Legumes grown under ideal conditions can add 30 to 120 kilograms of nitrogen per hectare, per year, back into the soil for use by other crops. The benefit of legume rotations to the following crops comes when the nitrogen-rich legume residue has decomposed and released nitrogen into the soil. Wheat crops grown in rotation with chickpeas can show a gross margin 50–90% higher compared to a wheat-on-wheat rotation.
Some grain legumes fix more nitrogen than others.The expected order of various crops in their contribution of N to the soil is faba bean > chickpea > soybean > mungbean. If you are looking for additional nitrogen for the following crop species choice is important as not all legumes fix additional nitrogen beyond their own needs. The conditions legumes grow in can also affect N fixation.
The amount of nitrate already present in the soil will affect fixation rates, with some legumes not fixing additional nitrogen in the presence of high nitrate.
The overall N contribution depends on effective N fixation and the total biomass produced, less the amount of N that is removed in the grain. The ‘rule of thumb’ is that 15–25 kg/ha of N is fixed for every tonne of biomass dry matter, and approximately 35 kg/t of N is removed in grain.
Higher levels of nitrogen fixation have been seen in chickpeas planted in narrower-spaced rows – 25 or 50 cm row spacings. Chickpea plants on 100 cm spacings did not fix as much N as they produced less biomass and relied more on soil nitrogen.
Inoculating legumes with commercially available rhizobia (the symbiotic soil bacteria that live in the root nodules of legumes) is an important step in getting effective fixation from a legume crop even if the legume species has been grown recently in the rotation. Different groups of rhizobia suit different varieties of legumes, so check which group is suited to the crop on the GRDC website.
The cost of inoculating legumes can be as little as $3 to $4/ha for seed-applied inoculants. Legume crop failure can occur when growers don’t use inoculants or delay planting after inoculating seed. Seed treated with inoculant needs to be planted within the manufacturer’s guidelines, or the rhizobia may not survive. Use a quality assured inoculant with the green tick logo. Check nodulation in the crop whether you have inoculated or not.
Legumes also have value as break crops that may reduce some weed and disease pressures, especially stripe rust and the root lesion nematode Pratylenchus thornei.
This article was prepared in response to discussions between growers, advisors and researchers at the More Profit from Crop Nutrition Roadshows 2016.
Photo courtesy of the GRDC.