Ted Floyd Creekcare

Urban Soils

Soil Organic Matter

Soil Carbon, Carbon Cycle & Carbon Sinks

Soil organic matter is the decaying remains of plants and animals and consists of carbon compounds.

Organic matter is important in forming healthy fertile soils. Plant nutrition and soil physical properties are improved in soils containing an adequate supply of organic matter.

Soils are a very good carbon sink and have the potential to reduce the adverse effects of global warming. Carbon dioxide is released into the atmosphere from decomposing organic matter and is absorbed during photosynthesis and added to the soil by growing plants. In well managed soils organic matter can be increased, creating a soil carbon sink.

Carbon exists in soils in many different forms. Soil organic matter is part of the carbon cycle and life cycle of eco-systems on land. Lime, gypsum, ash, charcoal, are mineral forms of carbon in soils.

Fertile soils provide an ideal medium for the healthy growth of many lifeforms. Most plants obtain water and essential nutrients from soils. Many small animals including worms, mites and insects live in the leaf litter and surface soils. A small number of large animals including wombats and rabbits burrow into soils to make their family home.

Soil Carbon CycleFertile soils are teaming with invisible microbes. Soils are an ideal home for, bacteria, actinomycetes, algae, protozoa and fungi. Many plant roots and microbes form special interactive relationships. In the rhizosphere surrounding plant roots, exudates containing food for microbes are released by roots encouraging prolific microbial growth. The microbes help the roots to absorb nutrients.

Small soil animals and microbes continually break down soil organic matter into gaseous CO2, water and minerals. The decomposition of organic matter releases essential mineral elements into molecules available to growing plants. Nitrogen, phosphorus and sulphur are important plant nutrients continually added to the soil by the decomposition of organic matter.

There is a continual cycle of carbon without the build up of large amounts of dead plant material. The accumulation of peat in swamps and the ancient formation of coal and oil occurs when the environment is unfavorable to micro-organisms.

Organic matter is made up of partly decomposed plants and humus. On the soil surface their is a layer of leaf litter. Many small animals, especially insects and worms eat the fallen plant material and deposit faeces rich in nutrients deeper in the soil. Many microbes feed on organic matter and then eat each other.


Humus is completely decomposed organic matter and cannot be recognized as plant remains. Small humus particles are amorphous colloids, smaller than crystalline clay colloids and the surface is electrically charged and attracts cations and anions. Many essential plant nutrients are stored on the electrically charged surface of humus helping to improve soil fertility. Water is absorbed by humus increasing the ability of soils to store water.

The dark colour of soils, especially top soils is due to dark coloured humus. The presence of humus tends to give soils a healthy, fertile feeling. A healthy soil contains 10% or more organic matter in the top soil and the sub soil may only contain 0.5%.

Leaf Litter

Leaves falling onto the soil surface build up a layer of leaf litter. Leaf litter is a very dynamic eco-system with many small animals and microbes. The leaf litter and soil is natures recycling centre where small animals and microbes eat fallen plant material releasing nutrients into a soluble form which is available to growing plants.

A healthy, dynamic leaf litter also improves the physical fertility of a soil. Small soil animals and microbes create good soil structure improving soil aeration, increasing water infiltration and reducing extreme temperatures. Burrowing animals, especially earthworms greatly improve water infiltration. Leaf litter protects surface soil from beating raindrops and erosion. A deep leaf litter is a natural mulch and is food for earthworms, every gardeners friend.


Leaf litter on the surface of soils is natures perfect mulch.

Mulches are a layer of loose plant material or other residue spread on the soil surface to help conserve moisture. Common materials used for mulching are wood or bark chips, straw, animal manures and compost. Garden waste if chopped into small pieces makes a good mulch. Artificial materials are sometimes used as a mulch. A layer of black plastic, paper or small stones will stop weed growth and conserve moisture.

Mulches made from plant material will be eaten and broken down by soil animals and microbes and incorporated into the soil organic matter. The speed of breakdown varies greatly depending on the mulch. Wood chips breakdown slowly and remain on the surface for a long time. Straw from horse stables will breakdown more quickly if it contains animal manure. Many mulches, especially mulches containing animal manures will add plant nutrients to a soil when they are broken down and buried in the soil. It is important not to introduce weed seeds or diseases to the garden from mulching.


