• Why is it important to me as a farmer?
• Background to soil organic matter and soil carbon
• Carbon in agricultural systems
• How much carbon can be stored in soils?
• Factors affecting soil carbon level
• Soil organic carbon in the Corangamite region

• What is the best practice?
• How can you achieve this?

• Soil organic carbon is the basis of soil fertility and is a part of soil organic matter
• It plays a key role in soil health through biological, physical and chemical functions
• Increasing organic matter in soil increases the amount of carbon in soil, and a wide range of soil health benefits result

• The amount of organic carbon in soil is a balance between the inputs and outputs of carbon
• Building soil carbon can take a long time

• Soil carbon has declined significantly in many soils since they were cultivated for cropping - consequently these soils may be less able to supply nutrients to meet plant demand
• High levels of organic carbon helps to maintain agricultural production through its positive role in maintaining soil health, raising fertility, reducing erosion and encouraging soil biota

• Soil organic carbon (SOC) is considered the basis of soil fertility
• Soils with higher SOC levels are generally more fertile, more productive and easier to manage than low SOC levels

• Soil carbon, or soil organic carbon as it is more accurately known, is the carbon stored within soil
• Carbon makes up approximately 60% of the soil organic matter (SOM), with the remaining 40% of SOM containing other important elements such as calcium, hydrogen, oxygen, and nitrogen
• SOM is commonly, but incorrectly used interchangeably with SOC
• Soil carbon enters the soil as soil organic matter
• Soil organic matter is made up of plant and animal materials in various stages of decay
• Un-decomposed materials on the surface of the soil, such as leaf litter, are not part of the organic matter until they start to decompose
• SOM makes up only a small fraction of the soil (normally 2 to 10% - compared to minerals which make up the bulk of soil), but plays a very important role
• SOM and SOC are a very diverse collection of materials that decay at different rates
• SOM retains moisture (humus holds up to 90% of its weight in water), and is able to absorb and store nutrients

• SOM is the primary food source for microorganisms and other forms of soil life in dryland agriculture
• Organic matter quality is important, as incorporating large amounts of high-carbon material (i.e. wheat stubble) can deprive plants of soil derived nitrogen in the short term
• Organic matter contributes to the development of the darker friable topsoil that retains moisture and cycles nutrients for plant growth
• The value of organic matter in soil health is hard to overestimate in that it can:
  • provide ground cover
  • increase soil stability
  • assist in cycling nutrients
  • provide habitat and erosion control
• Farming practices that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition which may not become evident for many years

• Soil organic carbon (SOC) is the basis of sustainable agriculture
• It is cycled through the atmosphere, through plants and animals, and through the soil
• The production of food affects the amount of carbon in the soil as harvesting plant and animal products removes carbon from the agricultural system

• By increasing the amount of carbon stored in the soil, we could significantly offset the amount of CO2 in the atmosphere and also improve the health of our soil
• Increasing organic matter in soil increases the amount of carbon in soil, and wide range of soil health benefits result

• There are a whole range of SOC levels in different soils
• For the surface soils, SOC ranges from about 10% in the alpine soils to less than 0.5% in the desert soils. In the Corangamite regions, SOC is estimated to be about 3-5% in grazing systems and 2-3% in cropping systems
• The amount of SOC stored in the soil profile can be considerable. For example, if there is 1% SOC over 30 cm soil depth, the amount of SOC stored over 1 hectare of land can weigh about 42 tonnes. Usually, the surface layer has the highest level of SOC which decreases with depth down the soil profile. The actual amount of SOC present in a soil is dependent on a number of factors
• The amount of SOC stored in soil is the difference between all SOC inputs and losses from a soil

• Determined by factors such as rainfall, temperature, vegetation and soil type
• The main inputs of SOC to soil in rain-fed farming systems are from plant material, such as crop residues, plant roots, root exudates and animal manure
• Losses of SOC from soil are from decomposition by microorganisms, erosion of surface soil and off-take in plant and animal production
• Decomposition and SOC:
  • Occurs when microorganisms use SOC in soil to obtain the carbon, nutrients and energy they need to live
  • During decomposition, SOC is lost from soil because microorganisms convert about half of the SOC to carbon dioxide gas (CO2). Without continual inputs of SOC, the amount stored in soil will decrease over time because SOC is always being decomposed by microorganisms

• Erosion of surface soil and SOC:
  • Losses of SOC from erosion of surface soil can have a large impact on the amount of SOC stored in soil
  • This is because OC is concentrated in the surface soil layer as small particles that are easily eroded
  • In Australian agriculture, erosion can cause the annual loss of 0.2 t/ha of soil from a pasture, 8 t/ha from a crop and up to 80 t/ha from bare fallow
• Off-take of OC in plant and animal production is also an important loss of OC from soil. Harvested materials such as grain, hay, feed and animal grazing all represent loss of OC (and nutrients) from soil

• Although there is very limited information available on soil organic carbon levels in the Corangamite region at this stage, preliminary results have shown that higher organic carbon levels are likely in higher rainfall areas featuring long-standing perennial vegetation, with minimal agricultural land use
• Figure 1 illustrates the difference in soil organic carbon levels at a farm scale under different management practises

Figure 1 - Two soils from neighbouring paddocks, soil �A� has high organic carbon levels, while soil �B� has lower carbon levels due to poor land management practises - Source: CCMA

Source: SoilHealthKnowledge.com.au

Soil OM is in a constant state of turnover, where it is decomposed and replaced by new organic material. Therefore the balance between inputs and the rate of loss will determine the relative flux in SOM content. In many instances, SOM is still declining and in �negative feedback� mode � thus current land management is typically taking more from the system than it returns.

