Mapping fertilizer recommendations for cocoa production in Ghana using soil diagnostic and GIS tools

The intensely weathered nature of soils used for cocoa production in Ghana, and the long history of nutrient depletion due to the absence of fertilization in this farming system have resulted in increased incidence of soil nutrient deficiencies in cocoa farms. The paper discusses the use of a soil diagnostic model combined with geographic information systems (GIS) to convert the current blanket fertilizer recommendations (0 N -165 P.O, -200 K.O kg ha1) into a more effective recommendation that accounts for local land resources and the actual nutrient requirements of cocoa trees. A digital map of cocoa nutrient requirements was created based on climatic and soil groups/associations data. The results showed that about 95% of the areas suitable for cocoa production are developed on only seven soil groups and four sub-groups (FAO classification). As inputs, 201 soil chemical analytic data were linked to land units to compute cocoa fertilizer formulae and doses. The number of fertilizer formulae was reduced to 32 using a fuzzy classification. Six fertilizer formulae would be sufficient to cover fertilizer recommendations for 52% of the cocoa growing areas, while 16 fertilizer formulae will cover 90% of the cocoa area. The current blanket fertilizer formula is only suitable for 6% of the cocoa growing areas in Ghana. In western Ghana, where rainfall is over 1800 mm, the climatic influence is predominant and high leaching of soils is probably responsible for greater needs for P, than for K, Ca, and Mg. In eastern Ghana, where soils have high exchangeable cation levels and base saturation, soil diagnostic method suggested application of N fertilizers. This integrated method could provide precision agriculture techniques for cocoa farmers in Ghana, and beyond, in order to sustain yields on cocoa farms. (Résumé d'auteur