Dahlen Long-term Nitrogen and Phosphorus Recovery Experiment

The long-term profitability of N and P use has been assessed at the Dahlen field experiment established in 1996. In 2012, the site was sown to wheat. The addition of 9 kg P/ha gave about a 1 t/ha yield increase (30%) while extra P gave no further significant responses. The first 20 kg N/ha gave a 0.6 t/ha yield increase (14%) and the highest yields occurred with 80 kg N/ha. The high N rate increased grain protein, but had no effect on grain test weights. Half the experimental treatments have had P application suspended, and even in the first year, a small (0.22 t/ha) but statistically significant yield decrease occurred in plots where no P fertilizer was added in 2012. High P application rates reduced grain Zn contents, but high N rates did not affect grain Cu contents. These sites would be considered non-responsive to added micronutrients.

The conclusions to date are that a set fertilizer strategy of 9 kg P/ha and 20 kg N/ha continues to be a sound investment, even in uncertain seasonal conditions. Long-term data on nutrient removal indicates that applications of 9 kg P/ha and 40 kg N/ha will about balance nutrient removal and will maintain soil fertility as measured by soil tests. A review of the changes in soil C over the period of the experiment has shown that applied N - even in excess - does not lead to a decline in soil organic matter. The impact of balanced nutrition - especially balancing N and P increased crop growth and so raises the input of N into the soil C cycle. The conclusion is that the best way to build soil C is to grow good crops with zero tillage and stubble retention.