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).