Adding N with Legumes


Mark Peoples from CSIRO talks about recent research, funded by GRDC,  into the value of legume rotations for adding nitrogen (N) to cropping soils, and explains some rules of thumb for growers in the southern region.

The work looked at two kinds of legume rotation:

  • pulse crops where the grain is harvested
  • brown manure crops where the legume is sprayed out prior to full maturity.

Both legume rotations have a dual purpose to the grower. The pulse crop offers a cash return on the harvested grain, along with additional N into the soil for future crops. The brown manure crops add N and organic matter to the soil and supports control of annual ryegrass.

Researchers found that, compared to what is measured after wheat or canola, the pulse crops contributed around 30 – 40 kg mineral N/ha to the soil, while the brown manure legume added around 90 kg mineral N/ha (measured to 1.6m depth).

Heading into a second wheat crop the legacy effect of the crop legume was an additional 20kg mineral N/ha, and an additional 30 -35 kg mineral N/ha from the brown manure.

The rates of N mineralised from crop residues were around 7 kg N per tonne of residue for the pulse crop, and around 11 kg N per tonne of residue from the brown manure crop. The rate of N mineralisation is affected by rainfall, with about 0.1 – 0.2 kg additional N mineralised with each millimetre of rainfall during the fallow period.

Further information

The GRDC:

Legume effects on soil N dynamics – comparisons of crop response to legume and fertiliser N -from GRDC update papers
Profitable pulses and pastures -An article in the recent GRDC Groundcover supplement

Acknowledgements

Mark Peoples CSIRO, Luke Gaynor, NSW DPI, Kate Wilson, Agrivision.

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