Colour is an obvious feature of soils that has always been used for describing and identifying soil. Soil Colour is determined by four main factors:
- the quantity and type of organic material present
- the nature and abundance of iron oxides in the soil profile
- inherited characteristics from the original parent material of the soil eg. the bright red basaltic soils of Australia derived from basalt
- the water content of the soil, which can markedly alter the colour.
The soil colour factors listed above, along with processes of soil formation, result in an enormous range of colour patterns in both Australian and New Zealand soils (as shown in the previous photo). If there are mottles present in a soil, then the colour, size and contrast of those mottles are very good indicators of a range of soil processes, including the degree of biological activity going on in the soil, the chemical reactions happening and the seasonal patterns of water logging.
These mottles can be easily be seen in the photo on the right. They appear as brown rust-like blotches.
Soil colour can be determined either in the laboratory or more practicably in the field using fresh, unsheared pieces of soil and standard colour charts.
The Munsell Colour System is the most commonly used and is similar to the colour description system used in most computer graphics packages. The System uses three descriptors of elements of colour- hue, value and chroma.
The diagram below shows the three axes of colour (hue, value and chroma) used in the Munsell Colour System.
The Hue refers to the dominant wavelength (or shade) of the colour and is usually denoted with an alphanumeric code. For example the range of hues between strong red through to yellow would be represented by the codes 10R, 2.5YR, 5YR, 7.5YR, 10YR.
The value represents the tone of the colour - the graduation from black to white is scored from 1 to 9.
The Chroma is the degree of colour saturation or difference from the tone. Thus a chroma of 0 has no saturation whereas 9 is intensely saturated.
The Munsell System also provides descriptive names for the various codes. For example a code of 10YR 6/6 refers to a brownish-yellow soil.
Exercise in using the Munsell Colour Chart
At first glance, the Munsell Colour System seems quite complicated. However, in the field, the system is very easy to implement. The photo at right can be used as an exercise for you to test your understanding of the system.
This photo contains a copy of two pages from the Munsell Chart and a photo of a soil profile.
See whether you can correctly work out the Munsell colour description of the soil shown inside the red square in the photo. The Answer is below in this article.
Have you worked it out yet?
Its really quite simple. Firstly identify the colour on the chart that is closest to the colour of the soil inside the red square.
Then work out which page the colour is on (its on the 5PB hue page), then decide which value row the chosen colour is on (Row 9/ in this case) and which chroma column the colour is in (in this case its in the /0 column).
Thus the correct colour description of this soil is 5PB 9/0.
Did you get this right? Well done!! Its very easy to use the Munsell Colour Chart after a little practice.
Once you understand the principles, you will be able to understand the lingo in the pedologists report when they describe the colour of your soil in technical terms and have an appreciation of the value of this information.
The colours found in a soil give an indication about the likely drainage condition of the soil.
A Horizon (Topsoil) Colours - dark brown and black coloured soils have the greatest amount of organic matter in them and dark brown soils usually have good drainage.
B Horizon (Subsoil) Colours - These are usually lighter brown in colour due to the absence of soil organic matter but this tends to depend on the soil parent material.
Red and orange flecks (mottles) in a soil indicate that a soil is sometimes wet. Mottles are often described separately to the soil layer and are described by mottle size, density and dominant colour (which often gives clues to the chemistry of the mottle materials).
Grey colours in soils indicate that a soil has long periods of being wet. This process is called gleying and is common in wet spots in both New Zealand and Australian soils.
