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

Soil depth (topsoil, subsoil, total solum thickness)

What is it?

Solum depth refers to total depth of soil (A and B horizons). It does not include the unconsolidated or partially weathered material which underlie the soil, where soil forming processes are not obvious (carbon horizons). It is often difficult to determine the lower limit of soil, and for many purposes depth of soil is considered to be the rooting depth of plants.

Topsoils (A horizons) are defined as the surface soil layers in which organic matter accumulates, and may include dominantly organic surface layers (O and P horizons).

A horizons are usually darker than underlying layers but they may also be horizons that are lighter coloured or have a lower content of clay when compared to underlying horizons.

Subsoils (B Horizons) contain less organic matter than topsoils, and may often have a zone of accumulation of clays, carbonates or iron and aluminium oxides. The structure, colour or composition is significantly different to the overlying layer.

Some soils do not have a B horizon (e.g. young soils developing on alluvium). In this case, solum depth is the depth of the A horizon.

The depth of soil horizons is measured in metres.

Why is it important?

Soil depth defines the zone available for growth of plant roots and determines the size of the soil water store. Available water capacity is a function of the depth of soil.

The depth of soil required varies for different crops, but in general shallow soils are less suitable for agriculture. Deep soils provide a much larger store of water.

The depth of topsoil is important because, with their higher organic matter content, topsoils generally have more suitable properties for agriculture, including higher permeability and higher levels of soil nutrients.

How does it vary and what is it related to?

Soil depth depends on:

In wet, humid areas, weathering of rocks to form soils is rapid, and soils tend to be deep. In arid zones, weathering proceeds very slowly, and soils are usually shallow.

Soil depth is usually strongly related to topography—soils on hillslopes (zones of erosion) tend to be shallow, those in valleys or depressions are deeper (zones of deposition). Wind, hillslope, gully and streambank erosion provide the source of sediments to these alluvial valleys. Deep soils are also associated with the volcanic landscapes, for example, the Ferrosols of the dairying region of the Atherton Tablelands and potato growing areas of northern Tasmania—these areas are well known for their deep red soils.

Soil depth classes used in the Australian Soil Classification are:

Class Soil depth (m)
Very shallow < 0.25
Shallow 0.25 - < 0.5
Moderate 0.5 - < 1.0
Deep 1.0 - < 1.5
Very deep 1.5 - 5
Giant > 5

Depth of the topsoil (A horizon) is determined by the relative rates of accumulation and decomposition of organic matter. It is also related to the activity of soil fauna—earthworm and termite activity results in mixing of organic material to greater depths.

Topsoil (`A horizon') depth classes used in the Australian Soil Classification are:

Class Soil depth (m)
Thin < 0.1
Medium 0.1 - < 0.3
Thick 0.3 - < 0.6
Very thick > 0.6

Subsoil depth is related to permeability, since this determine how easily water can penetrate and hence the depth of weathering and deposition.

Table A2 Percentage of land use categories at specified solum depths (m) across Australia.
Very shallow
< 0.25		 Shallow
0.25 - < 0.5		 Moderate
0.5 - < 1.0		 Deep
1.0 - < 1.5		 Very deep
1.5 - 5		 Total land
use class
area
(ha)
Conservation and natural environments 12 18 51 18 0 263 894 700
Production from native environments 3 14 47 35 1 443 032 600
Cropping 0 3 37 58 2 22 519 100
Grazing modified pasture 0 3 55 40 1 19 237 900
Horticulture 1 3 42 53 2 350 900
Irrigated cropping 0 1 24 70 5 949 000
Irrigated modified pasture 0 2 22 76 1 1 079 100
Total area 751 063 300
Table A3 Percentage of land use categories at specified topsoil thicknesses (m) across Australia.


 Thin
< 0.1 m		 Medium
0.1 - < 0.3 m		 Thick
0.3 - < 0.6 m		 Very thick
> 0.6 m		 Total land use class area
(ha)
Conservation and natural environments 4 76 19 0 263 894 700
Production from native environments 9 75 15 0 443 031 100
Cropping 2 70 26 2 22 519 000
Grazing modified pasture 1 65 33 2 19 237 900
Horticulture 1 68 29 2 351 000
Irrigated cropping 4 89 7 0 949 000
Irrigated modified pasture 0 90 9 1 1 079 100
Total area 751 061 800
Distribution of solum depth (m) across Australia. Distribution of depth of topsoils (m) across Australia. Distribution of depth of first subsoil (m) across Australia.

How can these maps be applied?

Estimates of soil thickness (Figure A4) are required to make calculations of soil volumes, for example, to assess total stores of soil carbon for greenhouse inventory or to assess total stores of nutrients.

Estimates of soil depths (figures A5, A6) are needed to calculate the amount of any soil constituent in either volume or mass terms (bulk density is also needed) (e.g. the volume of water stored in the rooting zone potentially available for plant use).

What is the level of uncertainty?

The scale of the various soil maps used in deriving this map is shown in Figure A2.

Several sources of error are possible when estimating soil depth and thickness of horizons for the lookup tables:

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