Laboratory and greenhouse assessment of plant-available N in organic materials

Laboratory indices (KCl extraction, thermal fractionation, pepsin extraction, C:N ratio and N mineralization) of organic materials were compared with plant availability of mineral-N produced from organic-N in a greenhouse experiment. Six types of organic materials [farmyard manure (FYM), pig manure (PG), poultry manure (PL), sewage sludge (SS), pressmud (PM) and compost (CP)] were compared with urea as a chemical reference fertilizer. Relative effectiveness of organic N (RE_o) was used to compare the fraction of N extracted with different N availability laboratory indices. RE_o values of organic materials ranged from 0.14 to 0.77 and decreased in the following order: PL > PG > SS > PM > CP > FYM. The KCl-extracted inorganic N, pepsin-extracted organic N and N mineralized during 42 days gave a positive relationship with RE_o, and the C:N ratio a negative relationship with RE_o. Among the different N availability laboratory indices, pepsin extraction of organic N, C:N ratio and N mineralization can be recommended for determining mineralizable organic N in organic materials. KCl-extracted inorganic N proved to be a useful index for organic materials having a low inorganic N fraction. Thermal fractionation did not provide a suitable index of plant-available N in organic materials.