Soil organic carbon – a measurable drop in farmed land
Author: Will Martel | Date: 26 Jun 2013
Recent Grains Research and Development Corporation-funded (GRDC) research in the northern region shows native remnant vegetation measures around three per cent soil organic carbon (OC) in the top 10 centimetres of the soil.
Soil that has been cropped long term is about 1% OC, which indicates long term cropping has decreased OC by 2%.This is a massive drop in the real value of land. A 2% decrease equates to at least 20 tonnes per hectare of OC lost from the soil (assuming bulk density of only 1.0).
To put this into dollar terms, 20t of OC can contain up to $4000 worth of nitrogen, phosphorus and other nutrients. What would your farm be worth if you deducted $4000/ha?
OC can decline rapidly in Australia, largely due to our hot and dry environment. Annual crops reduce OC because they produce less dry matter than perennial native systems. Only 25% of fallow rainfall is used by the subsequent crop.
This means that there are less plants growing (and sequestering carbon) and the soil is kept moister which leads to more decomposition. Cultivation exposes organic carbon 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.
OC is the ‘free’ nutrient source farms have relied but it is not free to rebuild and this is the challenge for the future.
The past two winter crops have seen a large reduction in grain protein. At this year’s GRDC update at Goondiwindi Dr Chris Dowling explained that we now have a “long term loss of soil nitrogen supply elasticity as a result of organic matter decline. The soil has lost the ability to give more in the good seasons.”
Along with nutrient supply, healthy soils rely on the physical and biological benefits of OC for crops and pastures, such as water holding capacity and disease suppression.
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 have shown to 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.
In higher rainfall areas (>700mm) there is potential to increase OC using an intensive no till opportunity cropping system. This 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.
Researcher Dr David Lawrence, Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) 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 will slow the subsequent decline.
To download information, visit www.grdc.com.au/GRDC-UpdatePaper-DalalWang2010.
Region North, South, West