Key Points
- Dairy shed effluent may be effectively used on intensive cropping land as a nutrient source to replace inorganic fertilisers. In our trials we found that 30-40% of the total N in the effluent is made available to the crop in the first year after application in the spring.
- In a recent trial looking at the long term release of N from effluent we found no increase in yield from residual effluent in the second year after it was applied. The no-nutrient controls yielded the same as the effluent treatments.
- This unexpected result suggests either unaccounted losses over the two seasons through volatilisation at application and/or leaching of N through the season or a release rate too small to be picked up in the trial. However, at the start of the second season there was no evidence of mineral N at depth indicating that winter leaching losses from the residual effluent were low. The release rate is influenced by the initial characteristics of the effluent.
- More work is needed to identify the potential of an ongoing effluent application strategy every year. Overseas research from one trial suggests that the nutrient supply reaches equilibrium after 6-8 years of annual applications and at best, only 60-80% of the N becomes available. This has not been confirmed under New Zealand field conditions.