Nitrogen mineralisation of composts in an incubation assay

To date it is not possible to predict the amount of organic nitrogen that will be mineralised after a compost is mixed into a soil. Nitrogen mineralisation is greatly influenced by the physical and chemical properties of the compost, its biological colonisation and the biological activity in the soil which is controlled by soil humidity and soil temperature. In an incubation assay under constant optimal temperature and humidity conditions for 149 days the possible mineralisation of organic nitrogen from 5 different composts and 2 reference materials was measured. Between 0.7% and 6.9% of the total organic nitrogen was nitrified, depending on the properties of the composts. With the tested composts, less than 32.5 kg mineralised N/ha and a total input of plant available nitrogen of less than 40 kg N/ ha can be calculated for the first vegetation period, if the Swiss regulation limits of 25 t compost dry matter per ha are observed. There was no obvious positive correlation between any set of analytical data of the composts and their capability of nitrification of the inherent organic nitrogen.