The study was conducted in the administrative region of Marahoué in Côte d’Ivoire (West Africa), specifically in the sub-prefecture of Bouaflé located in the centre-east of the region and the sub-prefecture of Kononfla in the south-east. The Marahoué region itself is located in central-western Côte d’Ivoire, between latitudes 6˚58'59.999" and 6˚37'12" north and longitudes 5˚45'0" and 5˚55'12" west.

The region receives between 1800 and 2000 mm of rainfall annually, with an average temperature of 25˚C to 30˚C and a humidity level of around 75 pc. These climatic conditions are conducive to agriculture and livestock farming [10] . The Bandama Rouge or Marahoué and Bandama Blanc rivers flow through the region [11] . It lies in a transition zone between the dense forest to the south and south-west and the wooded savannah to the north and north-east [12] .

The relief is made up of low plateaux, plains and hills reaching an average altitude of 260 m, giving the region a relatively flat topography. Geologically, the area belongs to the Birrimian granite and schist complex [13] . The soils are predominantly Eutric Ferralsols, with some differences between the forest and savannah zones. Acrisols are also present in the north and north-east, as well as Gleysols near watercourses [10] .

A total of six cocoa plantations, evenly distributed between the sub-prefectures of Bouaflé and Kononfla, were used to collect soil data. The Bouaflé sites are located in the villages of Guessanfla (N6˚55'73.3" W5˚45'76.8"), Krayaokro (N6˚54'91.1" W5˚45'71.2") and Simporéfla (N6˚53'51.0" W5˚45'74.0"), while those of Kononfla were located in the villages of Diénembroufla (N6˚38'54.6" W5˚38'01.3"), Koumoudji (N6˚38'22.9" W5˚38'24.5") and Kayéta (N6˚38'50.2" W5˚40'06.1").

In each plantation, two useful plots were delimited: the first, measuring 100 × 100 m, was located in a healthy part of the cocoa plantation and the second, of the same size, was located in an infested part ( Figure 1 ). The two plots were sufficiently far apart (at least 50 m) to avoid confusing the soils.

In each plot, three soil sampling points were identified: P1, P2 and P3. These points correspond, in this order, to points on the plot where the cocoa trees have high, medium and low vigour. Indeed, under the conditions of the study, the level of crop development (growth, health, infestation, yield) would better reflect the state of the underlying soil [14] .

At a given point, two successive samples were taken by placing the auger perpendicular to the soil and then rotating it until it reached a depth of 20 cm. The auger is then removed from the ground, along with the first soil sample. After the auger has been removed and the soil removed, the second soil sample is taken, cleaned thoroughly with water and then pushed back down to a depth of 20 cm in the hole dug in the ground for the first sample. It was the 0 - 40 cm layer that was explored, because 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. In total, six soil samples were taken from each useful plot, i.e. 12 samples from each plantation, giving a total of 72 soil samples taken throughout the study area. The samples were stored in plastic bags, labelled and taken to the laboratory for chemical analysis.

The soil chemical properties measured are listed in Table 1 along with the methods used. These analyses were carried out at the plant and soil analysis laboratory of the Institut Polytechnique Félix Houphouët-Boigny (Yamoussoukro, Côte d’Ivoire).

Based on the measurements taken, chemical balances that form part of the main indices of soil quality in cocoa farming were calculated. These indices specify that:

- exchangeable bases must be balanced according to 8 pc of potassium (K⁺), 68 pc of calcium (Ca²⁺) and 24 pc of magnesium (Mg²⁺) [19] ;

- the minimum saturation level of exchangeable bases must be over 60 pc [19] ;

- the optimum C/N range is between 10 and 12 [20] [21] .

An analysis of variance (ANOVA) was performed, followed by identification of homogeneous groups of soils in healthy plots on the one hand and infested plots on the other (Newman Keuls test). The analysis of variance also involved the Student’s T test, with a view to measuring the differences between the soils in healthy plots and those in diseased plots. All these analyses were carried out using XLSTAT software.

