Submitted to: African Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2011
Publication Date: 11/15/2011
Publication URL: http://handle.nal.usda.gov/10113/59992
Citation: Yost, R., McCarty, G.W., Doumbia, M. 2011. Loss on ignition: Measuring soil organic carbon in soils of the Sahel, West Africa. African Journal of Agricultural Research. 5(22):3088-3095. Interpretive Summary: Increased interest in implementing projects in the Sahel region of Africa for sequestration of atmospheric carbon dioxide in soil organic matter has intensified the need for methods that accurately measure soil C but are also suitable for use by often resource limited soil analysis laboratories in the region. The challenge is amplified by the low carbon content of the degraded soils found on agricultural lands in the region. Various indirect methods have been recommended for estimating soil C which can be readily implemented in a limited resource environment but they require more evaluation in terms of accuracy. This study compares soil carbon results obtained by loss on ignition using different oven temperatures. The LOI method, when using initial (preparation) temperatures of 105 Celcius and ignition temperatures of 350 Celcius, related closely with combustion measures (Adj. R2 of 0.89). The Soil organic content measurements by combustion, however, differed from those of Walkley-Black procedure. Indicating that new calibrations are needed for use in different parts of Africa.
Technical Abstract: 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 Walkley-Black methods was carried out with soils of Mali, West Africa, ranging from 2 to 25 g kg-1 SOC. The LOI method, when using initial (preparation) temperatures of 105oC and ignition temperatures of 350oC, related closely with combustion measures (Adj. R2 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. R2=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 II error, where the sample belongs to a different calibration set.