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The Dirt on Soil
Down & Dirty
Recipe for Soil
Name That Soil

Tropical rainforest Soils are as diverseas the plants and animals growing and living above them. Tropical rain forest soil is not like a prairie soil. Physical, chemical, and biological actions work together to create every soil on the planet. But there are many ingredients that can go into the basic recipe.


What's your type?Squeeze some soil between your fingers. Is it crumbly? Sticky? A soil's texture depends on the size of its particles. And living things depend on the right texture to thrive in the soil.

Soil Particle Sizes

Soil Particles
Gravel:larger than 2mm; feels coarse
Sand:2 - 0.05mm; feels gritty
Silt:0.05 - 0.002mm; feels like flour
Clay:smaller than 0.002; feels sticky when wet

Every soil type is a mixture of sand, silt, clay, andorganic matter.

Sand: Too Coarse.Soils with lots of sand have big spaces between the particles. They don't hold water or nutrients. Sand doesn't react with other chemicals. Sandy soils don't stick together very well. Plant roots can't hold onto this soil. But the big spaces do allow air into the soil. There are some plants that are able to grow in sandy topsoil by putting their roots deep, through the sand to the subsoil.

Silt: Too Light.This is material which is finer than sand, but still feels gritty. Silt is commonly found in floodplains and is the soil component that makes mud. Soils with a lot of silt make excellent farm land, but erode easily. This is the soil blown away in dust storms and carried down stream in floods.

Clay: Too Fine.Lots of clay makes the soil heavy and dense. The spaces between soil particles are very tiny. When clay soil is dry, it's almost as hard as concrete. Plant roots can't push through it. No air can get in from the surface. Most bacteria and other soil organisms that need oxygen can't breathe. But clay is important because it can change the soil chemistry. Clays give off minerals and absorb acids.

Loam: Just Right.The perfect soil for plants and soil organisms has about the same amount of sand and silt, plus a smaller amount of clay. This soil has enough large and small spaces for air and water to flow in. It also has enough clay to let it stick together and holdhumus. These clumps make another size of space. Plant roots can easily grow through these spaces. This is what farmers and gardeners are talking about when they call a soil "aloam" or "loamy." It's the nicest thing they can say about a soil. Really.

Pictures: Saskatchewan Interactive/Dr. Jeff Bettany | Point Pelee National Park, Canada |
Steven Allison-Bunnell | USDA/Larry Rana


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