Where I live the soil is red.

Is there a map or chart where you can see the average color of the dirt according to geographical location? What would the color be if all of the dirt on Earth was added equally to a pallet?

I understand that composition of minerals determines dirt color but what makes dirt its color is not the question I am asking.

Kata Tjuta, Northern Territory, Australia

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Sagada, Mountain Province, Philippines

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Gentry County, Missouri, United States

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    $\begingroup$ You know the second photo is just exposed rock, right? $\endgroup$ – lly Jun 26 '18 at 1:16
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    $\begingroup$ 'I understand that composition of minerals determines dirt color but what makes dirt its color is not the question I am asking'... Uh, yes, it is. Color varies according to composition, so the composition determines color. @Wanderweeer even gave you a map and you complained that it didn't have enough hues; he's giving you average values at various depths because that's what you have to do since soil color can vary over the span of a few feet... depending on its composition. $\endgroup$ – lly Jun 26 '18 at 5:05
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    $\begingroup$ The Gentry County photo appears to be a field with colorful flowers, not the color of the soil. $\endgroup$ – David K Jun 26 '18 at 17:19
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    $\begingroup$ @Muze Given that when I google "purple dirt missouri" the only reference I can find to purple dirt is that particular photo, I'd argue that either the coloring is artificial (due to fertilizers or pesticides or something), or the photographer didn't realize there were flowers down there, or it's simply a trick of the light. In any case, I don't think that's the actual color of the soil. $\endgroup$ – David K Jun 26 '18 at 19:22
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    $\begingroup$ @Muze You don't. I'm basing my statements on personal experience doing multimedia work and working in the printing industry for multiple years. In the first case, it could also be really warm lighting combined with high contrast. The second picture though shows almost unnaturally vibrant colors though, and lherefore looks to me like it's gone through some kind of processing (could also be a high contrast filter, but the shadows don't look like they have enhanced contrast to me, so saturation makes the most sense). $\endgroup$ – Austin Hemmelgarn Jun 26 '18 at 23:11

This gif, prepared by the United States Department of Agriculture - Natural Resources Conservation Services (USDA-NRCS) soil scientists at the National Soil Survey Center, has soil colors based on the Munsell Color System for the United States at different depths:

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The soil colors nearest the surface are darker due to more organic matter and are lighter at depth with varying colors by region.

Source: http://munsell.com/color-blog/soil-colors-national-parks-anniversary/

This link also has soil colors of select United States National Parks. For example:

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  • $\begingroup$ I don't know it is not showing the whole dirt spectrum of color in your answer? $\endgroup$ – Muze Jun 26 '18 at 4:01
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    $\begingroup$ @Muze You asked for the average color of dirt, not for every color of dirt... $\endgroup$ – Shufflepants Jun 26 '18 at 13:52
  • $\begingroup$ Most of the more unusual colors are very rare and only occur in small areas, so on a map that size you likely would not even see them. That map also has a very diverse set of colors, reds, whites, greens, yellows and browns. If you follow the link it shows many areas in more detail. $\endgroup$ – John Jun 27 '18 at 0:41

Soil color is largely determined by it's composition.

There are three main components in soil:

  • Gravel, sand, and silt. In essence, the type of stuff you find in sedimentary rocks. Usually, this is mostly silicate minerals like quartz, feldspar, and mica, and is usually pretty bland in terms of color. In theory, certain silt minerals (olivines for example) can produce greenish soils, though concentrations that high are generally uncommon. With sand involved, you can get lots of other colors (yellow, pink, purple, and even green), though this is usually only seen on beaches.
  • Clay. Clay's often account for a large percentage of the weight of soil. They also usually account for many of the color variations. Most likely, that red soil near where you live has a lot of hematite rich clay in it. Clay's mostly contribute red, yellow, and brown shades to soil, but can in theory give almost any color. Brightly colored soils are usually the result of a sesquioxide clay, and can in theory be almost any color (red from iron, blue from copper or cobalt, green from copper, nickel, or chromium, yellow from chromium or iron, purple from manganese, etc), though only iron is particularly common.
  • Humus. Humus is heavily decomposed organic matter. It has a rather distinctly dark brown (almost black) color to it, and also accounts for.much of the earthy smell of soil.

Beyond those, there are a few other things that may be in soil and impact the color:

  • Charcoal. Usually found only in certain climates (warm with lots of humidity, like tropical rainforests), gives a very dark color to the soil.
  • Fungus. There's a lot of fungus in soil. Usually, it's not concentrated enough (or colorful enough) to have an impact, but in places where there is a very high concentration, it can have a localized impact on soil color.
  • Shells. Either from diatoms (that's what diatomaceous earth is, lots and lots of microscopic shells), or very finely ground shells from bigger organisms. You've probably seen pictures of pink or salmon colored beaches, the same effect can also happen to a much lesser degree in soil too. Usually though, lots of shells just turns soil white.

So, what color would that translate to on average? Well, for topsoil, due to the generally high humus content, probably a rather dark brown. Averaging over all soil, most likely something significantly lighter, as deeper soil tends to have much lower concentrations of organic matter, but likely still tan or beige.

  • $\begingroup$ I would also add iron to the list of things that impact colour; Given how common it is on this planet I suspect it may be responsible for more redness than clay; nice answer though $\endgroup$ – UKMonkey Jun 26 '18 at 12:36
  • $\begingroup$ @UKMonkey Finely divided hematite (the mineral form of red rust) is almost always the culprit for making soil red. If it's very very dry (like, Mars levels of dry), it's technically silt, though that's a rare occurrence most places here on Earth. If it's moist as is the case in most locations on Earth, it ends up in a colloidal suspension, and is actually classified as a type of clay (part of what are known as sesquioxide clays). $\endgroup$ – Austin Hemmelgarn Jun 26 '18 at 15:31

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