Healthy soil starts with the right proportion of sand, silt, and clay
Gather your soil sample from the root zone rather than from the surface. You'll need at least one cup of soil.

My friend's plants were ailing, and she had no idea why. Because I could clench a handful of her garden soil in my fist and form a firm ball, I believed the soil was lacking in organic matter. To be sure, I tested a small sample to determine its texture—the ratio of sand, silt, and clay. As it turned out, her soil was mostly silt, which is easily compacted and poorly aerated.

Collect a soil sample

The ratio of sand, silt, and clay in soil determines its ability to hold moisture and nutrients. The test I use to determine a soil's texture is a low-tech alternative to sending a sample off to a lab. All you need is a straight-sided jar with a tight-fitting lid (an empty peanut butter jar or mason jar works well), some powdered dishwashing detergent, a ruler, water, a calculator, a soil sample, and a soil texture triangle.

A soil sample should be representative of a gardening area. If the soil in the area I'm testing is fairly uniform, as in a raised bed, I take a sample from the middle of the bed. If the area I'm testing is large, like a lawn, I make a composite by collecting small samples at evenly spaced intervals across the area. I then mix these small samples together to form a representative sample.

To collect an accurate sample, containing soil from where the roots actually grow, I scrape away the first two inches of soil and dig a hole 6 inches down. I place a trowel full of soil (free of large organic matter and rocks) from this hole into a plastic bag and label the bag with the soil's origin. This is especially important when I'm collecting a number of samples.

The soil must be sifted before testing, so I spread it out on an old cookie tray to dry for a day or so. Once the sample is sufficiently dry, I sift it through a wire-mesh sieve or an old colander to remove small stones and roots and to break down any lumps of soil.

Add soap and water, then shake

After I've sifted my sample, I place a cupful of the soil into a straight-sided jar and add a tablespoon of powdered dishwashing detergent. The detergent is a surfactant, which keeps the soil particles separate, resulting in a more accurate test. I fill the jar to the top with water, screw the lid on, and shake the jar for three minutes to thoroughly combine the soap, soil, and water, and to make sure no soil is stuck to the bottom or sides of the jar. I then set the jar on a flat surface to let the sediment settle. See Test your soil sample for instructions on how to proceed.

As the sedimentation progresses, I check the sample periodically to watch the layers form and to note the size of the particles settling out. Sand particles are the heaviest of the three and settle out of the solution after about a minute. The sand layer is coarser in texture than the silt and clay. Silt is the next heaviest particle and will settle out after about an hour. The silt layer is darker than the sand. Clay, the lightest particle in the mix, can take from one to two days to settle out of the solution. The clay layer that settles on top is fine textured and light in color.


Test your soil sample

Measure each layer of sediment

Calculate percentages and transfer the numbers to the soil texture triangle to determine the desirability of your soil.

To figure out the percentages of sand, silt, and clay in my sample, I measure the total amount of sediment with a ruler, once it has all settled. This number represents 100 percent of the soil sample. To derive the percentages of sand, silt, and clay in the sample, I measure the amount of each layer and divide by the amount of total sample. For example, if the total sample measures 1-3/4 inches and the sand layer measures 1 inch, I divide 1 by 1.75 and get 0.57, which translates into 57 percent sand. If the silt layer measures 1/2 inch, I divide 0.5 by 1.75 and get 0.29, which translates into 29 percent silt. If the clay layer measures 1/4 inch, divide 0.25 by 1.75 and get 0.14, which translates into 14 percent clay.

Amend based on soil type

Sample: The bottom of the triangle is labeled "percent sand." Because the sample contained 57 percent sand, I find 57 on the sand axis and make a dot (A). I then draw a line from my dot at 57 toward the clay axis. On the clay axis, I make a dot at 14 (B), because the sample contained 14 percent clay, and draw a line from there to the silt axis. On the silt axis, I make a dot at 29 (C) because the sample contained 29 percent silt. I draw a line from the dot to the sand axis. The three lines intersect in the sandy loam segment of the chart.

Once I've derived the percentage of each particle, I transfer my results to the soil texture triangle to determine my soil type. The chart indicates I have sandy loam.

For general garden use, soils that fall into any segment of the triangle labeled "loam" are good. If a soil tends toward the clay or silt side, soil fertility should not be a problem, but some organic matter should be added to aid in drainage. If the sample is sandy, fertility and moisture retention may be problems. For soils high in clay or sand, I add organic materials to make them more garden-worthy. Organic materials like well-rotted compost and leaf mold will increase the moisture-holding capacity of sandy soil. They will also add fertility to clay soil and reduce its tendency to clump into a monolithic mass of wet, slippery goo in which little will grow.

Analyzing the texture of soil is as important as studying the needs of the plants in your garden, and understanding the science behind soil texture makes that job much easier. Becoming acquainted with your soil may help you discover why your prized rhododendron is doing poorly and why your equally prized pussy willow is doing well. Or you may discover that you possess what every gardener covets -- a rich loam in which most garden plants thrive.


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