Balanced nutrition yields benefits

Fertilisers can be used efficiently if the right balance is struck between nitrogen and phosphorus.

That’s one of the findings from 18 years of research at Incitec Pivot Fertilisers’ long-term field site at Dahlen, near Horsham in Victoria.

Trial co-ordinator, Dr Rob Norton, Regional Director of the International Plant Nutrition Institute, calculated the phosphorus and nitrogen balances between 1996 and 2013, including nitrogen fixed with pulse crops.

Nitrogen input and output were nearly equal when 40 kg/ha of nitrogen was applied per year, provided phosphorus was used.

The rate of 9 kg/ha of phosphorus per year was almost balanced by the annual removal over the duration of the experiment.

Dr Norton said there was an annual excess of application over removal of about 2 kg/ha of phosphorus at this rate, and this contributed to an increase in soil test phosphorus from 24 mg/kg (Colwell P) in 1996 to 42 mg/kg (Colwell P) in 2013.

Dr Norton used two further measures to assess the efficiency of nutrient use.

These both pointed to 9 kg/ha of phosphorus as a highly efficient rate for the cropping system, as long as nitrogen was also applied.

The first was a partial nutrient balance, or the ratio of the amount of nutrient removed in produce to the amount of fertiliser used.

It showed that over the duration of the experiment, where 9 kg/ha of phosphorus and 80 kg/ha of nitrogen was applied, 88% of the phosphorus and 92% of the nitrogen was recovered by the crops. The includes two drought years (2002, 2006) where fertiliser was applied but no crops were harvested.

Where low rates or no nitrogen was applied, the recovery was higher, but this was because the nitrogen removed came from organic matter breaking down to release nitrogen for crop growth.

In addition, the returns in grain from applying 9 kg/ha of phosphorus with nitrogen were around 250 kg of grain per kg of phosphorus applied.

“This just shows that with the right balance of nitrogen and phosphorus, nutrient use efficiencies in long-term experiments can approach 100%,” said Dr Norton.

However, the nitrogen and phosphorus combination with the highest nutrient use efficiency was not the treatment which gave the highest productivity in the experiment.

Between 1996 and 2013 the highest cumulative grain production was achieved with 80 kg/ha of nitrogen and 18 kg/ha of phosphorus.

The 2013 barley yield results also showed that productivity can suffer where crops are forced to rely on soil phosphorus, rather than being supplied with starter fertilisers.

The experiment was split in two in 2011, and from 2012 onwards no additional phosphorus has been used on half of the plots.

Dr Norton said this new experiment was established to evaluate the rate at which soil phosphorus could be drawn down from the high soil test values that developed over time.

“Even with very high soil test phosphorus values, there were small but significant yield reductions where no phosphorus was applied with the seed,” he said.

“In 2013, yields were 4.28 t/ha where phosphorus had been stopped, compared with 4.41 t/ha where phosphorus was applied at various rates.”

This was consistent with 2012 results, where wheat yields were 0.22 t/ha lower where no phosphorus was used.

“This suggests that small amounts of fertiliser phosphorus every year are valuable even where soil phosphorus levels are adequate,” he said.

Dr Norton suggested that if the withdrawal of phosphorus was the same as the build-up, it may take three to four years before the soil reserves from the previous 9 kg/ha of phosphorus plots were reduced to the critical level of around 30 mg/kg Colwell P.

In 2014, the site is planted with canola, as part of the long-term rotation.