The nitrogen and phosphorus contents of organic materials as a factor in determining the trend of mineralization of these elements when plant materials are added to the soil, were studied in an incubation experiment over a period of 12 weeks. Nitrogen mineralization increased with decreasing C:N ratio. No nitrogen mineralization was recorded above C:N ratio of 16.1 and the critical ratio lay between this and 23.0, with the plant materials used. Initial phosphorus immobilization occurred when plant materials ranging in C:P ratio from 501 to 112 were decomposed in the soil. The release of the immobilized phosphorus as incubation progressed suggests that green manuring should be considered of long-term benefit as far as phosphorus is concerned.