Nitrogen mineralization from organic amendments is variable but predictable

To manage nitrogen (N) efficiently, organic growers must be able to predict the amount and timing of plant‐available N from organic amendments. In this study, we measured N mineralization from a variety of organic amendments, including composted animal manures and plant material, pelleted and granular organic fertilizer formulations, slaughter waste products, and hydrolyzed liquid fertilizers. In a laboratory incubation, we measured net N mineralization from materials mixed with either organically or conventionally managed soil at 23°C and 60% water holding capacity after 0, 7, 21, 42, and 84 d. We found that net mineral N change in the amended soils after 84 d of incubation fell into four categories: immobilization to 5% of applied N for yard trimmings composts, 15–30% for poultry manure composts, 35–55% for granular fertilizers, and 60–90% for quick release products. However, across all amendments the amount of plant‐available N after 84 d of incubation was well correlated with the carbon (C)/N ratio ( R 2  = 0.92). Within amendment types, the C/N ratio predicted N mineralization for yard trimmings composts ( R 2  = 0.91), manure composts ( R 2  = 0.81), and specialty fertilizer and slaughter products ( R 2  = 0.88) but not liquid products ( R 2  = 0.11). Soil management history did not consistently affect net N mineralization but may have influenced timing.