A Critical Re-Examination of the Factors Influencing Determination of Organic Matter in Soils

Organic matter is an undesirable constituent of the soil. Soils containing organic matter are associated with significant secondary compression and unsatisfactory strength characteristics. The estimation of organic matter in soils is hence a topic of concern for geotechnical engineers. The routine estimation of organic matter is carried out using the loss on ignition method put forward by ASTM and BS codes. The bureau of Indian standards has laid down the method for determining the organic matter present in soils, by the chromic oxidation equivalent of soil organic matter. This method is usually ignored due to the recovery of hazardous chromium whose disposal is dangerous to the environment. However, loss on ignition method also involves serious limitations like uncertainty in ignition time and temperature. This paper presents the results of the study conducted to identify various factors affecting the determination of organic matter using codal procedures. Tests were carried out on artificially prepared organic soils, prepared by mixing kaolinite, bentonite and sand with predetermined percentages of starch. Results have shown that IS code and BS code method I can be adopted when organic content in soils is less than 10%, and sample size is limited to 0.5 g. The loss on ignition method is the most reliable method with an optimum ignition time of 4 h and ignition temperature of 450 °C. The results of analyses performed on artificially prepared organic soils provide an insight into the selection of best method for determining organic matter in soils through an improved scientific perspective.