Carbon in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 22

carbon compounds are used as food by animals and other soil biota. This carbon is
subsequently made available in the soil as humus, compost, and chemicals such as
humates, fulvates and carbonic acid. These chemicals in turn can react with
inorganic chemicals in the soil, making them available to plants for uptake
through their roots. As this carbon ends up in the food web, it is converted back
into gaseous carbon (as carbon dioxide and methane) and returned to the
atmosphere to begin the cycle again (Figure 9).

Soil tests for carbon
Estimates of organic carbon (OC) in the soil are used to express the amount of
organic matter present.
Levels are highest in the surface layers, and in Australian soil may vary from
almost zero to 15% or above. Due to the harsh Australian climate, stable organic
carbon levels in the soil are usually quite low at around 2%, with reasonable levels
at around 4%. Soil carbon levels can be increased reasonably easily, but will not
remain at elevated levels without regular inputs, and in time will revert back to the
sustainable base level.
Soil test reports usually show organic carbon as a percentage of total soil mass.

photosynthesis

animals

CO 2

CO 2

residues, wastes
and manures

soil surface

earthworms and
other macrofauna

CO 2

CO 2

residues

nutrient
release

soil reactions

microbial activity nutrient

release

carbonates

CO 2 and
humus

Figure 9 The carbon cycle.