Soil Organic Carbon
Carbon in agricultural soils
Storage of carbon in agricultural soils through appropriate management actions has been recognised as an important tool to mitigate climate change. Carbon is the main element present in soil organic matter, on average making up 58% by weight.
Increased carbon storage in soil can:
- improve long-term farm productivity
- improve soil health
- increase the land’s resilience to a more variable climate
- help enhance Australia’s natural environment
- protecting against soil erosion
- act as a water purifying medium
- act as a sink and ameliorating agent for organic wastes and environmental pollutants
- support biogeochemical-nutrient cycles.
It is estimated that worldwide, soils contain approximately double the carbon present in the atmosphere and approximately three times the carbon contained in living plants and animals. It has also been estimated that globally, agricultural ecosystems have lost 50% to 70% of their natural soil organic carbon pool. The greatest accumulation of soil organic carbon typically occurs in the surface soil, with this organic enriched layer often 10 cm or less thick in South Australian soils. Therefore, it is important to identify and implement practices that minimise or reverse this decline in soil organic carbon, whilst balancing economic sustainability and global food needs.
Soil organic carbon
The amount of organic carbon in soil is a balance between inputs (plant and animal residues) and outputs (decomposition/mineralisation, erosion and removal of plant and animal products). Soil carbon can be built up with good management practices and a favourable climate, but it can also be lost as a result of some management practices and during times of drought.
Soil organic carbon is the carbon found in organic matter and includes all living and dead organic material in the soil, such as:
- plants
- soil organisms
- animal materials.
Soil organic carbon does not include fresh or undecomposed plant material greater in size than 2 mm or inorganic ‘carbonate’ matter. The inorganic carbon fraction of the soil is derived of weathered rock particles.
There are three pools or fractions of soil organic carbon:
- particulate - primarily comprises fresh plant and animal materials that are easily decomposed
- humus - materials that are less than 0.05 mm in diameter, and can persist in the soil for many decades, and store and release nutrients that can be directly utilised by plants and microbes
- resistant/recalcitrant/inert - charcoal and charred remnants of organic materials.
An adequate amount of soil organic carbon is important for a healthy community of soil microbes that provide many benefits to plant growth. Microbial activity, supported by soil organic carbon, helps turnover of nutrients and deliver them in plant-available forms. Microbial activity can provide disease-suppressing capacity to soils, through competitive exclusion and diverse defensive mechanisms against potential pathogens.
" Your soil type determines the potential storage of organic carbon. The climate determines the attainable storage of soil organic carbon. Your land management determines the actual storage of organic carbon in soil."
‐ Jennifer Carson, The University of Western Australia
How you can increase soil organic carbon in the Limestone Coast
The potential for storing more carbon depends on a soil’s measured base level of organic carbon and its storage capacity. Climate, soil type and condition, as well as land use and management practices affect the amount of organic matter that can be grown and put into the soil.
Ultimately, the opportunity to increase soil organic carbon depends on the:
- ability to grow or supply sufficient biomass (mostly in the form of plant roots) and maintain these inputs, otherwise soil carbon can also be lost
- ability of the soil to stabilise organic carbon (in combination with clay particles and soil aggregates)
- capacity of the soil to store more organic carbon
- capacity to convert organic carbon to more stable fractions.
Existing soil organic carbon stocks
Soil Organic Carbon Benchmarks for the Lower Limestone Coast
A recent study (Schapel 2022) undertaken in the Lower Limestone Coast region, and supported by the Limestone Coast Landscape Board and AgriFutures Australia, collected local information about soil types, climate and primary production systems to help identify soil organic carbon potential within the Limestone Coast region.
