Comparison of ignition method with Walkley and Black method for the determination of organic matter content in selected paddy soil samples in Sri Lanka

Abstract

Rice (Oryza sativa L.) is cultivated largely using external inputs such as inorganic fertilizers and pesticides. Apart from that, the addition of crop residues, organic matter, organic fertilizers, or soil amendments are also required to enhance the productivity of rice farming. Usually, organic matter content in soil is determined by Walkley and Black method which is costly and uses carcinogenic chemicals such as potassium dichromate. In order to avoid these disadvantages, soil organic matter content can be determined by obtaining the weight loss through ignition of soil samples using muffle furnace. Therefore, this study was conducted to determine the fidelity of ignition method compared to the chemical method for the determination of soil organic matter of selected paddy soil samples in Sri Lanka. A total of 35 soil samples were collected representing paddy lands in Gampaha (20), Rajanganaya (7) and Polonnaruwa (8) areas. Then the organic matter content of each soil sample was tested using both Walkley and Black method and ignition method. In the Walkley and Black method, organic matter content was determined by the oxidation of soil using potassium dichromate. In the ignition method, soil samples were heated to 375 °C for 3 hours using muffle furnace. Regression and correlation analysis were performed to study the relationship between two methods. According to the results both methods made close estimations of the organic matter content i.e. regression coefficient of 0.95 with a R2 of 0.89. Moreover, ignition method slightly underestimated (6%) the organic matter percentage compared to the chemical method. As the soil organic matter content determined using both methods were in good agreement, ignition method can be used as an alternative method for the testing of organic matter in paddy soil samples.