Facilitated by: Cassandra Douglas-Hill, Kangaroo Island Landscape Board 
Kangaroo Island, South Australia  | April 2026

• Plants require sunlight, water, nutrients and CO2 to photosynthesise
• CO2 is a trace gas in air (currently ~400 ppm, up from ~320 ppm pre-industrial)
• Plants capture carbon from CO2 and convert solar energy into stored carbohydrate energy
• All plant biomass - stems, leaves, roots, waxes, oils, lignin, cellulose - is derived from this process
• Around 100 water molecules are needed per carbohydrate molecule produced; the remaining water is transpired, which cools the landscape

• Actively growing plants exude sugars from their roots into the soil
• This feeds soil microbes (bacteria, fungi, earthworms, dung beetles etc.), driving nutrient cycling
• Microbes eat each other and organic matter, releasing nutrients back into the soil food web
• Nitrogen passed through the food chain remains largely attached to carbon, which stabilises it
• Plants change the type of root exudates through the season to recruit specialised microbes as needed
• Bare or compacted soils have much lower microbial activity, less soil structure, less aeration, and stay waterlogged longer

• Organic matter enters soil via root material, surface litter, and root exudates
• In most environments, organic matter breaks down relatively quickly through microbial respiration, releasing CO2
• Carbon either respires back to the atmosphere or stabilises as humus - there is no third pathway
• The carbon percentage on a soil test represents the balance between inputs and losses
• To convert carbon % to organic matter %, multiply by approximately 1.5
• Humus acts as a colloid, buffering pH and salinity, and contributes to soil structure and water holding capacity

• Sandy / light soils: minimum ~2% carbon to start driving functional soil improvement
• Heavier soils (more silt and clay): minimum ~3-4% carbon
• Low carbon = insufficient energy to drive the soil food web, regardless of nutrient inputs

Carbon numbers should always be interpreted in context of soil texture, topsoil depth, and observed structure - not read in isolation.

• Climate: cooler, wetter conditions slow breakdown and allow accumulation
• Soil texture: more clay generally means greater capacity to hold organic matter
• Vegetation management: the biggest single influence on carbon levels
• Tillage: increases oxygen, speeds microbial respiration and accelerates carbon loss
• Ground cover and canopy: reducing surface temperature slows respiration rate
• Root depth and diversity: deeper, more diverse roots build more carbon through the profile
• Excessive nitrogen: can over-stimulate microbial activity, accelerating carbon loss

• Nitrogen in soil lives in organic matter - not as a free soluble form
• Fertiliser nitrogen uptake efficiency is commonly less than 50%, and can be much less on degraded soils
• Unused nitrogen can: volatilise as a gas, leach (especially on light sandy soils), or be temporarily immobilised by microbes
• Microbes need carbon (energy) to stabilise nitrogen; without it, excess nitrogen cannot be held in the system
• Organic matter is the primary storehouse for nitrogen in the soil

• Some plants are specialist nutrient scavengers, technically known as bioaccumulators
• Example: chicory has a deep tap root and is known to access nutrients unavailable to other plants
• Useful for targeted nutrient cycling in deficient or compacted paddocks
• Different plants have different specialisations - some are generalists, others are highly targeted

• Keep ground cover and living roots in the soil for as much of the year as possible
• Avoid over-tillage - every cultivation event burns carbon and disrupts soil structure
• Do not apply excessive nitrogen to low-carbon soils - it will be lost, not held
• Build organic matter first to give nitrogen somewhere to live in the system
• Interpret soil test carbon numbers in context of soil texture, topsoil depth, and observed structure
• A visual soil assessment alongside a soil test gives a much more complete picture
• SAP testing (plant sap analysis) was offered alongside soil tests to understand what the plant is actually accessing