How to Assess Soil Composition

By Ann Whitman, Suzanne DeJohn, and The National Gardening Association from Organic Gardening For Dummies, 2nd Edition

4 of 6 in Series: The Essentials of Garden Soil

The health of garden plants depends on the soil's composition — the proper balance of mineral pieces, organic matter, air, and water. Knowing the type of soil you have can help you choose techniques to enhance its good qualities.

The best garden soil should have proper balance of minerals, water, organic matter, and air.

The relative amounts of clay, silt, and sand particles determine your soil texture:

Clay particles are microscopic and flat.
Silt particles are more angular and larger than clay but still microscopic.
Sand particles are the largest of the three types. They can be angular or rounded.

Determine the type of soil you have.

For most plants, the ideal mixture is approximately 40 percent sand, 40 percent silt, and 20 percent clay. Soil with this makeup is called loam, which provides a balance of water-holding capacity, drainage, and fertility. Soils composed of mostly one particle type can pose challenges for gardeners:

Clay soils are naturally fertile, but the individual particles are so small that they pack tightly, leaving little room for water and air. Clay soil drains poorly, stays wet longer than other soils, contains little oxygen, and dries as hard as concrete.
Silt soils have moderate fertility and medium-size particles and pore spaces that hold some water and air. They can pack tightly, especially when wet. They may get powdery or dusty when dry. Silt particles are easily carried away by runoff and are small enough to be blown away by wind.
Sandy soils contain few nutrients. Sand particles are large; water drains quickly from the pore spaces, and any nutrients that are present tend to leach out. Sandy soils don't pack tightly like clay and silt soils.

Unfortunately, except by trucking in huge amounts of soil, you have no way to change your soil's texture. You can take advantage of its natural assets, however, and compensate for its challenges by working on the soil structure.

Many factors affect soil structure, but the most important ones include the following:

Organic matter: Decayed plants and animals become humus, a substance that helps soil particles bind together. Adding organic matter improves the structure of sandy and clay soils.
Soil organisms: As they tunnel through the soil, earthworms, beetles, and other organisms open spaces between soil particles, allowing air, water, and roots to pass through easily. Encourage these beneficial soil organisms by providing food and habitat for them in the form of organic matter.
Rotary-tilling: Churning the soil through rotary-tiller blades breaks up compact soils. Rotary-tilling also changes the soil structure.

Overtilling pulverizes soil aggregates, damages soil life, and promotes too-rapid breakdown and loss of organic matter.
Working with wet soil: Avoid walking on, digging in, or rototilling saturated soil. Allow garden soil to drain to the dampness of a wrung-out sponge before working in it.

Adding organic matter to your soil every year is important, particularly for sandy or clay soils. Organic matter helps sandy soils stick together into aggregates that retain the proper amount of moisture and helps clay soils drain better. But even healthy, loamy soils benefit from annual additions of organic matter, which contributes to soil fertility.