Higher-yielding legumes leaving less nitrogen than before

Central Queensland growers were told during a Grains Research and Development Corporation research update day at Capella that newer high-yielding crops were using more nitrogen in producing their grain than previously measured.

As a result, growers are finding there is less nitrogen in the soil than expected.

Northern Grower Alliance CEO Richard Daniel said it was a common expectation that nitrogen production from rhizomes would leave an excess for the next crop planted.

"What's being seen the more it's looked into is that nowhere near enough is being left behind," he said. "As we're getting better crops, they are using more nitrogen themselves."

DAF research agronomist Andrew Erbacher said some of the findings had come from measurements at seven sites between Emerald and Dubbo, beginning in the winter of 2015, where water and nitrogen was measured before and after each crop.

Chickpeas and wheat were grown in different systems and the nitrogen left behind was compared.

"Chickpeas might produce plenty of nitrogen but it's going off the paddock in a truck," he said. "The bottom line is, don't bank on having extra nitrogen left for the next crop."

The findings were part of a farm systems project looking into changing management decisions and their long-term effects, and was presented at Capella as part of a discussion on the movement of nitrogen through central Queensland soils.

Mr Daniel said on-ground experience of the timing of fertiliser applications was giving different results to that predicted by scientific theory because it didn't move as fast in dryland systems.

Instead of spreading urea before planting in a stop-go approach, crops would benefit from regular applications.

"Rather than dialling up the nitrogen for the next crop, we're encouraging people to think about the whole rotation," Mr Daniel said. "Think of it as a drip feed, and the flow-on benefits in subsequent years."

Feedback shows the message of applying a certain amount every year regardless of rain was giving results, but soil tests were still necessary from time to time to adjust those levels.

Nitrogen mobility has been found to be very slow in the heavy clays of central Queensland, particularly when there is erratic rainfall.

The Northern Grower Alliance has been heavily involved in nitrogen management trials in wheat since 2012 and has created a large body of data on nitrogen uptake efficiency.

Mr Daniel said while central Queenslanders focused on sorghum crops, he believed the findings relating to wheat would translate.

"The implication is, if we drive down N levels in the soil and try and fill up a few months before planting, we'll never catch up," he said. "You come unstuck in terms of yield and protein."

Because the soil dries out in erratic rainfall conditions, it has to get wet again to move fertiliser down to a sufficient depth to be useful to crops.

That's either an expensive ripping exercise or means putting it on earlier to give it time to move.

"The findings change the way we think about nitrogen programs," said Mr Daniel. "The plant doesn't care which year of nitrogen it's using."

The comments coming back from growers and advisors is that it's a simple exercise to work out how much nitrogen is being removed, and then to budget for the baseline of what's being exported.

"Use that to give you a rough starting point and add 10pc more, and do soil tests from time to time," Mr Daniel said. "It's a simpler process than testing and making recommendations for individual paddocks."

He added that some growers had decided to put nitrogen on dry soil straight after their wheat or chickpea crop, to maximise its movement.