Peats have a very high organic matter content and form in swamps or cold climates when the decomposition of plant material is very slow. The organic matter content ranges from 40% to 90%. When the rate of plant growth is faster than the rate of decomposition, organic matter will accumulate and a peat will form.

In many natural ecosystems peats help to regulate water flow in creeks and rivers. Water is stored in peats, reducing flood peaks and during dry times water is released maintaining streamflow during droughts. Peats are famous for purifying water and the best Scotch Whisky is made using pure, fresh water flowing out of the highland peats. In the Blue Mountains of NSW, many hanging swamps help to regulate water flow in mountain streams.


Soils are an ideal medium for microbes to grow in. Bacteria prefer to grow in the thin layer of moisture surrounding clay particles and fungi grow better in large soil pores and can survive in dry conditions. Bacteria have a diameter of approximately 0.001 mm and fungi filaments about 0.005 mm. In comparison clay particles are less than 0.002 mm.

Microbes need a continual supply of food from plants and animals. Different microbes eat different plant materials. Bacteria prefer to eat smaller soluble compounds, especially sugars. Fungi feed on hard to decompose plant fibers and woody material including cellulose, lignin and plant fibers.

Benefits of soil organic matter

A gram of healthy soil contains up to 3,000 million bacteria and 500,000 fungi plus actinomycetes, algae and protozoa.

Microbes generally grow faster in fertile soils similar to the ideal conditions for plant growth. The soil needs to have adequate moisture, aeration and good drainage. Most microbes need similar inorganic nutrients to plants. Nearly all microbes differ from plants and cannot manufacture organic material by photosynthesis and need a supply of organic matter to grow on.

Healthy soils with dynamic microbial ecosystems are able to break down many organic pollutants.

Small soil animals

(Mesofauna or Invertebrates)

Most small soil animals are found in the leaf litter and top soil. Good soil aeration is needed for optimum growth and waterlogging greatly reduces the survival of animals. Insects, mites, spiders, centipedes, millipedes, earthworms and nematodes are found in soils.

Plant Roots

Roots anchor plants and stop them from falling over or blowing away and they absorb from the soil water and nutrients essential for growth.

Plants with deep roots can tap a larger reservoir of water and are more drought resistant. Plants with a shallow root system can only absorb water from the surface soil and are more susceptible to droughts.

Perenial plants with deep tap roots may have roots up to 40 metres deep. Annual grasses have shallow fibrous roots and the lawn grass Poa annua have roots only 15 cm deep.

Roots do not penetrate into dry soils. When watering plants sufficient water should be applied so the water wets the soil to a depth a little greater than the root depth. Plants should be encouraged to grow deep roots and to become drought proof.


The rhizosphere is the zone of soil surrounding roots containing a large, very active population of microbes. This microbial active zone can extend up to 1mm from the root.

Root exudates & secretion of border cells

Plant roots exude water and many organic chemicals including sugars, amino acids, organic acids, vitamins, plant hormones, growth substances, mucilage and proteins. Special chemicals can inhibit growth of pathogens and competitor plants. Benificial microbes are encouraged by exudates.

Up to 20% of carbon fixed by photosynthesis in plants is transfered to the soil as root exudates.

Exudates regulate microbiological activity and encourage beneficial symbiosis. Microbiological activity encouraged by exudates improves the uptake of nutrients by roots. Exudates defend roots from pathogens, reduce diseases and inhibit growth of competing plants.

Rhizophere and exudates improve water conditions in soils encouraging plant growth. Fungi filaments and mucilage stabilise soil aggregates and improve soil structure.

Soil Carbon Sinks

In June 2008 the Australian Government released a draft policy on Carbon Trading and Global Warming Control. This policy did not recognise soils as a potential carbon sink.

In the Australian Greenhouse Gas Inventory for 1990 carbon dioxide emissions from clearing native forests was 130 Megatonnes. Since then emissions from clearing forests have decreased to a quarter of this value. In the same time emissions from fosil fuels have increased by a similar amount.

The decrease of emissions from clearing forests has enabled Australia to nearly achieve the Kyoto targets even though energy use has rapidly increased. About half of the carbon dioxide emissions from clearing forests originates from soil carbon and half from above ground plants.

If carbon dioxide emissions from soils was ignored in the Kyoto target Australia would fail miserably. It would be wise if Australia encouraged soil carbon sinks in a carbon trading scheme so the reduction of emissions from clearing forests remains valid in International Protocols.