Organic matter in a cropped soil is a dynamic system. Organic matter levels found in many ultisols and oxisols of the humid tropics differ little from those in soils of the temperate region. Organic matter contents in cropped soils eventually reach a steady state where rate of addition equals rate of decomposition. Temperature, rainfall and cultivation determine the rate of change as well as the steady state level reached. Biological nitrogen fixation can be important in supplying nitrogen to crop plants once steady state organic matter levels are reached.