Legume break crops can provide significant fixed nitrogen (N) for subsequent cereal crops. From 2013 – 2015, Michael Moodie (Mallee Sustainable Farming) compared various legume crop productivity and N contribution in low rainfall environments. Trials were conducted in the Victorian and South Australian Mallee regions, where N is often limiting.
The trials compared dry matter (DM) production and symbiotic N fixation of chickpea, field pea, lentil, lupin, faba bean and vetch crops. Each trial was located on sandy-loam or loam soil. Trials were sown into moist soil in the first week of May each year. Growing season rainfall during the three trial years was 130-145 mm. These were less than the regional average of 175 mm.
Range of results
On average, peak shoot DM produced across species and seasons was 3-4 t DM/ha. Average grain yields were about 1 t/ha.
Most of the legume crops fixed about 60 kg shoot N/ha across the three seasons, with the exception of chickpea, which fixed significantly less shoot N than the other species.
The researchers also measured the N removed in harvested legume grains across the three seasons and estimated the likely contributions of fixed N by the nodulated roots. Using these measurements, they calculated that 12 of the 15 legume crop/season combinations provided agronomically significant fixed N contributions for subsequent crop use.
Average cereal grain yield in the region where the trials were located is 1.6 t/ha. This means about 32 kg/ha N is removed at harvest with these cereal crops. Using the trial results, a grain legume crop would need to be grown for every 1-2 cereal crops to replace the N exported from the farm with a typical cereal crop.
To confirm these trial results, the trial program has expanded to investigate legume productivity across a wider range of seasons and soil types. Future research is required to look at how to maintain a positive N balance across the farm in addition to using grain legumes. Options include legume pastures, forage or manure crops and N fertiliser strategies.
International Nitrogen Initiative conference 2016: