Put about 2 inches of soil in a straight-sided, flat-bottomed quart jar. Add 1 teaspoon of powdered dishwasher detergent to help disperse the soil particles. Add water to within 2 inches of the top. Screw on the lid tightly and shake the jar for 5 minutes or until the soil is thoroughly liquefied.
Place the jar in a spot where it can remain undisturbed for several days. After 1 minute, use a wax pencil to mark the level of sediment in the jar; this represents the sand in the soil sample. Make a second mark after 1 hour; the sediment between the two lines is silt. Clay particles will settle to the bottom last, taking up to a week. Some organic material may remain floating after all the mineral particles have settled out.
Use a ruler to measure the depth of sand, silt, and clay, as well as the total soil depth in the jar. (If the silt layer has become more compressed as the clay settles, measure by observing the slight variations in color instead of the wax pencil marks.) Use the measurements to calculate the percentage of each soil component.
For example, the jar above shows 1/2 (0.5) inch of sand, 5/8 (0.625) inch of silt, and 3/8 (0.375) inch of clay, for a total of 11/2 (1.5) inches. Divide each particle depth by the total soil depth to get the percentages:
0.5 ˜ 1.5 = 0.333, or about 33 percent sand;
0.625 ˜ 1.5 = 0.417, or about 42 percent silt;
0.375 ˜ 1.5 = 0.25, or 25 percent clay.
Learn More: How to Get an Accurate Soil Test