Ted Floyd Creekcare

Urban Soils

diagram with numbers of a tree with roots below ground with kangaroo

Carbon Cycle in Soils

  1. Photosynthesis in green leaves CO2 in air synthesised into sugars
  2. Sugars transported in stems to all parts of plant
  3. Exudates from roots deposited in soil and becomes food for microorganisms
  4. Plants eaten by animals and respiration produces energy and CO2 returned to atmosphere
  5. Leaf litter forms from dead leaves, bark and stems
  6. Soil organic matter
  7. Microrganisms
  8. CO2 is released during respiration by insects and microorganisms and
  9. Inorganic plant nutrients are released into the soil in a soluble form available to plants.


Vigorous plant growth, irrigated by water harvesting will establish carbon sinks in urban gardens.

Water cycle

Water harvesting: Water collected from the roof of a house and used to irrigate the home garden. Water can be stored in a water tank and used in dry times or water from the roof spread directly on the garden during rain periods.

When raindrops come to earth they are absorbed by the soil or if there is too many of them they will flow down hill over the soil surface.

Transpiration is the movement of water from soils up the roots and stems of plants and into atmosphere. Water movement up roots and stems is essential to carry plant nutrients from soils to the growing parts of the plant.

Excess rainfall is not absorbed by soils and flows downhill over the land surface into drains and creeks. Flooding is reduced when more water is absorbed by soils and less water allowed to enter drains.

Water harvesting reduces downstream flooding and reduces severity of droughts in gardens. Organic matter improves water penetration into soils and increases water storage in soils.

Irrigation increases plant growth. The use of town water may not be available in the future and to ensure good plant growth in urban gardens water harvesting is necessary. Water tanks are very useful and storage of water in soils will help plants to grow in dry times.

Water collected on the roof of a house and spread over a garden area of equal size to the house, doubles the total rainfall. For this to work the soil has to be well prepared. The ground surface needs to be very permeable to allow water to soak down into the earth.


Growing plants encourages water to penetrate deep into soils. A variety of plants with different root systems will allow water stored in the soil at different depths to be utilized.

Carbon sinks

Carbon dioxide in the atmosphere causes greenhouse effect and air temperatres to rise. A carbon sink is created when carbon is removed from the atmosphere and locked up in a form away from the atmosphere and not causing a rise in air temperature.

Photosynthesis: In green chlorophyll of plant leaves, organic compounds are synthesised using energy from sunlight, water from soils and atmospheric carbon dioxide CO2.

Photosynthesis is the basic process producing food and energy by plants. Animals eat plants and each other and there food and energy is originally synthesised by photosynthesis.

Respiration:Plants and animals use oxygen from atmosphere and living compounds to produce energy and exhale carbon dioxide CO2 to atmosphere. All living organisms respire including plants, animals and microorganisms.

In a mature native forest the rate of photosynthesis is nearly equal to the rate of respiration. There is a balance and no CO2 is added or subtracted from the atmosphere. If the forest is cut down photosynthesis in trees stops and respiration by soil microorganisms continues and extra CO2 is added to the atmosphere increasing the greenhouse effect. If trees are planted and nurtured in cleared land and photosynthesis is faster than respiration a carbon sink will form.

Carbon sinks can be formed from any plants growing at a faster rate than total respiration. In most farming land respiration is faster than photosynthesis so CO2 is added to the atmosphere.

Carbon sinks in urban gardens are created by living plants growing above and below the ground surface. Green plants absorb carbon dioxide from the air and manufacture plant material by photosynthesis in leaves.

Below ground level, organic matter in soils is a very important carbon sink. Leaves and stems fall onto the soil surface, forming a leaf litter. Many small animals and microorganisms break down large dead plant remains into small pieces, incorporating broken down plants into soil organic matter.

Benefits of soil organic matter

Organic matter in soils is found in varying stages of decay. On the soil surface plant residues occurs including leaf litter and mulches. Particulate organic carbon is small plant remains, humus is very small particles synthesized by microorganisms. Charcoal is mainly carbon and is resistant to decay, lasting for hundreds of years.

Plants are made from sugars, starches, cellulose, fats, oils, proteins and lignin. Sugars are very quickly eaten by microorganisms, cellulose is slowly converted to sugars and lignin is very resistant to decay and may survive in soils for hundreds of years. Organic matter is mainly dead, decomposing plant material. Only a small proportion originates from animals. Animal bones, CaCO3 is a inorganic compound containing carbon.

The amount of carbon is often equal above and below ground level. Carbon in soils increases in cold or wet conditions. Woody plants, containing lignin, forms humus resistant to microbial breakdown and will persist in soils for hundreds of years.

Carbon sinks will be bigger when there is extra plant growth in soils with desirable physical properties.

The build up of soil organic matter by returning garden cuttings, compost and mulches to soils will add nutrients to the soil. The addition of organic materials to soils will improve soil physical properties. Structure is stable, improving soil aeration and water movement. Water holding capacity is increased, improving drought tolerance. Damage from erosion and the beating action of raindrops is decreased.

Cultivation and digging soils can reduce soil organic matter. Digging soils increases aeration and this can increase the decay of organic matter. Cultivation kills plants and this can reduce the build up of organic matter.

Hardwood trees and shrubs and especially gum trees form long lasting humus in soils.

Well managed garden soils will contain up to 10% organic matter. A small gum tree can store up to 1 tonne of carbon with ½ half above ground and ½ half in the soil. It is possible to store 4 tonnes of carbon in 100 square meters of garden containing several trees.

During a hot summers day trees will cool a backyard garden and strategically positioned plants will cool a house. Tall trees provide welcoming shade and growing plants transpire water from their leaves into the air producing a cooling effect.