Abstract: Organic materials are important resources that could be used more efficiently in crop production. Improving the utilization efficiency of organic materials requires improved decomposition estimates, which could stem from a better understanding of decomposition patterns of different organic manures. Four organic amendments were studied for their decomposition patterns under field conditions of the Nigerian Guinea Savanna between 1998 and 2000. Mass loss from the organic materials, was found to increase in the following order, L. leucocephala prunnings> M. pruriens vines > maize stover > cow dung. The plant residues decomposed more rapidly, losing more than 50% of their dry weight within 28 days. Decomposition of cow dung occurred more slowly reaching 50% after about six weeks. A two pool, first order, three parameter negative exponential model was adequate to describe dry matter loss for the organic materials. The relative rate of decomposition for the labile pool (kL) were 0.33, 0.30, 0.55 and 0.52 g day- 1 for cow dung, maize stover, M. pruriens vines and L. leucocephala, respectively. At zero inorganic fertilizer application, average dry matter loss for three years was 51, 49, 35 and 30% for L. leucocephala, M. pruriens, cow dung and maize stover, respectively. The results indicated that organic materials biomass could be separated into two pools, each of which responds differently to varying levels of inorganic fertilizer.