“The amount of nitrogen fixed by a pulse crop is largely influenced by how well that crop grows.
“More crop biomass equals more nitrogen fixed by that crop provided it is well nodulated.”
These comments spoken by Nikki Seymour, Department of Agriculture and Fisheries, Queensland, at a recent round of GRDC Research Update seminars sums up the vital role pulse crops have in crop rotations and how to maximise that role.
Dr Seymour is involved in GRDC-funded research studying how to improve soil nitrogen levels by using pulses in rotations.
She said that the amount of nitrogen fixed by a legume does not equal the amount available for the next crop.
Nitrogen is removed in the harvested grain and nitrogen remaining in the crop residue (foliage and roots) then needs to be mineralised by microbial activity before it is available to the next crop rotation.
Average amounts of nitrogen fixed annually by crop and pasture legumes are around 110 kg/ha ranging from close to zero to more than 400kg nitrogen/ha.
Nikki Seymour said the amount fixed depends on the species of legume, the site and the seasonal conditions as well as agronomic management of the crop or pasture.
The legume crop uses this nitrogen for its own growth and may fix significantly more than needed, leaving a positive nitrogen balance in the soil for proceeding crops.
Dr Seymour noted that many agronomy and soil aspects affected pulse crops ability to build soil nitrogen.
High soil nitrate levels for example can reduce legume nodulation and nitrogen fixation by rhizobia.
In general legume reliance on nitrogen fixation is high when soil nitrate levels are below 50kg nitrogen/ha in the top metre of soil.
Above 200kg nitrogen/ha, nitrogen fixation is generally close to zero.
The addition of nitrogen fertiliser does not give any yield advantage in pulses and may reduce the amount of nitrogen available for the following crop, Dr Seymour said.
For example, trials conducted in Queensland in mung beans showed that nodulation was suppressed by applications of urea at 10 or more kilograms nitrogen/ha.
In addition, no yield advantage was gained by nitrogen addition pre-plant with nitrate nitrogen starting at about 80kg/ha in the top metre.
While there was also a history of mung beans at this site contributing to nodulation of the uninoculated treatments, there was a significant yield response to inoculation.
Because nitrogen fixation in pulse crops is closely linked to biomass production, Dr Seymour highlighted that research showed agronomic practices that improve crop growth all lead to improved nitrogen fixation by the crop.
For example, as far as is practical for a given pulse crop, sowing in the early part of the recommended sowing window commonly leads to greater crop bulk (and also commonly yield) and can significantly improve the amount of nitrogen fixed.
Narrower row spacing for pulse crops has particularly shown to improve nitrogen fixation compared to wide (1m rows) at equivalent plant populations.
Chickpea research noted that row spacing of 0.25m led to greater biomass, yield and increased nitrogen fixation compared with 1m rows.
Nitrogen fixation in mung beans is also better at narrower row spacing particularly for recently released varieties.
Further details of the research are available from proceeding of the various seminars and can be found on the GRDC web site.
Next week. How livestock enterprises can upgrade production as fast as grain businesses.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.