5. COLOUR AND COLOUR MOTTLING

5.0 Colour of soil horizons

How do you define the colour of soil horizons?

The colour of individual horizons should be defined when the soil is moist , as soon as visible moisture films disappear from a moistened sample. Specialists usually refer to the Munsell Soil Colour Charts to define the colour of horizons. For the purpose of this manual, a simpler system is used, including the following shades of colour:

Shades of colour
BLACK
  
GREY
  • Dark
  • Light
BROWN
  • Dark
  • Reddish
  • Light
YELLOW
  • Reddish
  • Yellow
RED
  • Yellowish
  • Red
  • Dark
 

What may colour indicate?

In the surface soil such as the A-horizon, darker shades usually indicate a higher content of organic matter than lighter shades. This, however, is not always true as, in high rainfall areas, dark soil colouring can be caused by poor drainage.

In deeper horizons such as the B-horizon, a brown colour usually means that the soil has good natural drainage. A black or dark grey colour usually comes from an accumulation of organic matter. In areas of high rainfall, this may again mean poor drainage.

A typical colour description for a soil profile
.
Horizon
Colour
A1
0-10 cm
Dark brown
A2
10-25 cm
Dark reddish brown
B1
25-70 cm
Dark reddish brown
B2
70-100 cm
Dark red
C
100-200 cm
Yellowish red

5.1 Colour mottling in soil horizons

What is colour mottling and how do you describe it?

The presence of spots or streaks of a particular colour, usually reddish, yellowish or white, which may exist in certain soil horizons is referred to as colour mottling.

  • If there is a definite background colour, colour mottling is described by determining and noting the amount and colour of the mottles*, such as, for example, reddish yellow horizon with common yellowish red and white mottles;
  • If the amount of mottles is so great that it is not possible to see a single background colour, define and note the various colours you see and add the word mottled, such as, for example, red, brown, mottled.

What does colour mottling indicate?

Colour mottling is strong whenever a soil is subject to waterlogging, even if the soil is highly permeable. However, colour mottling may also occur for other reasons.

If mottling occurs in a bright-coloured B-horizon, it is not related to a drainage problem.

If mottling occurs in any horizon with any dull colour, usually grey, it is a sign of drainage problems for a major part of the year, regardless of the absence or presence of water.

Abundant pale yellow mottles coupled with a low pH characterize actual acid sulphate soils.

Various colours of soil and mottling as they relate to drainage conditions are given below.

Soil colour/mottling
Drainage conditions
Warm colours, browns, reds and oranges Good drainage
Pale yellowish, pale and dark greys with rusty orange and/or grey mottling within the waterlogged horizon; grey colours are common as well Drainage seasonally poor
Water-table at 25- to 120-cm depth
Pale, dark and bluish greys, or pale brownish yellows with rusty orange, brown or grey mottling within the topsoil Seasonally swampy soil
Water-table at less than 25-cm depth

Colour combined with colour mottling may also be used as one of the visual indicators of soil permeability
(see Table 17B).