15  Horizon colour

15.1 Colour recording

The colour of soil components is recorded using the Munsell Soil Colour system (Munsell Color Company 2009), which conceptualises colour using Hue, Value and Chroma parameters.

Table 15.1: Examples of Munsell Colour codes and their approximate matches in other systems.

Munsell	RGB	Hex
10YR5/4	148, 118, 80	#947650
7.5YR3/2	88, 69, 55	#584537
5R3/8	130, 41, 48	#822930
N4/0	97, 97, 97	#616161

For accurately describing colour:

• The colour charts must be used in clear direct or non-dappled indirect daylight. Avoid describing colours in the early morning, late afternoon, in dappled light or in deep shade.
• Record the moist colour of each soil sample to standardise the readings across profiles. Use a spray bottle to gently wet up a sample, if necessary, or wait for a saturated sample to dry out slightly. In both cases wait until the sample surface is no longer glistening.
• Remove sunglasses and turn to keep the main light source behind your shoulder.
• The sample may not perfectly match any one chip; record the closest match available. Do not interpolate between chips.
• Do not record the colour of a texture bolus or a smeared sample as these can be altered by e.g. mixing in mottles or breaking down ped surface features.

For sands, which may have multicoloured grains from multiple parent materials, colour matching can be difficult, but try to pick the colour of the dominant mineral component. Additional colour information can be stored as free-text notes.

Warning 15.1: Colour vision acuity

Operators need full colour vision to use the Munsell colour system correctly. While diagnosed colour blindness is relatively rare, many people have minor colour-recognition limitations without knowing it. Taking a colour vision test periodically is recommended.

Note 15.1: Obtaining soil colour from photographs

Obtaining soil colour information from photographs is possible, but the photographs must meet certain requirements. See (Roudier et al. 2016; Kirillova et al. 2021) for more information.

15.1.1 Colour short codes

In some circumstances (e.g. rapid assessment of redox mottles), a series of single letter codes can be used for colour.

Table 15.2: Short codes for major soil colours, adapted from (Soil and Terrain 2009, p. 161). NB: The Munsell colour ranges given are a general guide only, do not interpret them strictly.

Code	Name	Description
R	Red	2.5YR or redder, with value < 7 and chroma > 2
O	Orange	5YR or 7.5YR, with value < 7 and chroma > 2
B	Brown	7.5YR or 10YR, with value > 4 and chroma > 2
Y	Yellow	2.5Y or 5Y, with value > 4 and chroma > 2
G	Grey	Any non-gley hue, value 4--7, chroma 1
D	Dark	Any non-gley hue, values ≤ 3 and chromas ≤ 2
P	Pale	Any non-gley hue, values ≥ 7 and chromas ≤ 2
L	Gley	Gley charts

15.2 Matrix colour

The matrix colour of the soil is the dominant ‘background’ colour. This excludes the colours of redox-induced mottles (mostly oranges and gleys).

For routine assessment, record the single dominant matrix colour per horizon. For detailed assessment, up to three matrix colours may be recorded if necessary. It is assumed that multiple matrix colours are co-dominant, otherwise the subdominant one must logically be recorded as colour patterns (Section 15.3).

Dry soil matrix colours are sometimes necessary as well, e.g. for diagnosing a bleached E horizon. If moist and dry matrix colours are both obtained for a horizon, append them with their moisture status as D or M.

Example: 10YR52 M / 10YR71 D.

15.3 Colour patterns

For most assessments, only pattern type and abundance are required. Further detail is optional, but descriptive measures for pattern shape, size and distinctness are provided below.

15.3.1 Pattern type

Soil colour patterns are most commonly driven by redox processes affecting iron minerals, but can also be a product of parent material weathering, water movement, or mixing.

Table 15.3: Types of soil colour pattern

Code	Name	Description
M	Mottles	Colour patterns induced by redox processes
R	Reticulate	Colour patterns induced by preferential flow (e.g. gammate patterns in pallic soils)
W	Weathering	Colour patterns induced by parent material weathering, most commonly seen in BC horizons and below (e.g. rock ghost patterns)
X	Mixing	Colour patterns created by biological (e.g. worm tunnels) or mechanical (e.g. landslide) means

15.3.2 Pattern abundance

Estimate the areal percentage of the cut exposure face occupied by the colour pattern, versus the matrix colour. Use the visual guides in Section 22.2.3 to help estimate accurately. Abundance data can be classified according to Table 22.4 where needed.

For example, M 7.5YR58 25% for a bright orange redox mottle.

15.3.3 Pattern shape

Table 15.4: Types of soil colour pattern shape

Code	Name	Description
B	Blotches	Rounded shapes, blobs, where colour patterns penetrate the soil mass
S	Stripes	Linear colour patterns e.g. root linings
C	Coatings	Patterns caused by ped surface features e.g. cutans

15.3.4 Pattern size

Estimate the average size of each pattern in mm, e.g. 5 ± 2 mm. Pattern size may be classified against Section 22.1.1 if necessary.

15.3.5 Pattern contrast

If both matrix and pattern colours are recorded using the Munsell system, the following parameter can be calculated automatically. Complete rules for automating this process are found in Schoeneberger et al. (2012), section 2-15.

Table 15.5: Types of soil colour pattern contrast

Code	Name	Description
F	Faint	Indistinct colour variation evident only on close examination. The secondary colour is typically of the same hue as the primary colour and will generally differ by no more than one unit of chroma or two units of value.
D	Distinct	Although not striking, the colour variation is readily seen. The secondary and primary colours usually: Have the same hue. but differ by more than one and less than four units of chroma, or more than two and less than four units of value, or Differ by one hue (2.5 Munsell units) and not more than one unit of chroma or two units of value.
P	Prominent	The colour variation is conspicuous. The secondary and primary colours normally differ by At least two full hues (5 Munsell units) if chroma and value are the same, or At least four units of value or chroma if the hue is the same, or At least one unit of chroma or two units of value if there is a difference of one full hue (2.5 Munsell units).

15.3.6 Pattern boundary distinctness

Table 15.6: Types of soil colour pattern distinctness

Code	Name	Description
S	Sharp	Knife-edge boundaries between colours.
C	Clear	Colour transition < 2 mm wide
D	Diffuse	Colour transition > 2 mm wide

Kirillova NP, Zhang Y, Hartemink AE, Zhulidova DA, Artemyeva ZS, Khomyakov DM 2021. Calibration methods for measuring the color of moist soils with digital cameras. CATENA [Internet]. 202:105274. https://doi.org/10.1016/j.catena.2021.105274

Munsell Color Company 2009. Munsell soil colour charts. [Internet]. 2009 revised. Grand Rapids, MI, USA: Pantone. https://munsell.com/

Roudier P, Manderson A, Hedley C 2016. Advances towards quantitative assessments of soil profile properties. In: Hartemink AE, Minasny B, editors. Switzerland: Springer International Publishing; p. 113–132. https://doi.org/10.1007/978-3-319-28295-4_8

Schoeneberger PJ, Wysocki DA, Benham EC, Soil Survey Staff 2012. Field book for describing and sampling soils, version 3.0. [Internet]. Lincoln, NE, USA: Natural Resources Conservation Service, National Soil Survey Center. https://www.nrcs.usda.gov/sites/default/files/2022-09/field-book.pdf

Soil NC on, Terrain 2009. Australian soil and land survey field handbook. [Internet]. 3rd ed. Collingwood, Vic.: CSIRO Publishing. https://www.publish.csiro.au/book/5230/