Friday, July 22, 2011

Soil Color: Why is my soil red? or tan or brown or white???

Ever wonder how you can tell if a particular soil will make a good garden?  Soil color can very quickly tell you if you have a good garden soil or not and what you might have to do to it to make it better.  Ever wonder why soils come in a variety of different colors?  Typically it has to do with where the soil came from.  For example, red rocks make red soil.  It also has to do with what is being put into or taken out of the soil by dust or water deposits.  How much water is typically in the soil also influences soil color.  As a naturally curious person looking at your soil in your yard or garden, along the hiking trail, or in a farmers field, soil color can tell you a lot about the land.  And knowing a lot about the land will help you be a better gardener, farmer, or help you explain the local habitat.  The following are the most common soil colors the average person will encounter.  A little understanding of these can greatly aid your gardening skills or your knowledge of
Reddish brown, iron containing, light colored soil
Reddish brown soil: Red soils that have built up an accumulation of iron oxide, which is essentially rust.  The more red the soil the older it is.  In some cases however red soil is derived from red colored rocks such as the red Coconino Sandstone parent material rocks around Sedona, Arizona.  Red soils derived from red rocks also derive their color from iron oxide though.  But how does soil turn red when the parent material rocks are not red?   Interestingly, dust containing iron is deposited on the soil where it oxidizes turning the soil red.  Over time more dust containing iron falls on the soil and the soil becomes increasingly red.  Red color also tells us that the soil is dry for most of the year.

Yellow, iron containing, light colored soil
Yellow soil: Also in indicator of iron in the soil but at much lower levels then red colored soil. 

Dark, humus rich soil
Dark soils:  Typically the darker brown to black the soil the more organic material the soil has in it.  Temperate climates with sufficient rainfall will have darker colored soils due to the build up of humus.  Humus is similar to compost in that it is simply broken down decayed plant materials.  Prairies have the darkest soils due to the huge build up of dead and decaying plant materials.  Dark soils also indicate the richest soils for growing plants and agriculture.  If you are a farmer of gardener you want a dark colored soil being the humus retains water, contains nutrients, and aerates the soil. If a soil is very black, it may indicates either magnesium from the parent material rocks in the soil.  More typically though it indicates the soil is saturated with water so it has very little oxygen in it, turning the soil black.

Light colored soils: Any light colored soil no mater what shade indicates the soil has a low humus content.  These are often rain forest or desert soils.  Rain forests are too hot and wet for humus to accumulate in the soil, it simply breaks down too much.  Deserts are also often too hot for humus to accumulate and don't have much plant material to produce it.  Light color often indicates the soil is leached, or nutrients have been washed out of it.  This all makes light colored soils nutrient poor.  For the gardener with light colored soil you can simply add compost to it, making your soil more productive.  The yellow and reddish brown soil pictures above are examples of light colored, humus poor soil.

Soil with high amounts of calcium carbonate.
Whitish soils:  White soils indicated calcium carbonate or in some cases salt.  This color may also be a result of whitish parent material such as quartzite.  Often in desert soils you will not be able to see white soil being it is buried under other soil colors.  If the surface layers of soil erode off though some of the white soil may be visible.  Calcium carbonate builds up in desert soils due to calcium falling on the soil as dust, rainwater then carries it deeper into the soil where it is retained.  As with iron, the older the soil is, the more dust has fallen on it, and therefore the greater the calcium carbonate content.  This only happens in desert like areas being wet areas wash the calcium out of the soil.  Salt is different in that is is deposited by flowing water on the soil, typically in playas which are seasonal lakes.  Every time water is washed into the lake it carries salt with it, then when the lake drys up the salt is left behind.  For the farmer or gardener these are very bad soils.  They can be remedied to some degree by adding compost though.

Gray colored soil.
Gray soils: Could indicate the soils parent material is gray colored or that the soil is wet most of the year.  In iron rich soils that are saturated with water much of the year, the lack of oxygen turns iron a gray color.

There are a few other soil colors that are a lot less common.  One of the more rarely seen and interesting is purple soil.  Purple soil has a strong sulfur smell and results from sulfur bacteria that grow when a soil is waterlogged for a long period of time.  Which brings up another interesting point, soil smell.  As odd as it might sound a lot of soils have different smells to them.  Anaerobic, soils without oxygen, have a sulfur smell to them.  While aerobic soils, soils with oxygen, have a different smell.  Soils with and without certain nutrients or humus also have different smells.  These smells are a result of different soil microbes like bacteria and fungi, which also contribute to soil color.  I have kind of learned some of these smells with experience and occasionally they can be useful.  Yes, soil covers all the senses; sight, smell, touch, sound in some cases, and some people tell me taste (though I haven't tried it)!


  1. This really helped with my science paper. Thanks.

  2. very good article,i am growing a vegtable garden high in horse manure content it rained today , and my soil appears black ,,,reading this article enforced what I already knew its very informative