Boorowa grazier builds soil carbon
News story from 'Climate Change, A NSW Department of Primary Industries special feature', in The Land, October 2008
Boorowa farmer David Marsh: "When you think that 85 per cent of land used for agriculture in Australia is grazed, we have a lot of opportunity to increase soil carbon levels just by changing our grazing management."
Our potential for restoring soil carbon is large, according to Boorowa farmer David Marsh who has seen first-hand the benefits of changing grazing practices to provide more organic matter both above and below the soil.
"Grazing management that allows plant communities to recover between grazing and become more diverse and perennial increases organic matter and soil carbon," he said. "Conversely, land that is losing cover is losing soil carbon."
Ten years ago David and his family changed their farming system from a mixed farming enterprise to a system of planned grazing.
They divided their 814 ha farm into 90 paddocks, each with a watering point, and now move their livestock in one or two mobs from paddock to paddock.
The animals move through each paddock, eat their fill and leave behind trampled vegetation, dung and urine that provides a food bonanza for soil organisms.
Each paddock is grazed for only 6-10 days a year, and the length of time stock stay in the paddock depends on the season.
"During the growing season, we only graze plants for three days or less, and let them fully recover before re-grazing. The time they take to recover depends on the time of year, and varies from 30-150 days," David Marsh said.
"When the plants are dormant, we assess how much feed is available so we know how many stock we can carry through to the next break without compromising groundcover.
"We plan for 150 days of no growth plus a further 60 days in case the break is late. This means we plan for dry conditions every year."
The impact of this system on plants and soils has been remarkable. Perennial plants have large basal areas and root structures that add volumes of organic material to soils. Bare ground reduced from 30 per cent in 2002 to zero in 2003, and 100 per cent groundcover has been maintained since then. The soils are soft and absorb all rainfall so that there is no runoff on the soil surface.
Soil carbon levels have increased in the top 10cm from an estimated 1.2 per cent under cropping to 1.92 per cent (LECO test).
"When you multiply 1.92 by pasture soil bulk density of 1.3 it gives us 25 t of carbon per ha in the top 10 cm of our soil," David explained. "This is an increase of 8 t per ha, equivalent to 30 tonnes of atmospheric CO2."
"Significantly, this increase has happened during ten well below average rainfall years. It is much easier to increase carbon in cool moist conditions rather than the hot dry conditions we have been experiencing, so we think we have done very well.
"We have a lot of evidence that Australias soil carbon levels have been reduced by around 70 per cent since agriculture began in this country.
"When you think that 85 per cent of land used for agriculture in Australia is grazed we have a lot of opportunity to increase soil carbon levels just by changing our grazing management," he said.
"The good news is that landscapes can bounce back very quickly if we change what we do - its changing ourselves thats the problem.
"It seems logical that if cropping and grazing processes can be managed to produce more biomass and better soil cover then this will lead to greater amounts of soil organic carbon being sequestered in our carbon-depleted soils."
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