Soils and carbon for reduced emissions
Soil Carbon Snapshot – Updated 2022
Agriculture Victoria has packaged together the latest science around soil carbon. This updated resource in booklet form, now includes links to 70 research references and soil carbon reports relevant for Australian agriculture. If you are trying to better understand the fundamentals of soil carbon in agriculture, this is a great new resource to start your journey.Download PDF
A healthy soil is productive, sustainable and resilient to withstand the impacts of farm management practices and changing climatic conditions. Healthy soils undertake many functions for healthy plant growth, including storing and providing water and nutrients, maintaining biological activity, maintaining good soil structure and the ability to resist erosion.
Soils can store carbon and soil carbon is strongly linked to soil quality and productivity. How much and for how long varies depending on factors such as soil texture (the clay and fine silt fraction), climate — temperature and annual rainfall (amount and distribution over the year), soil moisture and importantly farm management practices.
Soil organic matter makes up a small component of the soil mass, yet it has an important role in the functioning of the physical, chemical, and biological properties of the soil. Soil organic carbon is a measure of the carbon contained within soil organic matter. Soil carbon provides a source of nutrients through mineralisation, helps to aggregate soil particles (structure) to provide resilience to physical degradation, increases microbial activity, increases water storage and availability to plants, and protects soil from erosion. Ultimately, increasing soil carbon levels can lead to better plant establishment and growth. While increasing soil carbon is highly desirable, it is also easily lost, so maintaining what you have is important. Climate is a strong driver, affecting accumulations and decomposition of soil organic matter in soils.
The following management options aim to improve soil condition by improving soil structure, reducing losses of carbon and nitrogen from the soil and building soil organic matter. Improving soil condition will enhance a plant’s ability to access the nutrients it needs, capture and retain soil moisture for longer and reduce losses of nitrogen to the atmosphere, groundwater and waterways.
There are ways to increase soil carbon while also increasing productivity, water holding capacity and nutrient cycling. This will reduce input costs and produce wider natural resource management benefits.
Monitor soil nutrient levels:
- test your soil to check the nutrient status and structure of your soil and develop a plan to improve constraints to nutrient and water access e.g. physical (structure, compaction, drainage), chemical (pH, salinity, toxicities/deficiencies), biological (micro-organisms).
- monitor soil organic matter/soil organic carbon over time via testing.
- complete a nutrient balance/budget to match fertiliser requirements to crop/pasture demand.
- manage soil structure to maximise water infiltration and retention for plant uptake and aeration.
Consider application of soil amendments:
- addition of organic amendments (manure, crop residues) where practical and economically viable. Know the quality of any products, have them tested and ensure any claimed benefits are supported by sound evidence and research.
- manage application of gypsum on sodic soils to maintain/improve soil structure.
- manage livestock manure (dung and urine) to minimise nitrous oxide emissions — see Livestock section for further details.
Manage the soil resource:
- use direct drill, minimum/conservation tillage and controlled traffic techniques in cropping operations to avoid compacting soils and losing carbon and nutrients through soil cultivation and erosion.
- avoid burning crop residues and retain where possible.
- cultivate soils at an appropriate moisture content — not too moist for soils to smear, or too dry that the soil is pulverised.
- avoid bare fallows and have continuous plant cover where possible e.g. green/brown manure crops between seasons and crops can maintain groundcover, provide active root material and organic matter as well as use available nitrogen and avoid losses by leaching.
- manage irrigation and soil drainage to avoid waterlogging. Use irrigation scheduling and monitoring.
- rotate crops and include perennial pastures and legumes phases in rotations. In general, perennial pastures will improve or stabilise soil carbon more than annuals can.
- do not overgraze pastures. Ensure there is sufficient groundcover throughout the year (>50 percent cover). Consider stock containment areas to ensure improved pasture and groundcover management options.
- manage livestock movement and paddock rotations to distribute animal deposited dung and urine evenly and reduce compaction from hoof traffic.
Keep an eye on policy changes relating to potential incentive payments for carbon stored in soil. Always seek legal and financial advice first. Some questions to ask before selling carbon offers useful insights on what you might need to consider.
Soil carbon explainer video
This video summarises what we know from the latest Australian research on soil carbon science.
Many people ask, "So, what's the story with soil carbon?" We know it's important, and for a farmer, it can boost soil health, fertility, water holding capacity, and soil structure. In fact, you can think of it as a foundation block for productive agriculture.
