Soil Colour

FRST 151

Soil Colour
After texture, soil colour is the most significant and
important physical feature. From colour we can conclude
a number of very important soil attributes including…
• How well the soil allows gases and water to flow
• The extent of chemical weathering that goes on
• The soil’s overall productivity

Soil Colour
Most soils have a fairly bland dark grey
colour as a parent material but changes over
time as a ƒ of:
Parent material type
% of organic matter content
The amount of eluviation and/or
illuviation that has occured
The extent of oxidation and/or reduction
that has taken place and the amount of
oxides accumulation in the soil
Red colours are due to the presence of Fe +++
or ferric compounds and indicate that the
soil is well aerated.
Yellows signify intermediate aeration.
Blue & grey colours indicate poor aeration
and are due to ferrous compounds (Fe ++ ).
Eluviated “A” horizon over a reddish
“B” horizon typical of a Humo-Ferric
Podzol

Soil Colour
Reddish colours indicate the presence of Ferous (Fe ++ )
compounds and indicate that the soil is generally well
aerated.
Blue – Grey colours indicate the presence of FERRIC
(Fe +++ ) compounds that are often associated with
reducing conditions (anaerobic bacteria) and poorly
aerated soils.
Yellowish soils signify an intermediate state of aeration.

An other way of looking at it…
Mottles are spots or blotches
that are different from the matrix
or background colour – they are
usually red to orange and result
from reducing conditions – poor
aeration usually due to flooding or
saturated conditions.
Loss of electrons is oxidation and gain of
electrons is reduction
“LEO says GER”
Fe +++ (ferric) Fe ++ (ferrous)

Soil Colour
To ensure that colour deffiniton is consistent, the
Munsell Colour Notation is used. This standardization
system was developed by …
Albert Munsell in the early 20 th century
and is used by all soil scientist as the basis of soil colour
classification.
Munsell broke down colour into 3 parameters…
Hue, Value, and Chroma.

Using the Munsell Colour
Notation standardizes
colour description.
Colour Coding
USDA
Handbook #18

Munsell Colour Chart

Hue, Value, Chroma
The system recognizes 5 dominant
colours with 5 additional “half way”
colours such as yellow-red. These are
referred to as the Hues.
The Value describes the brightness
of the colour from black (0) to white
(10) and 5 being neutral grey.
Chroma is a measure of “purity” or
strength of the colour on a scale from
0 (weak) to 12 (very strong) –
although the later only applies to
some colours.

On the scale, 10R is very strongly red; from there, the
colours degrade down to pure yellow with a code of 5Y
(10R ->2.5YR ->5YR ->10YR ->2.5Y -> 5Y)

Soil Colour & Moisture
Soil colour is strongly affected by moisture.
As such, it is important to specify colours as either
wet, moist, or dry.

Reading the Chart…
Each of the 5 Munsell colours are divided into 10 subdivisions
to allow for “merging or compound” colors.
The Munsell Colour Code: 7.5 YR 5/6 – Strong Brown. When
identifying a colour in the Munsell colour book, read the Hue
in the top right hand corner of the page followed by the value
on the inner vertical side of the page followed by the chroma
on the lower horizontal side.
Newer Munsell Books have a section specifically designated for
“Gleyed” soils.

Mottling
Mottling is a blotchy colour
condition in the soil that is
indicative of less than perfect
drainage but not as imperfectly or
poorly drained that the blue/grey
colours indicate.
Mottles tend to be reddish in
colour and stand out against the
“matrix” colour of the soil.
Mottles are defined in terms of
their - abundance
- Size
- colour
- colour contrast to the
matrix
SOIL MOISTURE CLASSIFICATION
Generally, for forestry purposes, soil moisture is merely described as
either dry, fresh, moist, or wet. However, at a more extended level,
soil moisture can be classified as...
RAPIDLY DRAINED: No mottling or gleying is found
throughout the profile or found on steeper slopes.
WELL DRAINED: Soils is generally free of mottling within
the upper meter to the soil surface.
MODERATELY WELL DRAINED: Soil is often faintly
mottled in the lower B and C horizons and the C horizon
can be gleyed.
IMPERFECTLY DRAINED: Soil is often distinctly mottled in
the B and C horizons but can also be gleyed.
POORLY DRAINED: Soil is usually strongly gleyed. Soil is
saturated for most of the year.
VERY POORLY DRAINED: Soil is strongly gleyed, free water
remains at or within 30 cm of the surface most of the year.
Soil drainage is a function of slope, slope position, texture, depth of
soil, water table, field capacity, and infiltration capacity.

Gleying
Gleying is the extreme outcome or
result of reduction by anaerobic
bacterium.
These bactirum do not use oxygen
but a process called electron
transfer respiration – they use the
electrons from Fe ++ (ferrous iron) for
energy and create Fe +++ (ferric iron)
in the process.
In this case the soil develops a blue
grey colour.
Soils that develop this colour are
usually saturated for prolonged periods
of time during the year.
Blue grey colours typical of the
Gleysolic order