Soil color

Last updated September 11, 2024

Soil color is often the most visually apparent property of soil. While color itself does not influence the behavior or practical use of soils,^[1] it does indicate important information about the soil organic matter content, mineralogy, moisture, and drainage.^[2]

The development and distribution of color in soil results from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colorful compounds. Soil conditions produce uniform or gradual color changes, while reducing environments result in disrupted color flow with complex, mottled patterns and points of color concentration. Sometimes, a distinct change in color within a soil profile indicates a change in the soil parent material or mineral origin.^[3]

Causes

Dark brown or black

Dark brown or black colors typically indicate that the soil has a high organic matter content.^[4] Organic matter coats mineral soil particles, which masks or darkens the natural mineral colors.^[1]

Sodium content also influences the depth of organic matter and therefore the soil color. Sodium causes organic matter particles such as humus to disperse more readily and reach more minerals.^[5] Additionally, soils which accumulate charcoal exhibit a black color.^[6]^[7]

Red

Red colors often indicate iron accumulation or oxidation in oxygen-rich, well-aerated soils.^[4] Iron concentrations caused by redox reactions because of diffusion of iron in crystalline and metermorphic rock,in periodically saturated soils may also present red colors, particularly along root channels or pores.^[8]

Gray or blue

Soil in anaerobic, saturated environments may appear gray or blue in color due to the redox reduction and/or depletion of iron. In an anaerobic soils, microbes reduce iron from the ferric (Fe³⁺) to the ferrous (Fe²⁺) form. Manganese may also be reduced from the manganic (Mn⁴⁺) to the manganous (Mn²⁺) form, though iron reduction is more common in soil.^[8] The reduced iron compounds cause poorly drained soil to appear gray or blue, and because reduced iron is soluble in water, it may be removed from the soil during prolonged saturation. This often exposes the light gray colors of bare silicate minerals, and soils with a low chroma from iron reduction or depletion are said to be gleyed.^[1]

Green

Iron reduction may impart greenish gray colors, though certain minerals including glauconite, melanterite, and celadonite can also give soil a green color. Glauconite soils form from select marine sedimentary rocks, while melanterite soils are produced in acidic, pyrite-rich soils.^[9]^[10] Celadonite in hydrothermally-altered basalt within the Mojave Desert has been observed to weather into a green colored smectite-rich clay soil.^[11]^[12]

Yellow

Yellow soils may indicate iron accumulation as well, though in less oxygen-rich environments than red soils.^[4] Jarosite accumulation can also create yellow soil color and may be found in salt marshes, sulfide ore deposits, acid mine tailings, and other acidic soils.^[13]^[14]

White

White colors are common in soils with salt, carbonate, or calcite accumulations, which often occur in arid environments.^[3]^[15]

Multiple soil colors in a marsh soil in South Australia

Description

Most soil survey organizations utilize the Munsell color system to decrease the subjectivity in evaluating color.^[13] This system was developed by Albert Munsell, a painter, in the early 20th century to describe the full-color spectrum, though the specially adapted Munsell soil color books commonly used by soil scientists only include the most relevant colors for soil.^[16]

The Munsell color system includes the following three components:^[1]

Hue: indicates the dominant spectral (i.e., rainbow) color, which in the soil is generally yellow or red. Each page of the Munsell soil color book displays a different hue. Examples include 10YR, 5YR, and 2.5Y.
Value: indicates lightness or darkness. The value increases from the bottom of each page to the top, with lower numbers representing darker colors. Color with a value of 0 would be black.
Chroma: indicates intensity or brightness. Chroma increases from left to right on each page, with higher numbers representing more vivid or saturated colors. Color with a chroma of 0 would be neutral gray.

A general color name, such as yellowish brown or light gray, often accompanies the Munsell notation for soil samples. These qualitative descriptors correspond to one or more color chips in the Munsell soil color books; however, they are not formally part of the broader Munsell color system.^[13]

Because soil color (specifically the value) varies with moisture, it may be described at both its moist and dry state. Soil is considered moist when adding water no longer changes the soil color or as "dry" when the soil is air dry.^[17]

Related Research Articles

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al₂Si₂O₅(OH)₄) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

A pigment is a powder used to add color or change visual appearance. Pigments are completely or nearly insoluble and chemically unreactive in water or another medium; in contrast, dyes are colored substances which are soluble or go into solution at some stage in their use. Dyes are often organic compounds whereas pigments are often inorganic. Pigments of prehistoric and historic value include ochre, charcoal, and lapis lazuli.

