Twenty slurries, 20 farmyard manures (FYM) and 10 poultry manures were chemically analysed to characterize their nitrogen (N) fractions and to assess their potential organic N supply. The organic N fraction varied between manure types and represented from 14% to 99% of the total N content. The readily mineralizable N fraction, measured by refluxing with KCl, was largest in the pig FYMs and broiler litters, but on average only represented 7–8% of the total N content. A pot experiment was undertaken to measure N mineralization from the organic N fraction of 17 of these manures. The ammonium-N content of the manures was removed and the remaining organic N mixed with a low mineral N status sandy soil, which was sown with perennial ryegrass (Lolium perenne L.). N offtake was used as a measure of mineralization throughout the 199 day experiment. The greatest N mineralization was measured from a layer manure and a pig slurry, where N offtake represented 56% and 37% of the organic N added, respectively. Lowest (%) N mineralization was measured from a dairy cow slurry (< 2%) and a beef FYM (6%). The mineralization rate was negatively related to the C[ratio ]organic N ratio of the ammonium-N stripped manures (P < 0·01, r = −0·63).