Soil is primarily identified based on its texture. Soil texture identifies the ratios of sand, silt and clay contained in a soil mixture. By understanding information on the different types of soil, a gardener can learn to identify problems with soil and improve it using soil additives. Scientists use particle size to describe types of soil through the different components that make up soil textures.
The largest particles in a soil texture, sand particles range from 1/20 mm to 2 mm in diameter. Sizes of sand break down into subcategories including--from smallest to largest--very fine, fine, medium, coarse and very coarse. In a soil mixture, sand particles provide good drainage but do not hold nutrients well. When rubbing sand, it feels rough, chunky and grainy between the fingers.
Clay particles are the smallest particles measured in soil texture. These particles encompass anything less than 1/500 mm. When it is wet, clay feels sticky, almost like paste. When dry, clay is silky, like a powder. In a soil mixture, clay drains slowly, creating a wet environment, but it holds soil nutrients well.
Silt are the medium-size particles--smaller than sand but larger than clay--that make up a soil texture. These particles range between 1/500 mm and 1/20 mm in size. When rubbing dry silt, it feels smooth, but unlike clay, it does not feel sticky when wet. Silt stays wetter and holds nutrients better than sand, but it drains more quickly than clay.
Loam, often considered to be the ideal soil texture for the average plant, is actually an even mixture of sand, silt and clay. The water and nutrient holding properties of the small clay particles mix with the draining and aerating properties of the larger silt and sand to produce a healthy soil environment.
Soil types are broken down into several types and subtypes based on the dominant soil component in the texture. Major soil types include sandy soil, loam soil and silty soil. Subtypes of soil indicate which component is the second most dominant. Soil subtypes are many but include types like sandy loam soil, loamy clay soil and silty sand soil.
Though many experienced gardeners can roughly identify soil texture by feel, the only way to get a completely accurate assessment of soil texture is to have the soil tested. Testing soil can identify many properties of the soil, including information on the types of soil in the testing area, plus texture, nutrient and water retention, and chemical, organic and biological components found in the soil. Soil testing for the public is usually done at agricultural facilities at colleges and universities.