A Do-it-Yourself Guide to Testing Soil Texture
by Janice Hand

Since such a large aspect of successful gardening depends on soil, it’s important to know what kind of soils you have. How to find out? Of course, you can send in a soil sample and get it tested. Or, you can do it yourself.

For the do-it-yourselfers in our Garden Club, here’s how to accurately determine soil texture.

1.    Collect a soil sample.  To do this you need a few supplies:
  • A straight-sided jar with a tight lid,
  • Plastic tray or old cookie sheet,
  • Some powdered dishwashing detergent,
  • Ruler,
  • Water, and
  • Calculator.

To get the sample itself, scrape away the top 2 inches of soil, since roots do not concentrate there. Then, dig about 6 inches down and get a trowel-full of soil from your hole, taking care not to include large organic matter and rocks. You will want to take multiple samples from the same area, mixing them thoroughly.

Spread the soil on a tray and let it dry for several days. When it’s dry, sift the soil through an old colander or screen to remove small stones and roots. This will make sure that the soil does not contain clumps.

2.    Combine the ingredients. Put a cup of the sifted soil in the straight-sided jar and add a tablespoon of powdered dishwashing detergent. (The detergent keeps soil particles from clumping.) Add water to fill the jar to the top, screw the top on tightly, and shake for 3 minutes so that soil, detergent, and water are all thoroughly combined.  Next, set the jar on a flat surface.

 3.    Watch and measure sedimentation.  Check your jar periodically to see soil layers form; note the size of particles that settle out. You will see:

  • Sand is the heaviest and settles out in about 1 minute.
  • Silt is the next heaviest and settles out after about an hour. This layer is darker than the sand layer.
  • Clay, the lightest in both weight and color, needs 1 to 2 days to settle.

 4.   Measure and calculate.  You want to figure out the percent of soil types in the layered sample in the jar. To do this, first measure the total amount of soil. Then measure each layer—sand, silt, and clay. Divide each individual layer’s measurement by the total amount of soil. An example:

  • The total soil sample made up of the three layers measures 2.25 inches.
  • Of the components, sand makes up ¾ inch, silt makes up 1 inch, and clay makes up ½ inch. (Note that the total is 2.25 inches.)
  • Sand is 33% of the sample (.75 divided by 2.25 x 100), silt is 44% (1 divided by 2.25 x 100), and clay is 22% (.5 divided by 2.25 x 100).

 5.   Refer to the soil texture triangle.  Transfer your numeric results to the soil texture triangle below. (It’s not as hard as it looks.)  Using the example above, you would find 33 for sand (go to the bottom of the triangle and put a dot at about 33, then draw a straight line up and to the left toward “clay.” Then, find 44 on the silt axis and draw a line from there toward “sand.” Last, on the clay axis, find 22 and draw a line from there toward “silt.”  Congratulations, the three lines intersect in the “loam” segment of the chart!

6.     Adjust your garden soil.  Based on your test, you can now appropriately adjust your soil texture. An example:

  • For “sandy” samples, fertility and moisture retention problems are likely. Add organic material.
  • For “silty” samples, soil fertility should be good, but add organic matter to help with drainage.
  • For “clay” samples, soil fertility should be good, but you will need to add organic matter to help minimize clay’s tendency to clump into root-free zones.