Additional file 2. Figure S2. Significant (p-value < 0.01) variation of soil chemistry and climatic variables across African countries. Significance was calculated using the Kruskal-Wallis test for non-parametric data distributions, while pair-wise comparison was calculated using the pairwise Wilcox test. Significant results are indicated using the following nomenclature: * - p-value < 0.05; ** - p-value < 0.01; *** - p-value < 0.001.