According to wikipedia, the colour of soli is determined by proteins present in the soil!

Soil color is influenced by the amount of proteins present in the soil. Yellow or red soil indicates the presence of iron oxides. Dark brown or black color in soil indicates that the soil has a high organic matter content. Wet soil will appear darker than dry soil.However, the presence of water also affects soil color by affecting the oxidation rate. Soil that has a high water content will have less air in the soil, specifically less oxygen. In well drained (and therefore oxygen rich soils) red and brown colors caused by oxidation are more common, as opposed to in wet (low oxygen) soils where the soil usually appears grey.

The presence of specific minerals can also affect soil color. Manganese oxide causes a black color, glauconite makes the soil green, and calcite can make soil in arid regions appear white.

It sounds a bit strange to me. Pigments proteins do impart colour to the substances they are in (e.g. Haemoglobin in blood) but I wonder if there are any such proteins that could impart the variable colours to soil. Besides the following sentences in the mentioned paragraph do not mention of any protein.

It is possible that the word protein is there by mistake?


P.S. The book from where the sentence is quoted is unfortunately not available online.

  • $\begingroup$ It doesn't say proteins is a major factor just that it is one of many factors. My guess is that proteins are a major source of the color of organics present in soil, which makes sense carbohydrates and oils don't have very strong natural colors. So the bulk of the organic colors may be proteins and organics are a major factor in soil color. If wiki had a source for the claim I would give it more credence. $\endgroup$ – John Jan 15 '17 at 16:38
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    $\begingroup$ Fixed it, although the wiki page could still use work. $\endgroup$ – haresfur Jan 15 '17 at 22:54
  • $\begingroup$ @haresfur 'work'? I saw you changed it to organic matter. But isn't the colour mostly depending on the oxidation state of the elements? $\endgroup$ – Tyto alba Jan 16 '17 at 5:46
  • $\begingroup$ It depends on the soil. High organic matter soil like Organisols (Australian classification) or Histosols and Mollisols in the US classification have colour dominated by the organic matter. $\endgroup$ – haresfur Jan 16 '17 at 21:58

Soil is a collection of minerals and the color of soil is largely determined by the composition of its minerals.

Protein is derived from biological sources. The only way proteins could affect the color of soil is if the soil also contained large quantities of biological material.

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  • $\begingroup$ I also think the same 'the color of soil is largely determined by the composition of its minerals.'. I was wondering if wikipedia is wrong by mentioning that it's protein that determines the colour. $\endgroup$ – Tyto alba Jan 15 '17 at 13:28
  • $\begingroup$ soil can contain a large quantity ofr biological material but in those cases the color is dominated by humus. $\endgroup$ – haresfur Jan 15 '17 at 22:55

"Proteins" isn't even consistent with the rest of the sentence in that quotation! It isn't principally protein.

I think that paragraph is a paraphrase of the relevant paragraph from Elements of the Nature and Properties of Soils, a standard introductory textbook, §4.1:

Most soil colors are derived from the colors of iron oxides and organic matter that coat the surfaces of soil particles. Organic coatings tend to darken and mask the colors derived from iron oxides.

Because of the color changes that take place when various iron-containing minerals undergo oxidation and reduction, soil colors can provide extremely valuable insights into the hydrologic regime or drainage status of a soil. Bright (high-chroma) colors throughout the profile are typical of well-drained soils through which water easily passes and in which oxygen is generally plentiful. Prolonged anaerobic conditions can cause iron oxide coatings to become chemically reduced, changing high-chroma (red or brown) colors to low-chroma (gray, bluish or gray-green) colors, a condition referred to as gley.

There are unusual cases with no (or only) organic matter, or with so little iron that iron oxides don't give color, in which case we see the underlying minerals, which can also have varying redox states.

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