This data shows where there is potential opportunity to increase soil organic carbon based on surface soil texture class. For example, if your soil organic carbon levels are at or around the 25th percentile/lower quarter level shown above, there are likely to be significant opportunities to increase organic carbon (you will also need to take account of land use and climate). However, if your soil organic carbon levels are around the 75th percentile/upper quarter level, there is probably little opportunity to increase levels unless major land use change occurs. In addition, the wide range between low soil organic carbon levels and high levels within the loam, clay loam and clay texture classes indicates considerable potential for soil organic carbon increase in some soils in the region.
Soil Organic Carbon Benchmarks for the Upper Limestone Coast
A recent study (Schapel 2022) undertaken in the Upper Limestone Coast region, and supported by the Limestone Coast Landscape Board and AgriFutures Australia, collected local information about soil types, climate and primary production systems to help identify soil organic carbon organic carbon potential within the Limestone Coast region.
This data shows where there is potential opportunity to increase soil organic carbon based on surface soil texture class. For example, if your soil organic carbon levels are at or around the 25th percentile level shown above, there are likely to be opportunities to increase organic carbon (you will also need to take account of land use and climate). However, if your soil organic carbon levels are around the 75th percentile level, there is likely to be less opportunity to increase levels unless major land use change occurs (e.g. from annual cropping land use to permanent pasture). The relatively narrow range between low soil organic carbon levels and high levels within most texture classes indicates a significant but somewhat limited potential for soil organic carbon increase in many soils in the district.
Case Study - Koonara Wines, Dru Reschke
“For us it’s not about being organic, it’s about being nutrient dense. The more nutrients we can get into the plant, the less need there is for synthetic chemicals.”
Dru Reschke and his wife Nicole are the owners and operators of the Koonara vineyard in the Coonawarra. Dru kindly donated his time and expertise to appear in the Limestone Coast Landscape Board’s Carbon Explainer video series. He later shared some more information about his vineyard operation.
When did you start making changes to the way you farm, with a particular focus on carbon management, and what motivated you to make those changes?
In 2006, although the vineyard was still producing quality fruit, I noticed that there was a lack of resilience that resulted in the death of some vines during a particularly savage season of frosts. This prompted me to investigate other management techniques to improve the available nutrition in the soil, as well as create biodiversity of micro fungi and bacteria to break down that nutrition to feed the vines.
In terms of carbon management, we all need to do our bit to overt the crisis of global warming, and farmers adding carbon in the form of organic material into their soil will take the equivalent of 200 million cars off the road.
What are some of the land management changes you have implemented?
The vineyard was certified organic in 2017 after almost a decade of organic farming. But certification, and indeed organics, was really only a starting point. We have created a unique form of high-nutrition organics, which we call ‘Ecosystem Organics’. Another land management change has been populating the vineyard with flowers. The flowers we grow in the vineyard feed the five types of wasp that we have identified as having larvae which feed on our two major pests: vine moth and light brown apple moth. Wattles that bloom in winter are planted nearby to
provide a food source for the wasps all year round. The roots of the flowers have added about 1.5 per cent more organic material to the soil.
We apply our nutrients through irrigation, which reduces tractor use, and they consist of compost teas, silica and seaweed, plus many other additives. This mix strengthens cell walls in the leaves, reducing the need for excessive copper and sulphur sprays.
What have the results of those changes been?
Our pest and disease pressures have decreased to almost nil and we have improved the water holding capacity of the soil and minimised plant stress.
Has farm productivity improved? If so, how?
Our vineyard costs have decreased by over 30 per cent.
Are you considering selling carbon credits and/or achieving a net zero emissions position?
We won't chase carbon credits, only because we have 10 Ha of grapes, and it costs $15,000 to set up a project with the clean Energy Regulator, so the fact we have already increased soil a few percent means we will be unlikely to make many ACCU's.
Case Study - Treasury Wines, Nick Baverstock
“We’re naturally mining our soils by growing a product so we need to be putting back in. We’re trying to reduce the amount of artificial fertilisers we’re putting in and we’re really looking at organic based fertilisers or composts to improve vine health as well. That will improve the sustainability of our vineyards and the vines should last longer and yield longer as well.”
