Soils vary greatly from area to area and climate to climate. The soils that are found in the southeastern United States are greatly different from those found in the Northwest. For this reason, the USDA has created the texture triangle to help determine the contents of soil and to help standardize soil that is for sale so that consumers can determine what texture of soil they are getting. It is helpful to know the characteristics of the soil that is being used as this helps to determine how easily the soil will drain, its ability to stand up to erosion or wear and how much it will compact down over time. When used for gardening it is essential to have a soil texture that is within certain ranges in order to provide the correct nutrients, water and oxygen to plant roots.
Clay is one of the three components of soil. In average garden soil, clay makes up less than 30% of the soil base. There are several classifications of clay soils: clay, sandy clay, silty clay, clay loam and sandy clay loam. These are all variations of clay soil that include more or less sand or silt (organic matter) to vary the texture.
Clay soils compact easily and are made up of the finest textured materials. When wet, clay soils can be compressed and formed into shapes that will hold. When wet, clay soils feel sticky and have a plastic feel. These soils do not drain well. Clay soils are made up of at least 40 percent clay and are less than 40 percent silt and no detectable sand. Sandy clay contains 50 to 60 percent sand.
Silt soils are the second finest textured soils. Silt soils feel smooth and flour-like when rubbed between the fingers and when wet they can be compressed and molded into forms that will hold together, withstanding a fair amount of handling. When squeezed, wet silt soils do not form ribbons, but will squeeze out between the fingers and break apart into sections.
Silt soils do not drain well but they contain high amounts of clay and organic matter. Silt soils are classified as: silty clay, silty clay loam, silty loam and silt. Silty loam soil is a soil that, when in the field, tends to clump easily and when dry, breaks apart easily. Silt soils contain more than 40 percent silt, less than 40 percent clay and less than 40 percent sand.
Loams are the most balanced-feeling of the soils--containing balanced amounts of sand, silt and smaller amounts of clay. Loams are dark, rich and spongy. It is fairly smooth when rubbed between the fingers and is somewhat sticky and plastic when wet.
When formed into a mold, loams can be handled freely and will hold together. Classifications of loams are: clay loam, sandy clay loam, loam, silt loam, silty clay loam and sandy loam. Sandy loam soils drain fairly well while clay loam is slow to drain. Loams are made up of 20 to 60 percent sand, less than 40 percent clay and between 20 and 80 percent silt.
Sandy soils are grittier soils and the texture varies depending on the coarseness of the sand that is contained within. Sand soils drain well, but those with little silt dry out quickly. Two classifications of sandy soils exist: sand and loamy sand. Varying textures within these classifications include very fine loamy sand, fine loamy sand, coarse loamy sand, very fine sand, fine sand and coarse sand. Sandy soils contain less than 50 percent clay, more than 50 percent sand and less than 20 percent silt.