Figure 2: pH values, OC contents, Ca, Mg, K, and P concentrations as well as BS and CEC in soils of the Kaduna study area compared to West African background levels and critical limits. Boxplot diagrams show chemical characteristics of the Kaduna study area soils that have developed on upper pediments, lower pediments, river valleys, lower plains, and upper plains, respectively. Dotted lines represent West African background values, while solid lines show the critical limits both as given by Jones and Wild , Landon , Vanlauwe et al. , Raji et al. , Agbenin and Yakubu , Esu and Ojanuga , and Valette and Ibanga . Grey values indicate statistically significant differences between the pH, OC, Ca, Mg, K, P, BS, and CEC of the soils in the Kaduna study area and the West African background values. Black values indicate statistically significant differences between the Kaduna soil fertility parameters and the critical limits.