The rate of N mineralization from 35 samples of manure or compost was estimated by both aerobic laboratory incubation and lath house pot studies at Davis, Calif., in 1996–97. Each manure and compost sample was mixed at 2% by dry weight with a 1 loam soil: 1 coarse sand blend. The amended soil blends were moisture equilibrated under 0.025-MPa pressure then incubated aerobically at constant moisture at 25 °C for 3 (1996) or 6 months (1997); subsamples were collected monthly (1996) or bimonthly (1997) for mineral N determination. Four-liter pots were also filled with the amended soil blends and seeded with fescue (Festuca arundinacea). The pots were watered but not fertilized for 16 (1996) or 18 (1997) weeks in a lath house at ambient summer conditions. N mineralization from the pot study was calculated from total fescue biomass N plus mineral N from pot leachate, minus those quantities in pots of the unamended soil blend. N mineralization rate estimates from the two techniques were highly correlated (r2 = 0.79). Green waste composts typically mineralized <5% of total N, manure composts 5% to10%, and manures (poultry, dairy, and feedlot) 7% to 20%. After 4 months of incubation, N mineralization rate (expressed as percent of total N per month) from the composts and manures was similar to that of the unamended soil blend.