Soil Permeability Based Upon Particle Size

Sand

• The largest particle of soil is a grain of sand, which has of size of 0.5 to 2 mm. Because sand particles are large and irregularly shaped, they fit together loosely, leaving large spaces between constituent parts; these spaces allow water and other fluids to pass through the soil unobstructed. Nutrients dissolved in water also drain rapidly away, and, as a result, sandy soils tend to be light, dry and infertile.

Clay

• A particle of clay is less than 0.002 mm in diameter; approximately 100 times smaller than a grain of sand. Clay particles are so small that they tend to pack together very tightly, leaving numerous, tiny spaces between them. These spaces trap water within them; consequently, soils with a high clay content are heavy, dense and difficult to work with.

Silt

• A single particle of silt is approximately 0.05 to 0.002 mm in diameter, which is at 10 times smaller than a particle of sand. Though they are much smaller than sand, silt particles are shaped in a similar fashion; however, they tend to be coated with clay. Accordingly, silt soils behave like both sand and clay-- they allow some water to pass through, but hold enough fluid to keep the landscape from drying out.

Combination Soils

• In nature, it is unusual to find a yard or garden made from a single type of soil particle. Most soils contain a mixture of sand, clay and slit; the general permeability of the soil is determined by the relative amounts of each. While there is not much that can be done to change soil permeability permanently, good soil structure can be encouraged by incorporating organic matter, such as compost or manure, into the soil each spring. These materials help bind sandy soil particles together while keeping clay particles from clinging to each other.