Soil Organic Matter – Understanding the Basics
Introduction
If you’ve been exploring soil health for a while, you’ve likely discovered that soil organic matter (SOM) is a cornerstone of productive, resilient farming. It plays a vital role in nutrient availability, plant tolerance to diseases, pests and environmental stresses (such as drought, salinity, and waterlogging), along with overall farm profitability.
So what is Soil Organic Matter?
SOM includes all living and non-living organic materials in the soil:
- Living organisms: microbes, fungi, and plant roots (including root exudates)
- Dead plant and animal material: leaf litter, stubble, mulch, and manure
- Partially decomposed matter: organic residues in transition
- Humus: the stable, fully decomposed fraction that drives soil structure and nutrient retention1
Microbes and earthworms break down these materials, releasing nutrients and forming humus. Humus improves soil aeration, water retention, and nutrient storage. SOM is rich in carbon, hydrogen, and oxygen, the building blocks of organic molecules, but it also contains essential elements like nitrogen, phosphorus, and sulfur.
Why is SOM so important?
SOM enhances soil health in multiple ways:
- Improves structure: humus binds soil particles into stable aggregates
- Boosts drainage and aeration: larger aggregates create pore spaces for air and water
- Increases water-holding capacity: humus acts like a sponge, storing moisture for plant roots
- Supports nutrient cycling: microbial decomposition releases nitrogen, phosphorus, and sulfur
- Enhances nutrient retention: humus particles are negatively charged colloids (a colloid is any particle in the soil with a diameter of less than 0.001mm or 1 micrometer)2 that hold onto positively charged nutrients like calcium, magnesium, and potassium until plants need them. Humus has a greater Cation Exchange Capacity (CEC) than clay.1
What are the benefits of increasing your SOM?
- Reduces nutrient loss: SOM helps soil retain nutrients, reducing runoff and leaching
- Supports microbial life: a thriving soil biology improves nutrient availability
- Builds plant resilience: plants recover faster from droughts or floods
- Improves profitability: SOM boosts productivity while reducing reliance on synthetic inputs
Generating more SOM in turn creates more humus and helps build resilience in plant health, which means that in both dry and flooding periods plants can recover quicker3.
This video, produced by Terrain NRM provides an excellent explanation about Soil Organic Matter, its role in plant health and steps that can be taken to increase SOM.
How can I build Soil Organic Matter?
Start by setting a SOM target. A target of 2.5% SOM would be ideal, however, this may be a bit daunting in some regions, so start by trying to increase by 0.5% over 5 years (depending on your farming system).
Soil microbes are key to building SOM. However, synthetic fertilisers and certain chemicals can disrupt microbial activity and accelerate organic matter decomposition.
- Test your soil: establish your SOM baseline
- Adjust management practices:
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- Reduce tillage
- Apply compost or mulch
- Introduce multispecies cover or break crops
- Revise grazing to allow longer rest periods
- Add biology:
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- Use NutriSoil Biological Solution as a seed inoculant or foliar spray
- Apply NutriSoil Castings or compost extracts
- Reduce synthetic inputs:
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- Excess nitrogen accelerates SOM breakdown
- Buffer nitrogen with carbon sources and gradually reduce application rates
5. Maintain living cover: Root volume below ground contributes more to SOM than leaf matter above ground.
Conclusion
Without SOM, soil can’t hold nutrients or moisture effectively. Humus, the stable component of SOM, is key to making nutrients available to plants. Building SOM does take time, however small increases can produce big results. Soil Organic Matter contains about 57% carbon4.
Even small increases in SOM can yield big results:
- A 1% increase in soil carbon boosts water-holding capacity by 144,000L per hectare5
- That same increase can store nutrients equivalent to $1,500–$2,000 worth of synthetic fertiliser (based on fertiliser prices in Australia in 2017)6.
Used in conjunction with other SOM building practices, NutriSoil products can help you integrate biology into your system. For example, foliar applications of our Biological Solution enhance photosynthesis, increase root biomass, and build SOM.
Speak to the Nutrisoil team about how we can help you start the process of building your SOM, increasing your farm resilience, reducing input costs and increasing your profitability.
References
- https://www.dpi.nsw.gov.au/agriculture/soils/guides/soil-carbon/organic-matter
- Soil Colloids: What Are They (Earth Science Classroom)
- Soil Organic Matter: Everything you need to know about organic matter on your farm (Terrain NRM)
- Local Land Services Fact Sheet 6
- Integrated Crop Protection by Soil Wealth
- Carbon: Soil’s Secret to Saving Civilisation