Loss on ignition: Measuring soil organic carbon in soils of the Sahel, West Africa

Traditional methods of measuring soil organic carbon (SOC) are not adequate for accurate evaluation of C sequestration nor to meet environmental safety requirements. Methods that permit accurate assessment of SOC, which are environmentally healthy, and yet feasible for use in developing countries are needed. Initial and ignition temperatures were adjusted so that loss on ignition (LOI) measures of SOC matched measurements by combustion methods. A comparison of combustion, LOI, and WalkleyBlack methods was carried out with soils of Mali, West Africa, ranging from 2 25 g kg SOC. The LOI method, when using initial (preparation) temperatures of 105C and ignition temperatures of 350C, related closely with combustion measures (Adj. R of 0.89). The SOC measurements by combustion, however, differed from those of Walkley-Black procedure. The LOI calibration equation was %C (combustion) = 0.03 + 0.36 x %weight loss by LOI, adj. R=0.89. The Mali calibration of LOI with combustion SOC was tested on soils of Gambia, however, a systematic over-prediction suggested that re-calibration may be needed to ensure LOI method accuracy with soils different from the calibration set. This need for re-calibration is similar to the Type 2 error, where the sample belongs to a different calibration set.