<span class="mw-page-title-main">Munsell color system</span> Color space

In colorimetry, the Munsell color system is a color space that specifies colors based on three properties of color: hue, value (lightness), and chroma. It was created by Albert H. Munsell in the first decade of the 20th century and adopted by the United States Department of Agriculture (USDA) as the official color system for soil research in the 1930s.

<span class="mw-page-title-main">Iron(II) sulfate</span> Chemical compound

Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula FeSO₄·xH₂O. These compounds exist most commonly as the heptahydrate (x = 7) but several values for x are known. The hydrated form is used medically to treat or prevent iron deficiency, and also for industrial applications. Known since ancient times as copperas and as green vitriol (vitriol is an archaic name for hydrated sulfate minerals), the blue-green heptahydrate (hydrate with 7 molecules of water) is the most common form of this material. All the iron(II) sulfates dissolve in water to give the same aquo complex [Fe(H₂O)₆]²⁺, which has octahedral molecular geometry and is paramagnetic. The name copperas dates from times when the copper(II) sulfate was known as blue copperas, and perhaps in analogy, iron(II) and zinc sulfate were known respectively as green and white copperas.

A sapric is a subtype of a histosol where virtually all of the organic material has undergone sufficient decomposition to prevent the identification of plant parts. Muck is a sapric soil that is naturally waterlogged or is artificially drained.

Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe₃(SO₄)₂(OH)₆. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct during the purification and refining of zinc and is also commonly associated with acid mine drainage and acid sulfate soil environments.

Glauconite is an iron potassium phyllosilicate mineral of characteristic green color which is very friable and has very low weathering resistance.

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al₂Si₂O₅(OH)₄), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

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<span class="mw-page-title-main">Gleysol</span> Saturated soil type

A gleysol or gley soil is a hydric soil that unless drained is saturated with groundwater for long enough to develop a characteristic gleyic colour pattern. The pattern is essentially made up of reddish, brownish, or yellowish colours at surfaces of soil particles and/or in the upper soil horizons mixed with greyish/blueish colours inside the peds and/or deeper in the soil. Gleysols are also known as Gleyzems, meadow soils, Aqu-suborders of Entisols, Inceptisols and Mollisols, or as groundwater soils and hydro-morphic soils.

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Acid sulfate soils are naturally occurring soils, sediments or organic substrates that are formed under waterlogged conditions. These soils contain iron sulfide minerals and/or their oxidation products. In an undisturbed state below the water table, acid sulfate soils are benign. However, if the soils are drained, excavated or otherwise exposed to air, the sulfides react with oxygen to form sulfuric acid.

