A joint Holbrook Landcare and Murray Local Land Services project has recently been undertaken to investigate how well clovers in our region, and in particular subterranean clover, are fixing nitrogen to feed our pasture systems.
Forty paddocks from Walbundrie to Tumbarumba were surveyed to look at soil conditions and to observe the abundance of nitrogen- fixing nodules on the roots of clover plants. The nodules are formed when microscopic soil bugs called Rhizobia, form a mutually beneficial relationship with the clover plant, fixing nitrogen from the atmosphere for the plant whilst being supplied with a food source in return. This is a free source of nitrogen that drives pasture growth.
The survey found nodulation levels in the area were only moderate to low, which is a concerning trend in line with findings from other projects across NSW. It also showed that healthy looking sub clover plants above the ground do not necessarily mean well-nodulated roots. In fact, these plants are drawing nitrogen from the soil in the same way as a grass rather than securing atmospheric nitrogen.
The project also looked at the strains of Rhizobia present at each site. Current commercial strains are more effective than older or native strains, however, despite 63 per cent of sites surveyed being sown with the current strain, it was concerning that only 29 per cent of nodules showed that strain to be present.
There are many factors that can potentially affect nodulation and which Rhizobia are involved including inoculation practices, soil texture, pH and fertility, certain herbicides, waterlogging and seasonal conditions. As with most things in life, the story seems to be quite complex. The take-home message for now is that in most soils in this district, the clovers are likely not performing to their potential and this warrants further investigation.
At a time when many are considering new pastures, one of the other messages is to check the treatment dates on pre-inoculated seed to ensure it’s fresh. Rhizobia health and therefore its ability to form nodules and be effective is affected by temperature and storage time.
Soil factors likely to have an impact include soil acidity and the maintenance of adequate levels of phosphorus, sulphur and molybdenum for clovers to thrive.
Future work is planned to try and untangle the complex issue but it’s one well worth understanding. The upshot of having better performing clovers, with adequate nitrogen fixation, is a greater proportion of “free” nitrogen, and long-lasting perennial pastures that maximise animal performance and maintain good groundcover.