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