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References

1 2 3 4 5 Brady, Nyle C.; Weil, Ray R. (2010). Elements of the nature and properties of soils (3rd ed.). Upper Saddle River, N.J. ISBN 978-0-13-501433-2. OCLC 276340542.{{cite book}}: CS1 maint: location missing publisher (link)
↑ Owens, P. R.; Rutledge, E. M. (2005). Encyclopedia of soils in the environment. Daniel Hillel, Jerry L. Hatfield (1st ed.). Oxford, UK: Elsevier/Academic Press. ISBN 0-12-348530-4. OCLC 52486575.
1 2 Gardiner, Duane T.; Miller, Raymond W. (2008). Soils in our environment (11th ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. ISBN 978-0-13-219104-3. OCLC 85018836.
1 2 3 Singer, Michael J.; Munns, Donald N. (2006). Soils : an introduction (6th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. ISBN 0-13-119019-9. OCLC 57506906.
↑ "Interpreting Soil Colour". Victorian Resources Online. Retrieved 15 January 2017.
↑ Krug, Edward C.; Hollinger, Steven E. (2003). "Identification of Factors that Aid Carbon Sequestration in Illinois Agricultural Systems" (PDF). Champaign, Illinois: Illinois State Water Survey. p. 10. Archived from the original (PDF) on 2017-08-09. Retrieved 2019-01-06. While humus (especially in organomineral form) helps give soils a black color (Duchaufour, 1978), the literature shows correlation between forest and grassland soil color to BC - the blacker the soil the higher its BC content (Schmidt and Noack, 2000)
↑ Gonzalez-Perez, Jose A.; Gonzalez-Vila, Francisco J.; Almendros, Gonzalo; Knicker, Heike (2004). "The effect of fire on soil organic matter-a review" (PDF). Environment International. 30 (6). Elsevier: 855–870. doi:10.1016/j.envint.2004.02.003. PMID 15120204 . Retrieved 2019-01-04. As a whole, BC represents between 1 and 6% of the total soil organic carbon. It can reach 35% like in Terra Preta Oxisols (Brazilian Amazonia) (Glaser et al., 1998, 2000) up to 45 % in some chernozemic soils from Germany (Schmidt et al., 1999) and up to 60% in a black Chernozem from Canada (Saskatchewan) (Ponomarenko and Anderson, 1999)
1 2 United States Department of Agriculture, Natural Resources Conservation Service. 2018. Field Indicators of Hydric Soils in the United States, Version 8.2. L.M. Vasilas, G.W. Hurt, and J.F. Berkowitz (eds.). USDA, NRCS, in cooperation with the National Technical Committee for Hydric Soils. https://www.nrcs.usda.gov/sites/default/files/2022-09/Field_Indicators_of_Hydric_Soils.pdf
↑ Wurman, Eliahu (1960). "A mineralogical study of a gray-brown podzolic soil in Wisconsin derived from glauconitic sandstone". Soil Science. 89 (1): 38–44. Bibcode:1960SoilS..89...38W. doi:10.1097/00010694-196001000-00007. ISSN 0038-075X. S2CID 128889991.
↑ Frau, F. (December 2000). "The formation-dissolution-precipitation cycle of melanterite at the abandoned pyrite mine of Genna Luas in Sardinia, Italy: environmental implications". Mineralogical Magazine. 64 (6): 995–1006. Bibcode:2000MinM...64..995F. doi:10.1180/002646100550001. ISSN 0026-461X. S2CID 129237832.
↑ Reid, D. A.; Graham, R. C.; Edinger, S. B.; Bowen, L. H.; Ervin, J. O. (1988-10-01). "Celadonite and its Transformation to Smectite in an Entisol at Red Rock Canyon, Kern County, California". Clays and Clay Minerals. 36 (5): 425–431. Bibcode:1988CCM....36..425R. doi: 10.1346/CCMN.1988.0360507 . ISSN 1552-8367. S2CID 55931277.
↑ Velde, B. (2003-01-01), Holland, Heinrich D.; Turekian, Karl K. (eds.), "7.12 - Green Clay Minerals", Treatise on Geochemistry, 7, Oxford: Pergamon: 309–324, Bibcode:2003TrGeo...7..309V, doi:10.1016/b0-08-043751-6/07090-0, ISBN 978-0-08-043751-4 , retrieved 2023-05-05
1 2 3 Soil Science Division Staff. 2017. Soil survey manual. C. Ditzler, K. Scheffe, and H.C. Monger (eds.). USDA Handbook 18. Government Printing Office, Washington, D.C., USA. https://www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Survey-Manual.pdf
↑ Cogram, Peter (2018-01-01), "Jarosite", Reference Module in Earth Systems and Environmental Sciences, Elsevier, doi:10.1016/b978-0-12-409548-9.10960-1, ISBN 978-0-12-409548-9 , retrieved 2023-04-18
↑ Brady, Nyle C. & Ray R. Weil Elements of the Nature and Properties of Soils, page 95. Prentice Hall, 2006.
↑ Owens, P. R.; Rutledge, E. M. (2005). Encyclopedia of soils in the environment. Daniel Hillel, Jerry L. Hatfield (1st ed.). Oxford, UK: Elsevier/Academic Press. p. 514. ISBN 0-12-348530-4. OCLC 52486575.
↑ Buol, S. W. (2011). Soil genesis and classification. R. J. Southard, R. C. Graham, P. A. McDaniel (6th ed.). Chichester, West Sussex. ISBN 978-0-470-96062-2. OCLC 747546196.{{cite book}}: CS1 maint: location missing publisher (link)