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Steady, or carbon goes
THE KEY to managing a soil’s organic carbon content is to maintain the levels that are already there.
The challenge? Contending with the continual decomposition of soil organic matter, of which soil organic carbon makes up roughly 58%.
In a recent podcast with Wheatbelt Natural Resource Management‘s Aimee Mouritz, Murdoch University’s Emeritus Professor Richard Bell discussed soil carbon and what Western Australian farmers could do to slow loss of the resource.
“Something like 2% of soil organic matter will break down in a year so our first aim is just to hold those levels steady. We shouldn’t underestimate the importance of that,” Professor Bell shared on Get the Dirt.
“Before storing more carbon, you’re already on the back foot because – even if you do nothing – you’re losing carbon year by year.”
Soil organic carbon plays major roles in soil fertility, nutrition, water retention, soil structure, microbial activity, crop resilience and climate change mitigation.
How atmospheric carbon dioxide enters and stays in the soil – a process known as carbon sequestration – is front of mind for farmers, particularly those navigating Western Australia’s sandy Wheatbelt plains.
It’s front of mind for researchers too.
Led by the Western Australian No-Tillage Farmers Association and with the support of the Department of Primary Industries and Regional Development, the Soil CRC project New farming methods to sequester soil carbon ran from 2022 to 2025. It included trial sites at Kweda, Bullaring, Wathingarra and Coorow, the latter from 2023.
While no significant increases in soil organic carbon were detected in that period, Professor Bell said that the trial site monitoring would continue – emphasising the need for long-term investigations.
“[To] frame this with a bit of basic soil knowledge, 1% soil organic carbon in 0 to 10 cm is 13 tonnes of soil organic carbon per hectare (when the soil has a bulk density of 1.3 g/cm3),” he said.
“As a ball-park figure, whenever you add soil organic carbon to the soil, 80% of it is probably lost through decomposition and goes back into the atmosphere. So that means that, even if we want to increase the soil organic carbon by 0.1%, you’re going to have to add six or seven tonnes of extra carbon.
“If you think about that in terms of farming systems and the sort of treatments that we apply, unless we’re able to find or add or produce that extra amount of carbon in the system, then it’s going to be a slow grind building up carbon.”
Professor Bell said that rainfall, temperature and clay content of the soil were the three factors affecting soil carbon variation across the country.
How to increase soil organic carbon
1. Slow down decomposition. “Of course, no till was probably the biggest thing that we did to slow down decomposition, but adding clay is another way in which farmers currently try to store more carbon by slowing down decomposition. In our project, we’ve also been looking at some other novel products as an alternative to subsoil clay.”
2. Increase external inputs. “That could be compost or manures. Transport costs are always a big factor to consider with those sorts of organic inputs. Where they make sense economically, where they’re available, that’s definitely something you can do.”
3. Increase crop biomass. “Grow more carbon on the soil. That relates to many of the innovations around soil management … overcoming soil constraints so that you produce a bigger crop and harvest a bigger crop. You also leave more biomass from crop stubbles as well as the root system.”
4. Get carbon into the subsoil. “The topsoil, the upper 5 or 10 cm, may be relatively saturated in carbon. To build more carbon, you often have to think about how you can get it into the subsoil.”
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