Primarily due to oxidation. The same process that turns an avocado brown when you cut it and leave it exposed to oxygen. Many elements in soil also change color while oxidized. As a geologist I tend to see browner soils predominantly near the surface or a characteristic of young soils that have more recently been deposited (like alluvial soils). Topsoils tend to be very dark brown because they contain a lot of roots which makes the soil porous and therefore aerated. Also they are organic rich which adds to the darker brown to black color. Also deeper soils that drain well tend to be brown like sands since they don’t hold onto water they get more exposure to air.
As a color nerd, I can tell you brown is more a type of color.
Brown can be anything from a dark orange, to a grey-ish purple, to bright greyish red, or even just straight up grey, surrounded by colors that somehow make it look brown.
If you’ve ever drawn digitally, and couldn’t find brown in the color wheel, this is why.
From my arts background, I’d say it’s because you just about mix any colours together and it really doesn’t take long before you end up with brown. Brown being a tertiary colour, which you get from mixing at least three primary colours together (of course dirt is made up of many components)
Thinking on it more, dirt is usually made from plant matter, which is already brown (bark, wood), or green/yellow/red (mix any of these colours and the pigment looks some shade of brown!), As well as minerals like silicone (white- wouldn’t effect the hue of dirt only the tint), or granite, iron, nickel, so on.. which if you think about most are greyish or reddish colours.
So again it depends on the composition of the dirt but it does come down to the fact most of these colours mixed up just end up brown.
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