Table 2 shows the soil pH values for the Bouaflé and Kononfla zones soils. The 0 - 20 cm layers of the Bouaflé soils have a similar acidity in healthy plots (pH = 6.6 - 6.9) and infested plots (pH = 6.2 - 6.5) (P_ANOVA > 0.05). The same observation was made at Kononfla. The pH in this part of the soils of the two types of plots is also similar in Bouaflé and Kononfla (P_Test-t > 0.05). At the level of the 20 - 40 cm layer, the soils of the same plot type were also identical in Bouaflé and Kononfla (P_ANOVA > 0.05). A difference was observed between soils in healthy plots and soils in infested plots in Bouaflé (P_Test-t < 0.05), unlike in Kononfla (P_Test > 0.05). The difference shows that soils in infested plots have a reaction close to neutrality, whereas soils in healthy plots are generally not very acidic.

- For ANOVA, means assigned the same letter on a line are identical. - For the T Test, the P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

For all the soil layers explored, there was no significant difference between the soils of plots of the same type (P_ANOVA > 0.05) ( Table 3 ) or between the soils of healthy plots and those of infested plots (P_Test-T > 0.05) ( Table 4 ). In healthy plots, C levels were close to 1.00 pc in the 0-20 cm layer, equivalent to 1.72 pc of organic matter. In this layer, average N levels fluctuated around 0.08 pc. In the 20 - 40 cm layer, C and N levels in healthy plots are approximately one-third to one-half of the averages observed in the 0 - 20 cm layer.

On all sites, the rates are similar for the same types of plots and for the same layers (P_ANOVA > 0.05) ( Table 5 ). However, levels were higher in the 20 - 40 cm layers, especially in the healthy plots (0.82 g/kg). All in all, the levels remain below 1 g/kg and above 0.56 g/kg. Comparison of the soils of the two types of plots, across all the sites in Bouaflé and Kononfla, nevertheless revealed a significant difference between the soils (P_Test-T < 0.05) ( Table 5 ).

- Means assigned the same letter on a line are identical. - P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

- P values in bold in the table indicate a significant difference between soils at the 5 pc level.

- For ANOVA, means assigned the same letter on a line are identical. - For the T Test, the P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

Table 6 displays the Ca²⁺, Mg²⁺ and K⁺ levels as well as the measured CECs. Of all these variables, none significantly distinguishes soils in the same types of plots in Bouaflé (P_ANOVA > 0.05). In Kononfla on the other hand, the levels of Ca²⁺ and K⁺ differentiate the soils, the soils most supplied with Ca²⁺ being in healthy plots of Kayéta (64.44 ± 2.57 mmol(+)·kg⁻¹) and Diénembroufla (63.55 ± 1.27 mmol(+)·kg⁻¹), when the most supplied in K⁺ is the soil in the infested plot of the Koumoudji site (3.65 ± 0.05 mmol(+)·kg⁻¹).

Comparison of the soils of the two types of plots shows differences in the 0 - 20 cm layers in Bouaflé and 20 - 40 cm layers in Kononfla ( Table 7 ). Indeed, in Bouaflé, the CEC and the Mg²⁺ rate are sometimes higher in healthy plots or in infested plots. In Kononfla, the difference observed is induced by Ca²⁺ with higher levels in healthy plots (P_Test-T < 0.05).

The balances between the exchangeable bases relative to their sum are worth 4 pc of K⁺ in all layers, 80 and 70 pc of Ca²⁺ between 0 - 20 and 20 - 40 cm from the ground, then approximately 15 and 25 pc of Mg²⁺ in layers 0 - 20 and 20 - 40 cm ( Table 8 ). The exchangeable base saturation rates exceed 60 pc on all plots. Even if these variations in cationic equilibrium states between soils are not significant, both on the scale of the same types of plot (P_ANOVA > 0.05) and on the scale of plots of different typologies (P_Test-t > 0.05) ( Table 9 ), we note overall that the greatest values of the variables come from the soils of healthy plots.

The balance between organic carbon and nitrogen indicated by the C/N ratio varies between 12 and 13 in the 0 - 20 cm layer, then 12 and 13 in the 20 - 40 cm layer ( Table 8 ). These variations do not significantly distinguish soils from the same types of plots (P_ANOVA > 0.05) ( Table 8 ), similarly for soils from different types of plots (P_Test-T > 0.05) ( Table 9 ).

- Means assigned the same letter on a line are identical. - P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

Table 7. Variance of the CEC and exchangeable bases levels in the soils of the two types of plots in Bouaflé and Kononfla.

- P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

- Means assigned the same letter on a line are identical. - P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.

- P values in bold in the table reflect a significant difference at the 5 pc threshold between the soils in healthy plots and in infested plots.