DISCOVERING pasture legumes with better phosphorous efficiency has been a focus for The University of Western Australia senior research officer Daniel Kidd for six years, culminating with a PhD on the benefits of serradella.
Mr Kidd has spent more than half a decade working on two phosphorus efficiency in pastures projects, collaborating with the likes of CSIRO, New South Wales Department of Primary Industries, grower groups and Murdoch University.
"It all came about because there is a lot of work being done on crops, but pasture systems are pretty inefficient with the way they use phosphorus, so we wanted to look at how we could improve the efficiency of them," Mr Kidd said.
"A lot of phosphorus goes in, particularly on pastures with legumes as they don't need nitrogen, but the actual requirement of the plant isn't known.
"People are trying to move towards efficient systems, especially in cropping with variable rate fertilisers, but for pastures that attention to detail often isn't there."
The focus of Mr Kidd's research was originally on subterranean clover - a pasture legume grown across all of southern Australia that is well adapted, handles grazing and has had about 40 to 50 cultivars produced.
Subterranean clover is the most widely used pasture legume in southern Australia and Mr Kidd wanted to improve phosphorus efficiency across a large area, which would have a large impact.
"The plan was to look at all of the root traits that help plants acquire phosphorus, including root morphology, associations with microriser and other impacts that happen in the soil," Mr Kidd said.
"We looked extensively at subterranean clovers, screening about 80 per cent of the genetic diversity and found big differences between subclovers but they didn't compare anywhere towards the phosphorus efficiency or acquisition efficiency of grasses and some other pasture legumes."
With the discovery that the phosphorus efficiency of subterranean clovers couldn't be improved, Mr Kidd turned his attention to serradella - a very phosphorus efficient plant that is grown quite a bit in Western Australian cropping and livestock systems.
Serradella grows well on sands and infertile soils because it has good root systems.
"It is grown in a few areas around southern Australia but it's potential is far greater than what is being achieved at the moment, which is due to a lot of management problems," Mr Kidd said.
"Farmers can struggle to control weeds with it and it is an upright species so it gets grazed out easier than subterranean clover.
"There are also other constraints that I investigated in my PhD including whether serradella will tolerate water logging, cold temperatures and how acid and aluminium tolerant it is."
Overall, Mr Kidd's research aim was to lower the phosphorus inputs into farming systems and pastures.
"With subclovers you need about 30 to 35 milligrams per kilogram of phosphorus in the soil to have maximum growth, but with serradella that was about 20-21mg/kg which is almost a 30 per cent reduction," he said.
"It's not that the serradella needs less, it just has a better root system so it can explore for soil to pick more up.
"If we said to farmers you can reduce your fertiliser inputs by 30pc, that's an attractive thing for them to look at."
One of the big takeaways from the research was the discovery that a lot of farmers held their paddocks at really high phosphorus levels, up to two or three times more than what's required for the plants to grow.
With phosphorus fertilisers not reducing in price and a lot of talk about them being a finite resource that might run out one day, being able to reduce the level of input required will be useful going forward.
"We're hoping to increase the range of adoption of serradella across southern Australia, so it would be good for consultants to know about the plant, where it can grow and what it's limitations are," Mr Kidd said.
"In the context of regenerative agriculture, if you're not having a lot of inputs then you're going to need really efficient species.
"If people are going to get into regenerative systems, they're going to need plants which can access phosphorus and fertilisers better than the ones they currently have and serradella, or other nutrient efficient species, would be perfect for those kinds of systems."
