Refining Soil Organic Matter Determination by Loss-on-Ignition

Abstract

Wet oxidation procedure, i.e., Walkley-Black (WB) method, is a routine, relatively accurate, and popular method for the determination of soil organic matter (SOM) but it is time-consuming, costly and also has a high potential to cause environmental pollution because of disposal of chromium and strong acids used in this analysis. Therefore, loss-on-ignition (LOI) procedure, a simple and cheap method for SOM estimation, which also avoids chromic acid wastes, deserves more attention. The aims of this research were to study the statistical relationships between SOM determined with the LOI (SOM LOI) and WB (SOM WB) methods to compare the spatial variability of SOM in two major plains, Shahrekord and Koohrang plains, of Chaharmahal-va-Bakhtiari Province, Iran. Fifty surface soil samples (0–25 cm) were randomly collected in each plain to determine SOM using the WB method and the LOI procedure at 300, 360, 400, 500 and 550 °C for 2 h. The samples covered wide ranges of soil texture and calcium carbonate equivalent (CCE). The general linear form of the regression equation was calculated to estimate SOM LOI from SOM obtained by the WB method for both overall samples and individual plains. Forty soil samples were also randomly selected to compare the SOM and CCE before and after ignition at each temperature. Overall accuracy of the continuous maps generated for the LOI and WB methods was considered to determine the accordance of two procedures. Results showed a significant positive linear relationship between SOM LOI and SOM WB. Coefficients of determination ( R 2) of the equations for individual plains were higher than that of the overall equation. Coefficients of determination and line slopes decreased and root mean square error (RMSE) increased with increasing ignition temperature, which may be due to the mineral structural water loss and destruction of carbonates at higher temperatures. A temperature around 360 °C was identified as optimum as it burnt most organic carbon, destroyed less inorganic carbon, caused less clay structural water loss, and used less electrical energy. Although the trends of SOM in the kriged maps by the two procedures accorded well, low overall accuracy was observed for the maps obtained by the two methods. While not suitable for determination where high accuracy is required, determination of organic carbon through LOI is likely suitable for exploratory soil surveys where rough estimation of organic matter is required.