While oceans are the world's largest carbon sink, our soils contain more stored carbon than is found in all the vegetation, and atmosphere combined. Every additional ton of soil carbon that we create, can remove the equivalent of 3.67 tonnes of CO2 from the atmosphere. Since CO2 is the most significant greenhouse gas, this is important, but, in reverse, every tonne of soil carbon lost from our soils will emit the equivalent of 3.67 tonnes of CO2 back into our atmosphere. It's important that we look after our soil carbon, and better understand it.
Let's look at it more closely. Soil carbon comes in both organic, and inorganic forms. Here, our focus will be on soil organic carbon, which typically makes up 58% of the total soil organic matter content. It's this component that we can most readily influence. Stored soil carbon is a bit like inheriting a bank account. With the size of your soil carbon bank balance being mainly driven by natural, primary productivity. With the two most influential factors being climate, and soil type and depth.
Climate is a key driver. The largest soil carbon stores occur where there is high rainfall, and cooler temperatures. Think about peat bogs. Similarly, the lowest soil carbons stores occur in low rainfall, hot areas, more like a [inaudible 00:02:02] desert. Soil type and depth is also important, where clay based soils hold more soil carbon, than sand. Soil carbon levels can vary from rich peat soils, with greater than 10% soil carbon, right down to highly cultivated and sandy soils, which can have as little as half a percent.
Although land management tends to be the minor player, it can influence carbon content over time. Our soil carbon wealth account is made up of three fractions. The labile, or particulate fraction. The humus fraction, and the resistant carbon fraction. The labile, or particulate fraction operates like an access account. It's readily available for use by soil microbes, which makes it the least stable, shortest lived, and the easiest to lose.
Humus is a bit slower, and more like a long term deposit, or investment in real estate that adds to your wealth, being stable over years, and decades. Resistant carbon is the equivalent of being locked away in a vault, for your great grandkids. It's very stable, and can last for hundreds of years.
A balanced soil carbon account generally requires a regular supply of plant residues, and organic matter, providing regular deposits into the account. Meanwhile, soil microbes eat away at the organic matter, using some nutrients for themselves, and releasing remaining nutrients for plants to access. It's a bit like having regular expenses taken from your account. That's why we have to keep making regular deposits, so that we can maintain a soil carbon account balance.
If your deposits match your withdrawals, then you'll have a stable soil carbon account. Just like a bank, if your deposits are greater than your withdrawals, your account will grow. Unfortunately, since the introduction of agriculture in Australia, it's been more common to be losing carbon from soils, rather than increasing them, with soil carbon levels often declining from natural levels. Across the Australian wheat belt, it has been estimated that over 60% of soil carbon has been lost from the top ten centimeters of soil. This is largely because little carbon is produced during the fallow period. Compared to what could be achieved by either permanent pastures, or native vegetation, which can accumulate some carbon input most of the year, after each rainfall event.
Land management practices, such as cultivation, stubble burning, annual cropping, overgrazing, and erosion, are all activities which tend to cause soil carbon loss. It's a bit like a banking situation, where our deposits are less than our withdrawals. Our balance shows a loss over time. Thankfully, many farmers are keen to try and turn this around. Using perennial pastures, cover crops, and other ways of increasing additions of plant biomass, and organic matter, which can help to maintain, or sometimes even increase soil carbon levels.
However, farmers need to be aware that this will not always increase the soil carbon level. When we add more inputs to our account, the soil microbes sometimes just increase their activity. Resulting in more carbon turnover, but, not necessarily adding more stored carbon in our bank. It's a bit like expenses, where we can earn more money, but, just end up spending more. That's why it's important that we measure your bank balance over the longer term, to see if your soil carbon wealth account is growing or reducing.
Earlier, we talked about the fact that soil microbes use nutrients, and release some to the soil as they eat away at soil carbon. This can result in crop and plant benefits, via mineralization. When nutrients in the soil organic matter, and carbon bank are released. Overall, a balanced farming system ensures we put back what we take out. This means that while it's a great idea to try and grow your soil carbon, it's important to remember, that when you store carbon, it also locks away other nutrients with it. This might require the addition of extra nutrients.
Also remember that any land use or management changes, which increase soil carbon, will need to be maintained indefinitely, if you want to keep that higher carbon bank balance over the longer term. There you have it. There's been a lot of great research on soil carbon across Australia, and how to look after it. To find out more, just head to these sites. This cash, can I keep this?
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