Assessing the relationship between soil quality parameters of Nigerian alfisols and cocoa yield

Soil quality is widely regarded as an important factor in sustainable agriculture and is often assessed by evaluating the pertinent physical, chemical and biological properties of soils. The aims of this study were to (i) establish a minimum data set (MDS) of soil quality parameters capable of monitoring changes in soil condition and ecosystem functioning in smallholder cocoa production systems on tropical alfisols in southwest Nigeria; and (ii) examine the relationship between selected MDS of soil quality parameters and cocoa yield along a chronosequence of 3-, 7-, 15-, 22-, 30-, 40-, 60- and 80-year-old farm plots. To achieve these aims, five plots were identified for each plantation age group and bulk soil samples were collected at a depth of 0–20 cm. The samples were analysed for a total data set (TDS) of twenty-three separate soil quality parameters. Principal component analysis (PCA) was employed to reduce the dimensionality of the TDS to five principal components. The PCA results showed that five principal components with eigenvalues > 1 explained more than 78% of the variability in the soil properties. Within PC1–PC5, eleven (11) soil quality parameters with high loadings were identified and selected as the MDS. A stepwise multiple regression analysis of cocoa yield and MDS revealed that soil organic matter and cation exchange capacity are the key parameters influencing cocoa yield. To maintain the yield as well as long-term sustainability of smallholder cocoa farming systems, inorganic fertilizer application is needed. Considering the poor transportation system in rural areas and prohibitive costs associated with inorganic fertilizer application in cocoa plantations, it is suggested that organic materials such as cocoa pod husk should be used to improve soil health. Another proposed approach for improving the N content of cocoa soils is to inter-crop nitrogen-fixing legumes.