The size of the mineral particles in soil creates its texture and defines its type. The three sizes of particles (in order of increasing size) are clay, silt, and sand. Loam, the most desirable of garden soils, is a blend of all three particle sizes.
“Clay,” “silt,” and “sand” mean only the size of mineral particles, not what they are composed of. Clay particles are less than .002 millimeters (1/12,500 inch) in diameter, smaller than bacteria. Silt particles range from .002 to .05 millimeters (1/22,500 to 1/500 inch). Sand particles are from .05 to 2 millimeters (1/500 to 1/12 inch). The largest sand particle is 1,000 times larger than the largest clay particle.
Classifying a soil by texture helps gardeners determine water and air circulation and water retention properties. The range of particle sizes is responsible for the pores in the soil, and the pores are where air and water reside. It also gives a clue as to how easy it will be to till the soil.
Soil with large pores drains quickly and doesn’t hold much water. Soil with very fine pores absorbs water slowly, drains slowly, and holds lots of water. In general, sandy soils drain faster than clay soils, but there are exceptions. Clay soils with strong structures aggregate into crumbs that increase the porosity of the soil, allowing water to enter and drain quickly, and they still hold lots of water.
As an exception at the opposite extreme, sandy soils with the wrong proportions of silt and clay can have very low porosity. Imagine a crate full of basketballs. There are large spaces between the basketballs because they are all the same size. Now imagine that you add golf balls to the crate of basketballs. Even though the crate was full of basketballs, golf balls can filter between them and fill the voids. You could add still smaller particles—BBs, for instance—to fill the pores between the golf balls. By adding particles of many sizes, you can fill the pores so there is little room for water or air. Some sandy soils drain poorly because their pores are packed with silt and clay.
Loam—a mix of particle sizes—contains approximately 10 to 20 percent clay, 25 to 70 percent silt, and 20 to 65 percent sand. The best loams for gardening are those that also include substantial amounts of humus (partly decomposed organic material), which helps the sandy soils retain moisture and nutrients and opens up the clay soils for air and water penetration.
Soils are seldom 100 percent clay, silt, or sand. To properly describe their consistency, scientists developed the soil texture classifications, which classify soil according to the percentages of particle sizes it is composed of. For example, if your soil is about 15 percent clay, 65 percent sand, and 20 percent silt, it would be classified as sandy loam.
Judging Textural Class by Feel
You can make a quick judgment of textural class by feeling the soil. Soil scientists and technicians become quite accurate with this method, but it takes lots of practice.
Wet the soil if it is dry, and rub a sample between your fingers. Sand feels gritty. The more sand in the soil, the grittier it feels. Silt feels soapy, and clay is sticky and leaves a strong smear on your hand.
You can estimate the amount of clay in the sample by its structural strength. Sand has very little cohesiveness, so a ball of it squeezed in your hand will fall apart at the prod of a finger. Clay is very sticky and cohesive; a ball squeezed in the hand will deform, but not break when prodded. Silty soil has a little cohesiveness, but a ball breaks easily in the hand. The stronger a wad of soil is, the more clay it contains.
A Simple Analysis of Textural Class
For a rough estimate of your soil textural class, make this test. It depends on the fact that large particles fall through water faster than small ones.
Take a sample of the soil you want to test, crumble it well, then dry it in an oven set at 200 degrees for 24 hours. See Taking a Soil Sample for sampling instructions. When it’s dry, grind or pound it with a brick or rolling pin to make the mix as fine as possible.
Put a pint of the soil in a quart jar and add enough water to saturate the soil. Mark the depth of the soil on the side of the jar. Add a tablespoon of water softener, such as Calgon, to the jar and almost fill it with water. Screw on the lid and shake the jar vigorously to dissolve the water softener and mix it with the soil. Let the jar rest for 10 minutes to allow the water softener to separate the soil particles.
After 10 minutes, shake the jar again to get all the soil into suspension in the water. Then set it down, look at your watch, and let the materials settle.
Measure the depth of the soil that has settled out after 40 seconds. This is sand, which settles out quickly.
Then measure the depth of material again 6 hours after shaking. These are the intermediate-size particles, or silt, plus the sand that had already settled out. The mark you placed on the jar to show the soil depth can be used as an estimate of the clay portion of the soil or, for a more accurate reading, you can let the jar settle until all the clay has settled out and the water is almost clear, which might take as long as 2 weeks.
Divide the depth of each layer by the total depth of the soil to estimate the percent of that texture. Percentages of sand, silt, and clay determine soil texture.
See the Soil Texture Triangle posted by Wilkes University to find your soil textural classification. Click on the triangle at the point where the percent of sand and silt intersect. Then click on “Calculate” to learn many interesting things about your soil, such as how much water and air it holds.