- New GRDC-funded research confirms that soil organic carbon levels decrease significantly when land is cropped.
- Modern farming practices that maximise water use efficiency for extra dry matter production are key to protecting soil organic carbon.
- Pasture rotations, reducing tillage and avoiding burning or baling stubble are important practices for northern grains region growers.
New research has measured drops in soil organic carbon (OC) levels of Queensland’s long term cropping land to the tune of two per cent compared to native remnant vegetation.
Will Martel, Grains Research and Development Corporation (GRDC) northern panellist and grower at Wellington, NSW says this worrying trend should be a wakeup call for land managers across the region.
The GRDC-supported research carried out by Dr David Lawrence, Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) shows soil under native remnant vegetation measures around 2-3pc OC in the top 10 centimetres, while soil that has been cropped long term measures about 1pc OC in the same depth.
“This indicates long term cropping has decreased OC by 1-2pc and this is a massive drop in the real value of land,” Mr Martel said.
“A 1pc decrease equates to at least 10 tonnes per hectare of OC lost from the soil (assuming bulk density of only 1.0) and to put this into dollar terms, 10t of OC can contain up to $2000 worth of nitrogen, phosphorus and other nutrients.
“What would your farm be worth if you deducted $2000/ha?”
Dr Lawrence says OC can decline rapidly in Australia, largely due to the nation’s hot and dry environment.
“Annual crops reduce OC because they produce less dry matter than perennial native systems and only 25pc of fallow rainfall is used by the subsequent crop,” he said.
“This means that there are less plants growing (and sequestering carbon) and the soil is kept moist which leads to more decomposition.
“Cropping exposes OC to decomposition relative to the extent of cultivation.
“Annual crops also have a relatively high harvest index meaning a high percentage of nutrients are exported from the paddock.”
Dr Lawrence says OC is the ‘free’ nutrient source farms have relied but it is not free to rebuild and this is the challenge for the future.
Along with nutrient supply, healthy soils rely on the physical and biological benefits of OC for crops and pastures, such as water holding capacity on lighter soils and disease suppression, he says.
“What do we do about the decline in OC? In a continuous cropping system reliant on fallow it is essential to maximise the water use efficiency through reducing tillage and not burning or baling stubble.
“Repeated applications of manure and other organic fertilisers may increase OC over time, but the effects from one-off applications do not last long.
“Rotating legumes into the system increases nitrogen throughout the profile that will enable the subsequent crop to produce more dry matter.
“The use of nutrient budgeting in contrast to the set and forget method will also maximise water use efficiency and dry matter production.”
Dr Lawrence says in higher rainfall areas (more than 700 millimetres per annum) there is potential to maintain OC at higher levels using an intensive no till opportunity cropping system which relies on converting a high percentage of rainfall into plant growth.
“In a mixed farming system, farms can increase their OC by growing the best pastures possible.
“Balancing nutrients, biology, soil structure and grazing will allow them to return the maximum amount dry matter to the soil.
“Grass-legume pasture can increase OC by 0.5t/ha/year. At present this is the fastest way to build OC and return the soil to a more resilient state.”
Dr Lawrence says the looming challenge is how to maintain this rebuilt soil organic matter when the land returns to cropping.
He says the hope is that modern systems with adequate nutrients and reduced tillage for good water storage will slow the subsequent decline.
Dr Lawrence will present his research findings at the upcoming southern Queensland GRDC Updates at Inglestone on 16 July and Pittsworth on 18 July.
Caption: Modern farming practices that maximise water use efficiency including no till are important for protecting soil organic carbon
Dr David Lawrence, DAFFQ Principal Extension Officer
07 46 881617 or 0429 001759
Rachel Bowman, Senior Consultant, Cox Inall Communications
07 3846 4380 0412 290 673
GRDC Project Code
North, South, West