Comparison of Organic Carbon testing method in organic fertilizers using the Walkley & Black and Combustion Method

Abstract

This study compares organic carbon content in 30 organic fertilizer samples analyzed by the Walkley & Black method and the Combustion method. The mean organic carbon contents obtained from two methods were 37.46+0.07% and 37.25+0.05% by weight, respectively. The organic carbon contents were distributed in the range of 35-45% by weight. Moreover, the deviation from the mean and the error values of the Walkley & Black method tended to be negative. In contrast, the Combustion method had a well-distributed error around zero. Walkley & Black method is a chemical method based on the oxidization of the carbon content in the organic fertilizers with potassium dichromate and sulfuric acid as the catalyst. The more significant discrepancy of the Walkley & Black method was the result of its incomplete chemical reaction, on which the method was based. In contrast, the Combustion method, which used direct heating at 950^o, achieved better complete conversion, as reflected in a lower measurement uncertainty (0.05% by weight). However, both analysis methods passed a statistical test: t-test for means of pair two samples, and the t-stat was less than the critical value. We can conclude that both approaches provide the same measured values for measuring organic carbon in organic fertilizers. Thus, the Combustion method with the automatic CN analyzer can achieve more precise values and lower measurement uncertainty. In addition, the procedure is more straightforward, more convenient, faster, more economical, and consume less resource.