Clay Soil Classification

Clay soil can bake to a hard crust under hot, dry conditions.

All gardeners dream of a nicely situated plot made up of rich, dark soil. But what many gardeners have to deal with is often less than ideal, and soil quality can range from sand to clay. When faced with clay soil, gardeners do have choices, and it helps to understand just what clay soil is before beginning to improve it for gardening.

Ball Test

Go out into the garden and pick up a handful of soil. Squeeze it hard and shape it into a ball. If it feels dry and crumbly, chances are it is high in sand content. If it's somewhat wet and forms a loose ball, it most likely is comprised of silt and organic matter. If it compacts into a tight, dense, sticky ball, then it is made up of mostly clay, which will make successful gardening difficult.

Clay

Clay soil is made up of tiny bits of weathered rock. The bits are smaller than the particles that make up sand. It has a powdery consistency and if you rub it between your fingers, it feels greasy. If you add water to it, you can actually use it as modeling clay.

Types

There are two basic types of clay soil: residual and sedimentary. Residual clay is the direct result of rocks being weathered and dissolved into a powder. This powder mixes with rainwater and other elements in the soil. Sedimentary clay is formed when the rock particles are washed away by rain and deposited as a layer on top of other soil. If you dig a hole in your backyard and notice the soil is a different color and texture at different depths, one or more of those layers is sure to be composed of clay.

Advantages

Clay soil does have some helpful properties in the garden. Because it is dense and heavy, it is able to hold onto rainwater longer than either sandy or silt-based soils. Clay soil also stores nutrients more efficiently than other soil, and it doesn't wash away as readily as sandy soil or soil with a high silt content.

Disadvantages

Clay soil takes longer to warm in the spring, so it can delay planting. It also expands and contracts more during the cold months because of its high water content that freezes and thaws all winter. During dry spells, clay soils can harden and crack, uprooting some plants and damaging the roots of others. Clay soil is also more difficult to cultivate, and it packs down hard when walked on.

Solutions

Adding large amounts of organic material such as well-rotted compost, manure or peat moss can help improve clay soil. Be sure to work the organic matter into the soil to a depth of at least 1 foot, and mix it in completely. The combination of clay soil and other organic matter will help plants grow stronger and healthier.

References

• Types and Classifications of Clays
• GardenLine: Soil Texture: From Sand to Clay

Resources

• NRCS: Soil Formation and Classification