Every soil type is a mixture of sand, silt, clay, and organic 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 flood plains 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 most 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 hold humus. 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 "a loam" or "loamy." It's the nicest thing they can say about a soil. Really!
Soil Particles (see image)
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
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