality
of the soils, in particular their chemical fertility as noted [9] on the soils of Gogbala in the North
of Côte d'Ivoire.
The results of the particle size analysis confirmed the predominance of silts by the loamy
texture of these soils. These results testify that the soils at Zépréguhé are suitable for
agricultural practice in accordance with the studies of [24-25-26] which reported that the
loamy texture of the soils is excellent and suitable for most crops.
The soils are very weakly acidic to almost neutral, with pH values ranging from 6.50 to 6.90.
These pH values are in favour for good biological activity and good availability of the majority
of mineral elements for plants, because they are better dissolved in them [27]. Since pH is a key
factor in the life of soil microorganisms and determines the availability of nutrients and plant
nutrition [21-28], it does not alone condition the cycle of matter. Indeed, the type of material,
temperature and humidity also contribute. Thus, the low availability of mineral elements in
these soils would be explained by the poor availability of organic matter in the previous market
gardening and cassava-maize on the one hand and by a poor biological activity under rubber
due to an excess of humidity. on the other hand.
The low level of carbon and nitrogen characterizes the soils of the lower part of the
toposequence subject to continuous development without external inputs. Indeed, shifting or
successive cropping systems influence the dynamics of organic matter [29] and do not give the
soil enough time to build up its nitrogen and carbon stock [30]. The lower part was occupied by
the cultivation of cassava-maize on the lower mid-slope and market gardening at the bottom of
the slope. These speculations cover little soil and have a low level of renewal of organic matter
with very little plant deposit for cassava-maize and permanent ploughing in the case of market
gardening. On the other hand, the upper part of the study plot is occupied by a rubber plantation
more than seven years old and a field of sorghum, a crop introduced by people from northern
Côte d'Ivoire and Burkina Faso. While rubber trees provide strong litter, a source of slowly
decomposing organic matter, harvesting sorghum leaves stumps and stumps that also
decompose slowly. The low carbon and nitrogen content and the low availability of K+ and Mg2+
are explained by a poor biological activity at these levels of the toposequence. These results are
in agreement with those of [31] which states that the more natural vegetation gives way to
agricultural land, the more there is a reduction in the production of biomass and organic matter
in the soil. Organic restorations through long-term fallows make it possible to restore the
fertility of soils depleted by several years of successive crops [32].
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Yéo, L., Tokapieu, M. G., & Ambéyin Touré, D. S. (2024). Qualities of Soil Upper Layers Under Rubber Plantation and Various Previous Soil Histories
in Zépréguhé (Daloa, west-central Côte d'Ivoire). Discoveries in Agriculture and Food Sciences, 12(1). 42-56.
URL: http://dx.doi.org/10.14738/dafs.121.16511
Permanent soil cover in rubber cultivation reduces material loss. The same is true for sorghum.
Indeed, sorghum can be used as a previous crop providing plant cover to fight against erosion,
due to its powerful root system, high biomass production, soil cover provided by cover crop
residues dead.
The available phosphorus content is very low in all soils. These results are similar to those
obtained by [33] under cocoa trees in the south-west of Côte d'Ivoire. The very low content of
assimilable phosphorus is typical and specific to many soils of inter-tropical regions because of
their strong fixing power ([34]. The low level of assimilable phosphorus would also be due to
the low content in these soils of matter which is the main cause of the bioavailability of
phosphorus in soils in tropical environments. Indeed, the work of [35] showed a similar effect
of organic matter on the bioavailability of phosphorus in soils under cocoa crops.
Soil contents of calcium, magnesium, potassium and sodium and the sum of bases are low in
most soils. Only the upper slope and upper mid-slope soils have an acceptable Ca2+ content for
the first and an acceptable Mg2+ content for the second. This poor supply of exchangeable
cations on the whole would be due to the export by the harvests over several years without a
manure of restitution and the bad decomposition of the organic matter. This result is similar to
those of [36] carried out on tropical soils in Burkina Faso which show the low sum of the bases
of the soils under cultivation and of [37] obtained after analysis of the soils under cashew trees
in Côte d'Ivoire. This low content will have a negative impact on productivity because these
cations are involved in important physiological processes for plants, such as photosynthesis,
fruiting and cell permeability. The cation exchange capacity is low at the bottom of the slope
and lower mid-slope, associated with low saturation in the upper part, confirming low plant
nutrition.
All these physical and chemical characteristics denote an ferralsol in this area. The soils present
highly desaturated complexes reflecting their low availability of cations. Similar results were
obtained by [9] on the soils of Gogbala in northern Côte d'Ivoire.
The K/CEC ratio is low in soils of all treatments according to [21]. Indeed, according to this
author, the expected values of this ratio must be between 2 and 3. It therefore presents an
imbalance for good potassium nutrition of plants. The Ca2+/Mg2+ ratio is satisfactory in the
rubber plantation at the upper of the slope and in the lower part of the toposequence, i.e., on
the cassava-maize and market gardening plots. On the other hand, on the upper mid-slope,
difficulties in assimilating calcium could be encountered. At the bottom of the slope, the low
availability of Mg2+ would limit the assimilation of potassium for certain plants due to a high
K+/Mg2+ ratio. The high (Ca2++ Mg2+)/K+ ratio over the whole toposequence confirms that the
assimilation of calcium and magnesium can constitute factors limiting agricultural productivity
in this zone. Indeed, the optimum of this last ratio must be between 12 and 15 for a good
nutrition in calcium and magnesium [38-26].
Land occupation by crops in this area was made according to a fertility gradient from the top of
the slope to the bottom of the slope with a succession of rubber-sorghum-cassava/maize- market gardening crops. This fertility gradient could, from the point of view of organic matter,
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be due to speculation and the management of agricultural residues. Indeed, the rubber tree
constitutes, through the seasonal loss of leaves, a reservoir of organic matter that is annually
renewed, just like the cultivation of sorghum for which the harvest residues are kept on the
plot. Thus, agriculture can mitigate climate change by implementing beneficial management
practices that increase soil carbon storage capacity and limiting practices that contribute to soil
organic carbon loss to the atmosphere. through crop choice and residue management.
CONCLUSION
The soils of the study area presented a cropping suitability ranging from low (bottom of the
slope) to moderately good at the top of the slope. If this gradient comes from the position on
the toposequence, it does not remain the only justification. Speculation and residue
management have had a strong contribution to this differentiated fertility on the toposequence.
The introduction of sorghum in the area seems, in addition to satisfying a food need imported
from the regions of origin, to bring an improvement in the management of soil fertility if crop
residues are kept on the ground. The cultivation of rubber trees has the advantage of favouring
the protection of the soil against erosion, while maintaining a microclimate that can slow down
the decomposition of organic matter, just like the previous sorghum. Rubber and sorghum
manage soil fertility better than the other crops in this study. The previous cassava-maize
appeared to have the most detrimental effect on organic matter management and, in turn, on
overall soil fertility. The bottom of the slope, which immediately presents a physical constraint
due to its richness in sand, occupies the lowest level of fertility due to continuous exploitation
by crops with a low level of organic matter constitution. The multiplicity of species on the plot
and the repeated number of plantings is a factor in soil depletion because various nutrients are
simultaneously taken up in large quantities by these vegetables.
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