• Eliminate unnecessary cultivation (introduce zero or minimum tillage)
• Include pasture rotations where possible and use pastures in the inter-rows for tree crops and vines. Plant perennial species where possible
• Adopt appropriate grazing management strategies that minimise the impact of grazing on soil structure and maximise organic matter returns
• Maintain and conserve ground cover - To maximise wind erosion control, stubble should cover a minimum of 70% of the soil surface, preferably be standing (anchored by roots)

• Grow high yield, high biomass crops and pastures, and in continuous cropping systems maximise crop frequency to increase organic matter returns to the soil
• Maintain soil fertility with inorganic and organic fertilisers to maximise production
• If available locally, import manure/compost or other organic amendments
• Monitor to assess whether management change is depleting or restoring the carbon resource

• The CSIRO has investigated management strategies for building soil carbon, and rated each practice for its potential to increase soil carbon levels (Kirkegaard et al 2007)
• The levels of positive (+) or negative (-) influence are indicated for each practice
• It is advised that farmers should increase soil carbon to enhance productivity first, rather than focus on the potential of carbon as a financial stream in carbon markets

• Ley farming is a system of rotating crops with legume or grass pastures to improve soil structure and fertility and to disrupt pest and disease lifecycles. It has been practised in many parts of the world for centuries
• For the practice to be successful, it must be well managed
• Provided for rapid C build-up
• Perennial plants provide a constant C input
• No disturbance
• Recycling residues/manure
• Less erosion

• The return of manure and recycled organic materials to the soil is considered the practice with greatest potential to increase SOC levels
• Can be difficult in extensive agriculture, but the use of recycled organic materials is gaining momentum in the Corangamite region
• Its ability to improve soil carbon levels depends on the amendments �quality� � mature composted products are best

• Improved C stabilisation (N,P,S)
  • Studies in the GHCMA region (Kirkby 2010) demonstrate that sequestering carbon into the stable SOC pool requires predictable amounts of N, P and S and that carbon sequestration will be limited when these nutrients are insufficient despite large amounts of carbon input
• Increased plant productivity
  • A healthy growing, well managed perennial species is one of the best practices to enhance soil carbon
• A healthy plant contributes increased residue inputs

• Increased decomposition rates
  • More rapid breakdown of organic matter in the soil to the more stable humus state
• Increased plant productivity
• Increased residue inputs

• Increased residue inputs
• Depends on crop and tillage used

• Increased residue inputs
  • Generally, stubble retention without other changed practices will contribute to a maintenance of soil carbon levels, rather than an increase
• Depends on grazing/tillage used

• Residue input and quality is key
  • Contributes to a more diverse microbial community
• Diversity may increase yield
• Large, deep rooted plants are best for increasing SOC

• Effects on biology are short term
• Unlikely to influence soil C

• Detrimental to SOC levels
• Increased disturbance/soil mixing
• Residue burial speeds decomposition
• Can influence water and temperature
• Increased erosion risk

• Carbon and nutrients are removed and lost off-site

• No C inputs
• Microbial respiration continues
• Moist soil favours decomposition
• Cultivation favours decomposition
• Higher erosion risk

• Only a minority of pastures are predominantly perennial species
• Increasing the proportion of perennial species in pastures will increase organic carbon in soils at depth
• Perennial species have a much higher biomass and therefore provide greater amounts of organic carbon to the soil
• Grazing regimes that encourage pasture regeneration, such as �graze and spell rotation�, are effective contributors to the maintenance of higher organic carbon levels

• Maintaining soil organic carbon levels is as important in cropping paddocks as it is in pasture paddocks
• Stubble retention contributes to the maintenance of higher carbon levels in soils. Incorporating pasture phases into the rotations also helps maintain higher organic carbon levels
• The use of organic fertilisers/amendments to supplement inorganic fertilisers has the greatest potential to enhance soil carbon levels in the Corangamite region

• How should we manage our soils to increase soil biology?
• How do I manage the impact of tillage?
• When and why should I need to apply lime?

• What organic fertilisers work?
• How do I manage stubble?

• Johnston, T. Understanding Soil Biology. DPI Healthy Soils Modules. DPI Victoria.
• Clarkson T, Department of Primary Industries on behalf of the Corangamite Catchment Management Authority (2007). Corangamite Soil Health Strategy 2007. Corangamite Catchment Management Authority, Colac, Victoria.
• Soil Biology. � Department of Primary Industries, Victoria.
• Why Soil Organic Matter matters. - CSIRO.
• How Much Carbon can Soil Store..� Soilquality.org.au
• Using biological activity to improve soil. Section D7 - SOILpak - southern dryland farmers. Department of Primary Industries, NSW.

• Woady Yaloak Catchment Group. Evaluating alternative fertilisers and biological products for pastures and crops. Result of the 2009 and 2010 seasons
• Soil biology in agriculture. (2004 Workshop Proceedings).
• Kirkegaard J, Kirkby C, Gupta V. (2007) Management practices for building soil carbon. � CSIRO.
• Name Soil Organic Matter � Best practice principles for managing SOM. -SoilHealthKnowledge.com.au.
• Chan,K.Y., Oates,A., Liu,D.L., Li,G.D., Prangnell,R., Poile,G. and Conyers,M.K. (2010). A farmer�s guide to increasing soil organic carbon under pastures. Industry and Investment NSW, Wagga Wagga, NSW.

Page Updated: September 2013
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