Pasture legumes need to form nodules in order to fix nitrogen.
Nodules are formed by bacteria (rhizobia) that enter the plant root. The rhizobia feed off the plant and in turn supply it with nitrogen. The nodules break down in late spring and summer and release nitrogen stored within them for other pasture species.
These were the lead comments detailed by Local Land Services pasture agronomist Dr Belinda Hackney at a recent field day held at Tooraweenah on the property of Brendan Butler and sponsored by agribusiness Rohr’s Gilgandra.
Dr Hackney particularly emphasised the need to check if legumes were adequately nodulated.
Surveys she and Janelle Jenkins (Riverina) had conducted across 140 random pasture paddocks had unfortunately found that 90 per cent of them had inadequate nodulation and more than 20 per cent had no nodules at all.
Reasons for poor nodulation and what to do about it was the focus of field day discussion.
Dr Ron Yates, a specialist in rhizobia studies from the Western Australian Department of Agriculture speaking at the day said an important consideration was that different strains of rhizobia have different requirements for survival.
Rhizobia strains for sub clover for example Dr Yates said are less tolerant of acidity than strains for serradella. As soil pH declines, sub clover nodulation can be greatly reduced.
While the more recent available sub clover strains are more acid soil tolerant than their predecessors, its optimum pH is 5.0 to 5-7 (calcium chloride method). When pH is less than 5.0, sub rhizobia really starts to struggle, he said.
Dr Yates said increasing soil pH through lime application will be necessary to optimise nodulation in more acid sensitive species such as sub clover.
It may also be necessary to introduce more rhizobia into poor paddocks.
Care at establishment, Belinda Hackney said, is critical for long-term good pasture nodulation.
Rhizobia are living bacteria and unless managed correctly (eg. if using peat inoculum; fresh, stored in a cool area and correctly applied to seed and into the soil) poor results invariably occur.
Where legume content is low and poorly nodulated, reintroduction of suitable legumes with sound nodulation application is an obvious option to consider. Dry granules have been successfully used in summer in WA and NSW with good results.
Hard-seeded species such as serradella biserrula and gland clover can be sown in January-February Dr Hackney said.
The logic for summer sowing is that untreated “hard” seed, often harvested by landholders from their pastures, will break down “hardness” via soil temperature variation and establish well from autumn rains. Appropriate clay based inoculants can survive adding with them.
Some researchers believe herbicides such as triasulfuron and clopyralid have residual adverse effects against rhizobia, sometimes applied more than a year before pasture sowing.
Soil aspects such as phosphorus and sulphur deficiency can also badly impact on nodulation.
Molybdenum is an important factor for good nitrogen fixation.
While only required in small amounts, its deficiency can significantly reduce nodulation.
Brendan Butler said most farmers attending the day assumed that if legumes appeared healthy they would be fixing nitrogen.
“We now know we need to dig a little deeper to find out for sure,” he said.
Next week. Long term pre-breeding research essential for strong grains industry.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
