How to manage nitrogen in no-till residues

Crop residues influence the supply and uptake of nitrogen (N). In the long term, residues will improve soil fertility. Stubbles can immobilise soil N as they break down. Moisture makes a big difference to the rate of stubble decomposition and mineralisation of nitrogen in the soil.

Nathan Craig was involved in a three-year study looking at N cycling through crop residues. Residues from a no-till wheat monocrop were compared with chickpea-canola-wheat rotation stubbles at the WANTFA site. The stubbles varied in quantity and quality (carbon (C) to N ratio). Overall, residues improved the supply and uptake of N.

Legume stubbles can tie-up N

At and above a C:N ratio of 30:1, the potential for tie-up (immobilisation) of soil N increases. Legume crops leave residues with lower C:N ratios than cereals. We used to assume legume residues always had C:N ratios below the 30:1 threshold. Sometimes legume stubbles can be above 30:1, so they also have the potential to tie-up N.

Extra fertiliser N can be needed when new crop growth coincides with N tie-up in decomposing stubbles. Additional N can be considered with residues from legumes as well as other crops. Adequate soil N at seeding improves crop potential.

Extra N fertiliser to offset N tie-up in residues is less likely to be needed after a wet summer. In Western Australia, summer rainfall makes a substantial difference to available N levels in the soil at sowing. With summer rainfall, more N is mineralised from soil organic matter. Stubbles decompose faster, reducing their C:N ratios. This means more available soil N and less potential for N tie-up in residues.

Using N with surface residues

  • Test soil N AND the residue C:N ratio to assess the potential for N tie-up.  
  • Reduce contact between the fertiliser and stubbles to minimise tie-up of N. It helps to get the N fertiliser into the soil.
  • Fertiliser at sowing can be ‘banded’ >5cm below the seed. Banding fertiliser too close can damage seeds.
  • Place in-crop N at the base of plants in the crop row or consider foliar N for in-season applications.

 

More

extensionAUS

When do retained stubbles increase the need for nitrogen?

What happens to urea with high residue loads?

Give seeds the best chance by avoiding fertiliser damage

 

GRDC

Study puts spotlight on Nitrogen supply to wheat

 

Acknowledgements

This work was conducted as as part of Nathan’s PhD studies with University of Western Australia (UWA)

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