Dahlen Long-term Nitrogen and Phosphorus Recovery Experiment

Since its establishment, crops planted included barley (1996, 2000, 2004, 2008) , chickpeas (1997), canola (1998, 2002, 2006), wheat (1999, 2003, 2007) and lentils (2001, 2005). Canola was chosen as the 2010 crop, following lentils in 2009 and barley in 2008.

With the good seasonal conditions, canola showed a strong response to phosphorus with the first 9 kg P/ha doubling yields and the next 18 kg P/ha plots raising yields to over 3 t/ha (see Figure 1). At the highest P rate (36 kg P/ha), establishment was poor probably due to fertiliser damage and so yields were somewhat lower.


Figure 1 – Response of canola to applied phosphorus at Dahlen (Source: Incitec Pivot Fertilisers, 2010).

The response to nitrogen showed a 17% yield increase to 40-80 kg N/ha (see Figure 2), and split nitrogen applications showed a small benefit over applying all the nitrogen upfront at sowing. The small nitrogen response is probably due to nitrogen fixation from the lentil crop in 2009 and possibly a carryover from the barley in 2008, which failed due to dry conditions.


Figure 2: Response of canola to applied nitrogen, Dahlen (
When first established in 1996, the site had a Colwell P of around 22 mg/kg and a moderate to low site PBI. Since 1996, there have been 15 crops sown and 12 harvested, with crop failures in 2002 (wheat), 2006 (canola) and 2008 (barley). At the end of the 2006 canola crop, a nutrient audit was undertaken at the site, with the inputs of phosphorus from fertiliser balanced against the phosphorus taken away in the grain. Figure 3 shows two trends from this study. Firstly, around 9 kg P/ha/year was in approximate phosphorus balance, with a slight surplus of 4 kg P/ha/year. This surplus appears as a rise in soil phosphorus test value from 22 to 33 mg/kg over the 15 years.

The results indicate that over the relatively low yield decade of the 2000’s, the use of 9 kg P/ha approximately balanced the grain phosphorus removal and raised soil test levels to a desirable value from what was a low value. That fertiliser strategy matched input with output and a separate economic analysis of this site reported some time ago showed that 9 kg P/ha also provided the best returns.


Figure 3: Colwell soil P value (mg/kg) and apparent P balance (kg/ha/year) for the period 1996 to 2007 at the Dahlen.