Australian Natural Resources Atlas

Natural Resource Topics

Australian Agriculture Assessment 2001

Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001

Appendix 2. Australian Soil Resources Information System

Percent organic carbon (topsoil and subsoil)

Organic carbon estimates the amount of organic matter in a soil as a percentage by weight. Soil organic matter content is an indication of natural soil fertility , and is a balance between input of surface litter (fallen leaves and dead organisms) and the rate at which microbes break down organic compounds.

Carbon is essential for plant growth. Organic matter is also important since it binds soil particles together into stable aggregates. It is also involved in adsorption of cations. Cations such as calcium, magnesium and sodium are important in plant nutrition.

How does it vary and what is it related to?

Soil organic carbon varies with depth. Levels are highest in the topsoil and generally decrease exponentially with depth. Organic carbon commonly ranges between 0% and 15%. Most Australian soils contain less than 5%.

Distribution of organic carbon across Australia?

Organic carbon does not appear to closely reflect soil types. Rather it varies as a function of climate and land use. It is highest in forested and cultivated areas, and generally follows continental rainfall and temperature patterns. Organic carbon is highest in the high rainfall, temperate regions of Tasmania, Victoria and Western Australia, along the coast of New South Wales and in the wet tropics of Queensland; and lowest in arid and semi-arid inland regions. The Australian Soil Resources Information System map estimates for organic carbon appear anomalously high in portions of the Northern Territory.

Table A10 Organic carbon by land use categories for topsoil (weight %C) by percent of land use type.
< 0.3 0.3 - 0.5 0.5 - 1.0 1.0 - 2.0 2.0 - 5.0 > 5.0 Total land use
class area (ha)
Conservation and natural environments 0 3 29 34 26 8 54 814 100
Production from native environments 0 7 42 33 15 2 184 376 100
Cropping 0 1 31 55 12 1 22 241 000
Grazing modified pasture 0 1 16 44 37 2 18 482 400
Horticulture 0 1 24 31 39 5 351 500
Irrigated cropping 0 0 23 68 8 0 948 700
Irrigated modified pasture 0 0 12 67 19 2 1 080 000
Total area* 282 293 800
Table A11 Organic carbon by land use categories for subsoil (weight %C) by percent of land use type.
< 0.3 0.3 - 0.5 0.5 - 1.0 1.0 - 2.0 2.0 - 5.0 > 5.0 Total land use
class area (ha)
Conservation and natural environments 22 34 29 12 3 1 54 814 100
Production from native environments 20 43 29 7 0 0 184 376 100
Cropping 24 53 17 5 0 0 22 241 000
Grazing modified pasture 17 41 32 9 1 0 18 482 400
Horticulture 5 46 35 14 1 0 351 500
Irrigated cropping 20 56 17 8 0 0 948 700
Irrigated modified pasture 2 59 34 4 1 0 1 080 000
Total area* 282 293 800

* Area of river basins containing intensive agriculture

Distribution of organic carbon (%) in the topsoil within the river basins containing intensive agriculture. Distribution of organic carbon (%) in the subsoil within the river basins containing intensive agriculture.

Soil organic matter content is an indication of natural soil fertility. The carbon:nitrogen ratio is an especially useful indicator of the source of organic matter, its state of decomposition and its potential contribution to soil fertility. Very high carbon:nitrogen ratios (> 25) indicate that organic matter accumulation is occurring faster than decomposition. These high ratios are observed in peats and forest litters. Carbon:nitrogen ratios between 12 and 16 suggest that organic matter is well broken down. Cultivated soils usually have a carbon:nitrogen ratio between 10 and 12. Carbon:nitrogen ratios below 10 usually occur only in the subsoil.

Soil organic carbon is also important in determining soil erodibility and maps of organic carbon can be used in estimating erodibility by combining with maps of soil texture and permeability (see soil erodibility - Figure A25).

Level of uncertainty

Models for organic carbon are reasonably robust.

Error diagnostics

Error diagnostic Topsoil Subsoil
Number of points used 11483 5100
R2 0.489 0.370
Relative error 0.65 0.77

Point distribution and other error diagnostics indicate that the topsoil model is considered to be good for South Australia, Western Australia, Murray-Darling Basin and central Queensland; but poor for the Northern Territory, Carpentaria and North Queensland, New South Wales (outside the Murray-Darling Basin), and Victoria. Similarly, the subsoil model is strongest for Western Australia, the Murray-Darling Basin, south and central Queensland, and Tasmania; and weakest for Northern Territory, western Victoria and northern New South Wales.

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