How Is the Soil Texture Triangle Used to Classify Soil Textures?

About Soil Particles

• When rocks weather into smaller and smaller pieces, the result is gravel, sand, silt and, if the mineral is right, clay. Clay minerals split into flat plates that stack without much space between, one of the reasons clay soil is difficult to garden in. Gravel is defined as rock fragments from about 0.08 inch to 3 inches in diameter. Sand is 0.002 to 0.08 inch, silt is 0.00008 to 0.002 inch and clay particles are smaller than 0.00008 inch.

Soil Triangle

• Natural soils, of course, are mixtures of many sizes of rock fragments in various proportions. The soil triangle names each type by the percentage of each size found in that soil. Each side of the triangle corresponds to a mineral size and the percentages increase as you move across the silt, sand or clay line. If you follow the lines that correspond to each percentage on the triangle, the three intersect in a point within a soil type. For instance, a soil with 60 percent sand, 25 percent silt and 15 percent clay is a sandy loam texture.

Assessing Soil

• Take a quart jar outside and fill it a quarter of the way with finely pulverized soil, with no gravel, roots or other extraneous material. Add water until the jar is three-quarters full and then add 1 tsp. of nonfoaming dishwasher detergent. Screw on the lid and shake for 10 to 15 minutes to break the soil up even more. Set the jar in an out of the way place where it will not be disturbed. After one minute, mark on the jar the level of the particles that have settled, the grains of sand. After two hours, mark the level of silt that has settled. When the water is completely clear, which may take two to three days or more, mark the level of clay on the jar.

Using the Triangle

• After the water in the jar has cleared and the soil has separated, measure the depth of the layers and the total depth. Calculate the percentage of each layer. For instance, if the total depth is 3 inches, the sand layer is 1 1/2 inches, the silt is 1 inch and the clay is 1/2 inch, then the soil is 50 percent sand, 33 percent silt and 17 percent clay.

  Using the soil triangle, find the point on the sand line corresponding to 50 percent. Do the same with the silt line and the clay line. Follow the lines that cross the triangle from one side to another to find the point of intersection. These percentages are just inside the "loam" range, so that's the texture of the soil.