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Rate more critical than timing of N in poor sandy soils

The efficient use of nitrogen (N) remains critical for closing yield gaps and managing risk in low rainfall and sandy soil environments. In a recent GRDC Communities podcast, Dr Therese McBeath discusses the benefits to wheat and canola from upfront nitrogen (N) applications, even after a legume crop. It seems that on low fertility sandy soils, applying N at the optimal rate will maximise productivity and reduce risk, but the timing of application is less important. 

Dr McBeath has been involved in trials over five years in the low rainfall cropping region of South Australia. The trials were conducted on low fertility, sandy soils and experienced seasons ranging from decile 1 to decile 9. 

Bridging the gap

Including legumes in the rotation does increase the supply of N and reduce the crop yield gap attributed to N. However, the best productivity increases in these trials came from combining a legume crop the previous season with additional N fertiliser. Manipulating the rate of N from both sources provided substantial productivity gains in both canola and wheat.

In the past, a rule of thumb suggested that N could be applied in-crop to wheat and canola (at the first node/bolting stages respectively) in response to seasonal conditions. But 40-60 kg N/ha from all sources is needed first just to get the crops to these stages. 

The trials showed in these regions, it was better to get N into the system earlier – before mid-tillering in wheat and soon after establishment in canola. So on these soils, an upfront N application at or soon after sowing targeting an average crop, with a top up later if the season looks good, is an attractive management option. 

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Benefits to wheat and canola of upfront N even after a legume

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