Timing of organic fertiliser application to synchronise nitrogen supply with crop demand

To achieve high nitrogen (N)-use efficiency, N availability from organic fertilisers must be synchronised with crop uptake. In order to estimate when previously unmineralised N is plant-available in relation to fertilisation time-point, net N mineralisation was studied in incubations under natural temperature conditions. The fertilisers studied were meat and bone meal (Biofer), dairy slurry, dairy manure, chicken manure, and a by-product from yeast production (Vinasse). The fertilisers were mixed with soil and incubated in plastic bottles placed in topsoil in south-west Sweden on different dates throughout the year, simulating fertiliser application in autumn, early spring, spring, and early summer. Bottles were sampled for analysis of NH(4)-N and NO(3)-N on three to seven occasions until late autumn and the experiment was repeated in two consecutive years. Dairy slurry and dairy manure had a very slow, almost negligible, net N mineralisation after application. Slurry with rather high ammonium content should therefore be applied as close to crop demand as other circumstances allow, whereas dairy farmyard manure with very low mineral N content can be applied off-season. Chicken manure had a considerable proportion of mineral N initially, but released further mineral N after application. Vinasse and Biofer had almost no mineral N initially, but much of the N present mineralised rapidly. About 65% of total N in Biofer, Vinasse, and chicken manure was in mineral form within 30-50 days or 450 growing degree-days (GDD), after which net mineralisation ceased. This indicates that these three types of fertiliser should be applied at least one month before the end of crop N uptake and that autumn application is associated with a risk of N leaching unless a crop with high N uptake is present during winter.