Nick Baverstock currently manages an 850 Ha vineyard in the Coonawarra. Nick kindly donated his time and expertise to appear in the Limestone Coast Landscape Board’s Carbon Explainer video series. He later shared some more information about his vineyard operation.
When did you start making changes to the way you farm, with a particular focus on carbon management, and what motivated you to make those changes? Or maybe this this is an area that has always been a focus, if so, why?
Climate change, sustainability of the asset and consumer expectations has motivated me to make changes, with more of a focus over the last 10 years.
What are some of the land management changes you have implemented?
Revegetation of non-viticulture land, use of organic inputs into the vineyard such as compost. Recycling of waste products such as old dripline, posts etc. Reduction in chemical use.
What have the results of those changes been?
Healthier vines, a more aesthetically pleasing environment to work in. A happier environment to work in, particularly around revegetated areas.
Has soil health improved?
We have seen improved soil health in areas we have applied compost and reduced herbicide use.
Has farm productivity improved? If so, how?
Difficult to say if productivity has improved, still on the journey.
Was there a transition period where productivity fell?
Still in the transition phase.
Are you considering selling carbon credits and/or achieving a net zero emissions position?
Not at this point.
Case Study - Verco Trading, James Verco
“Early on I liked to think that I could maximise production by focusing on the animals, then I thought a bit more about it and decided I should be maximising plant production, and then later on I realised that in order to maximise plant production I needed to focus on soil health.”
Where are you located?
Pooginagoric, 20kms south of Bordertown, mix of sandy loam over clay flats and clayspread sand hills, 450mm annual rainfall.
What are your farming enterprises?
We produce 1st cross ewe lambs, fat lambs, and fine merino wool.
When did you start making changes to the way you farm, with a particular focus on carbon management, and what motivated you to make those changes? Or maybe this this is an area that has always been a focus, if so, why?
Carbon specifically 2-3 years ago. Motivation came after learning about the landscape function benefits that higher soil carbon levels should provide us. Higher water holding capacity, higher water infiltration rates, higher nutrient holding capacity, pH buffering, higher cation exchange capacity. There only seemed to be positive outcomes for higher C levels.
What are some of the land management changes you have implemented?
Ceased drenching our sheep, maximising our mob size as often as possible - the only time we don't have all together is for mating and lambing, increasing stock density with portable electric fencing when required, sowing diverse cool season and warm season annual pastures/cover crops for grazing, not using any artificial fertilisers, drastically minimising herbicide and pesticide use, sowing perennial pasture species, mixed plantings of native trees and shrubs in shelter belts/riparian zones, reduced supplementary feeding.
What have the results of those changes been?
Increased health and vigour of perennial pasture species (sown, volunteer and native), increased predator insect and spider activity, decreased 'flight zone' of our sheep - generally they seem calmer and more inquisitive, sheep have a broader grazing preference.
Has soil health improved and are you seeing an increase in soil carbon?
I don't have any quantifiable evidence that soil health has improved or carbon has increased. We have undertaken soil chemical, biological, function and visual testing to give a starting point or 'baseline'.
Has farm productivity improved? If so, how?
Up to now I don't feel productivity measured in quantity has improved, partly because I think we were a little overstocked previously and compensating with a lot of supplementary feeding. I feel the quality of our produce is steadily improving, a recent surprise was our wool test results showing tensile strength had increased significantly and micron was finer than previous years. I would like to think these results and decreased expenses in chemical and labour (to spray paddocks and to make, shift and feed out hay) should make our business more profitable, even if we produce the same amount or slightly less product.
Was there a transition period where productivity fell?
Yes our productivity declined slightly, due mostly to our management decisions around decreasing our stocking rate and carrying capacity to suit our property better and to reduce supplementary feeding requirements.
Are you considering selling carbon credits and/or achieving a net zero emissions position?
Yes we are considering both of these options but not actively pursuing either of them.