Evaluating Different Methods of Organic Carbon Estimation on Nigerian Savannah Soils

The Walkley–Black (WB) method is a commonly used method of soil organic carbon determination in the Nigerian Savannah. This research evaluated four different methods of determination: Walkley–Black, modified Walkley–Black, elemental analyzer (CHNS), and permanganate on Nigerian savannah soils. The soil was sampled from four main regions in the Savannah; Derived Savannah, Northern Guinea Savannah, Southern Guinea Savannah, and Sudan Savannah. Soil samples were collected from major soil orders in the region and the amount of organic carbon was evaluated using the four different methods. The results were compared with WB (which served as a standard/reference method) to determine the recovery factor (RF). Linear regression and correlation between the methods and soil properties was also carried out. Significant results (p < 0.05) were obtained between the different methods. The results showed that amongst all the methods used, the use of WB and permanganate recovered high amount of organic carbon. Significant correlations (p < 0.001) was observed between the methods. The amount of carbon recovered using permanganate, modified WB and CHNS varied between 0.37–1.48, 0.42–1.78, and 0.52–7.34, respectively. The use of WB gives almost accurate results especially Ultisols with a possible underestimation of soil organic carbon for Alfisols (Sudan savanna). Poor results obtained in some of the studied soils are more related to the inherent poor status of the soil than the inefficiency of the method.