Mapping cacao fertiliser requirements in Côte d'Ivoire

In Côte d'Ivoire, soils in cacao plantations are depleted due to the absence or underuse of fertilisation. A digital map of 130 landunits was created from soil and climatic parameters. A soil diagnosis software was combined with GIS (geographical information system) to convert the current unique fertiliser “Engrais cacao” into a greater number of recommendations more adapted to local conditions, thus more actual. Cacao fertiliser requirements were calculated from soil samples taken in mature cacao plantations in each landunit. The relationships between nutrient requirements and soil chemical parameters enabled building a map of the actual cacao fertiliser recommendations. Soils with identical characteristics were compared regarding their cacao nutritional needs. Highly significant correlations between soil nutrients were found; particularly, Ca and Mg were highly correlated with K, making it possible to calculate the Ca and Mg amounts in fertiliser formulae as function of K (i.e. Ca = 8.5×K and Mg = 3×K). The final map contains 23 N-P-K-Ca-Mg fertiliser formulae. Among them, the currently recommended blanket fertiliser represents 16.5% of the cacao areas. The comparison of our results with a previous study, done 40 years ago, evidenced that the soil nutrients under cacao have significantly decreased over the period, reinforcing the need for fertilizers.