Soils and Irrigation Management
4. Soil Colour (SO4 - 0801)
Colour can be a useful indicator of some of the general properties of a soil, as well as some of the chemical processes that are occurring beneath the surface.
Soil colour is usually determined by:
- the amount and state of organic matter
- the amount and state of iron oxide
- soil aeration
Two 'rules of thumb' are that the: the darker the soil is, the more organic matter it contains; and a dry soil that leaves your hands 'dirty and dusty' has a high organic matter content.
Black and brown soils:
These soils are often associated with high levels of organic matter, although minerals that are present can also give a dark colouring.
Red and brown soils:
These soil colours indicate good drainage. Iron found within the soil is oxidised more readily due to the higher oxygen content. This causes the soil to develop a 'rusty' colour. The colour can be darker due to organic matter in the soil.
Yellow and yellow/ brown soils:
These soils often have poorer drainage than red soils. The iron compounds in these soils are in a hydrated form and therefore do not produce the 'rusty' colour.
Grey and blue grey soils:
These colours are associated with soils that have very poor drainage or suffer from waterlogged conditions. Iron and manganese compounds are in their reduced form due to the lack of air.
Light grey soils:
These soils are often referred to as bleached or 'washed out'. The iron and manganese particles have been leached out due to high amounts of rainfall or vertical and lateral drainage.
(Source: Manual of Canegrowing, BSES, 2000; Chapter 6, pg 113-114)
| Soil Property | Black | Light Grey | Red | Brown | Yellow | Grey/Blue Grey |
| Drainage | Often slow | Well drained | Well drained | Well to moderate | Less well drained | Poorly drained |
| Water-logging potential | Medium | Low | Low | Low | Low to medium | High |
| Organic Matter Accumulation | High | Low | Medium | Medium to high | Medium to low | Low |
| Leaching of nutrients | Low | High | Medium | Medium | Medium | Low |
| Nitrogen loss (denitrification) | Medium | Low | Low | Low | Low to medium | High |
The Rural Water Use Efficiency Initiative is a joint venture between BSES, CANEGROWERS and the Queensland Government through the Department of Natural Resources and Mines. For more information, please ring your local Rural Water Use Efficiency Officer.