IT is cheap, in abundant supply as an industrial waste product, and now new research could help deploy slag from steel making as a crucial defence mechanism for grazing and cropping.
Root herbivores, commonly known as the pesky larvae of a range of exotic beetles, are a rising threat, munching their way through grasses of all kinds.
The driving factors of this damaging trend are twofold.
Crucial supplies of plant-available silicon (calcium silicate) in the soil are dwindling after decades of farming. At the same time, rising CO2 is filling plants with more and more carbon, reducing their appetite for silicon and in turn, the hearty root stock it propagates.
“I am increasingly being contacted by farmers who are frustrated by herbivore insects, such as chafer grubs and scarab beetles, that are destroying pastures from the ground up,” said Dr Scott Johnson.
He heads the Western Sydney University team that is investigating the potential of adding silicon to soil for plants to use as a defence against root herbivores.
I have been sent photographs from farmers and agronomists, showing that they can grab their grass and roll it back like a doormat because these insects have completely severed the roots from the stems
Working within the Hawkesbury Institute for the Environment, Dr Johnson has received $900,000 for his research from the Australian Research Council.
“I have been sent photographs from farmers and agronomists, showing that they can grab their grass and roll it back like a doormat because these insects have completely severed the roots from the stems,” Dr Johnson said.
Plants draw silicon from the soil to coat their tissue, build cellulose and lignin in their cells, and protect against insect attack.
“For insects, grasses that contain large amounts of silicon are unpleasant to eat. It would be similar to eating an ice-cream with a thick outer coating of sand,” Dr Johnson said.
“Silicon can be taken from soil and incorporated into plant tissues to support structure and cells. It can stop crops lodging in a stiff wind, for example.
“It also frees plant to put its resources into producing chemical defences against pests and disease. Basically, it’s a cheaper way for the plant to grow, without having to manufacture structural compounds like cellulose and lignin.”
Greater uptake of silicon helps plants use water more efficiently, and lose less through transpiration in the foliage.
The research will also investigate the impact of elevated atmospheric carbon dioxide (CO2) on Australian grasses, which could compromise plants’ natural defences.
Elevated CO2 makes carbon more available to plants. This can cause them to switch to ineffective carbon defence mechanisms to ward off insects. It is also widely reported to suppress the Jasmonic Acid (JA) signalling pathway, which grasses use to mount a silicon defences against root attack.
“We have found that when when we’ve grown plants under elevated CO2, like the levels we’re forecast to see in 30 years’ time, they don’t take up as much silicon because they’re getting more carbon from the CO2.
“That seems to make plants more susceptible to insect pest which feed on them.”
The research could pave the way for beneficial reuse of a plentiful and potentially cheap industrial waste product.
The silicon will be introduced to the soil in granular form, using slag, a by-product from steel making furnaces.
“Slag is a rich source of silicon and it's in plentiful supply. A single Australian steel mill can produce more than a million tonnes of slag every year, making it an easy resource to come by,” Dr Johnson said.
Slag is a relatively clean by-product, but contains small trace metals and the research will investigate long-term impacts.
The research will lead the world in enriching our understanding of silicon’s role in plant growth.
“Internationally people are using silicon as a limin agent to promote plant growth, but we don’t really understand why it promotes plant growth, and any other knock-on effects,” Dr Johnson said.
The research will also determine the safety of deploying silicon in food crops, and where extra silicon is deposited within the plant.
Four metre wide open-top chambers will be built to house the grass. The chambers will be enriched with CO2 and silicone to test how they interact.
“We will need to grow more crops with less inputs in the future,” Dr Johnson said.
“We’ve already got data that shows crop losses will be greater in future conditions under climate change. We need to research intervention strategies now.
“Hopefully this work leads to less chemical interventions, which will be good for farmers and the environment.”