The principles and processes of N availability from inorganic nitrogenous fertilizers and its utilization by crops are by far well established. Information on crop-specific fertilizer requirements and appropriate timing and application strategies enable farmers to maximize crop yields and to reduce risks for nutrient losses. Increasing the use efficiency of added N either as inorganic or organic fertilizers or their coupled application remains an important issue for research activities. Even enrichment of soil C and N pool, the recycling of which are closely linked, is imperative not only to improve soil health but also to build-up organic matter in agricultural soils of the tropics. The release of organically bound-N is rather slow, and added organic materials enrich soil N pool through immobilization/fixation processes, depending on the chemical composition of organic materials, soil and environmental conditions. Integrated plant nutrition system (IPNS) is a current approach of applying organic materials coupled with inorganic fertilizers. This helps to reduce the uncertainty of nutrient supply from the organic N pool for crop uptake. The basic requirements of optimizing N fertilization are to quantity N being available from organic amendments during a crop growth period, N removed by a crop and inorganic N previously optimized for a specific crop. To achieve these goals, research scientists have been involved in assessing the plant residue quality using biochemical compositions (lignin, polyphenol, dehydrogenase activity, etc.) of plant residues to predict decomposition characteristics (k) and nutrient release, whose determinations are too expensive and time consuming. Recently, new equations have also been proposed, termed as the organic material quality index (OMQI), which can predict k, net N mineralization and nitrification in upland soils having distinct physico-chemical properties. Only pH and C/N ratio of organic materials from either source can be used in the equations developed by others to calculate OMQI based on the coefficients of the relative contribution (CRC) of the input variables. The equation for OMQI is: [1/(a * pH + b * C/N)*100] where ‘a' and ‘b' are the CRC of the input variables. Those can be calculated using the F-values of the regression analyses and CRC is the value of the given factors/sum of F-values of these factors. The OMQI could be an advancement of IPNS to estimate k values and mineral N to be available for crop uptake. The prediction capabilities of such indices can be improved greatly by considering soil factors. Application of organic materials to agricultural fields following these indices would ensure the enrichment of soil C and N pool as well as minimize N loss, and thus, the improvement of soil and crop productivity.