Appendices - Soil Essentials: Managing Your Farm's Primary Asset - page 173

Appendix 27. Soil colour

The colour of a soil is an important descriptive characteristic of a soil. Colours also
provide an indication of soil drainage, degree of leaching and organic matter content.
Soil colour is generally determined by organic matter and the iron compounds
in the soil. Decayed organic matter gives the soil a dark colour. In freely draining
soil (oxidising environments), soil colour can be attributed to: humus (black); iron
(red); and silicates and salts (white) (Figure 27). The colours of poorly drained
soils that are in reducing environments (rather than oxidising) tend to be bluey-
green. Soil with little iron in them are pale grey in colour (Corbett 1969).

Summary of soil colour and its implications
Source: Queensland Department of Primary Industries (unpublished data).

Soil colour Interpretation

Topsoil
Dark brown surface

Indicates presence of organic matter (cultivation can disturb this feature
in cropping soils)

A2 horizon (pale
middle horizon
between topsoil and
subsoil)

The pale colour indicates the strong leaching of iron and manganese
compounds into the subsoil. This layer tends to be poorly structured and
becomes waterlogged in winter, sets hard in summer and is acidic.

Subsoil
Bright red or yellow Well-drained and aerated soils. The presence of iron oxides gives the soil

its red colour. The redness is free ferric oxide produced under oxidising
conditions, and found in wet warm climates and in regions with summer
drought. The redness tends to disappear from waterlogged soils.

Pale grey, olive or blue Poorly drained soils that lack oxygen

Mottled red, yellow
and/or grey

Mottling is common in north-east Victoria and in high rainfall zones. The
mottles result from seasonal waterlogging. During anaerobic conditions
the iron is reduced to a ferrous state (Fe 3+ Fe 2+ ). This results in a grey/
blue/green pigment 'gleying'. This reduced iron can be leached out of a
soil, leaving bleached areas with no pigment at all. Under aerobic
conditions, the red and yellow iron minerals develop, converting ferrous
(Fe 2+ ) to ferric cations (Fe 3+ ). The redder and yellower areas indicate the
drier zones where more oxygen is available.

Dark brown and black The dark brown and black soils are rich in humus, but the colour of the

soil does not necessarily indicate fertility, although the lighter colour soils
do lack organic matter.

The colour of a soil is described using the Munsell Colour Chart. This provides
a standard system for categorising soils based on colour. The colour test is done on
a freshly broken surface of moist soil. All samples are wetted up so as to maintain a
uniform moisture content as the moisture content of a soil can vary from dry to
moist at any given time. This ensures a consistent sampling approach.