Soil carbon: the basics
Soil organic carbon is a complex and varied mixture of materials and makes up a small but
vital part of all soils.
What is soil carbon ?
Soil carbon, or soil organic carbon (SOC) as it is more accurately known, is the carbon
stored within soil.
It is part of the soil organic matter (SOM), which includes other important elements such as
calcium, hydrogen, oxygen, and nitrogen.
Soil organic matter is made up of plant and animal materials in various stages of decay.
Un-decomposed materials on the surface of the soil, such as leaf litter, are not part of the
organic matter until they start to decompose.
Different types of soil carbon
However, although determining the amount of soil organic carbon in soil is important for
understanding soil health, knowing the type of organic carbon present is also important as
this can greatly impact soil productivity.
“We have established that the amount of each organic carbon fraction varies significantly
across soil types and some fractions can be altered by management practices”
Dr Jeff Baldock, CSIRO Land and Water
CSIRO scientist, Dr Jeff Baldock and his team have identified four biologically significanttypes or fractions of soil organic carbon:
The different types of soil organic carbon not only differ in size but are also composed of
different materials with different chemical and physical properties and different
Each fraction of soil carbon has different functions, most of these are due to the relative
stability and biological availability of each fraction:
Particulate organic carbon
Recalcitrant organic carbon
The amount of each type of organic carbon in Australian agricultural soils varies significantly.
In rainforests or good soils organic carbon can be >10 per cent, while in many poorer soils
or soils which are heavily exploited, levels are typically <1 per cent.
The proportion of some fractions can also vary due to management practices. This is
important as different fractions decompose at different rates and contain different quantities
of nutrients, which will have an impact on the health and productivity of the soil.
Balancing carbon inputs and outputs
The amount of organic carbon in soil is a balance between the build-up which comes from inputs of new plant and animal material and the constant losses where the carbon is decomposed and the constituents separate to mineral nutrients and gases, or are washed or leached away.
Carbon levels build up where water, nutrients and sunlight are plentiful.
Carbon is lost where: