5 Things to Know About Clay Soil

Red kid boots in wet, clay soil.

Clay soil has some remarkable characteristics. Of all soil types, clay alone can be used naturally to form barriers against moisture and erosion. In a pure clay form, it is not well suited for most plants, but the natural plasticity of clay makes it easy to mix. Here are five interesting facts about clay soils, why they are special, and how they can be used.

1. The Make-up of Clay

There are three main types of soil: clay soil, sandy soil, and loamy soil. Clay and sand are effectively opposites, with loamy soil being ideal for the growth of plants. Where sandy soil is made up of visible grains, clay soil is much finer, with the individual grains much more difficult to discern. Because they are so small, clay grains are able to align, binding with other clay particles to form a substance that is able to resist water seepage. This alignment is the reason that clay feels slippery, as the grains slide almost without friction against each other. The common misconception is that things slide easily on clay, but the fact is that clay binds to other materials, and then slides against itself.

Tip: Never dig or till clay soils when they are wet, as doing so will cause compaction.

2. Types of Clay Soil

While most clay soil is inorganic, it can also be found as sedimentary deposits that have eroded from other locations and mixed with organic matter as it was transported. This type of clay tends to be found in mountain valleys, or in low lying areas that have intermittent flows of water. Organic clay mixtures are darker in color, and are often streaked with darker lines formed by layers of decomposed plant matter.

3. Uses for Pure Clay

If conditioning the clay soil is not what you have in mind, clay can be used to define garden borders, preventing excess flooding during heavy rains and channeling the garden drainage to a desired location. Clay forms a natural barrier to moisture and can be used as the central fill for berms, dikes, or small dams. Additionally, clay can be formed into different shapes, and fired or baked. Sun-dried bricks can be made by shaping small blocks of clay and leaving them in direct sunlight.

4. Conditioning

To condition clay soils for use in planting, the addition of organic matter is required to increase how much air and water can move through the soil. It will also add valuable nutrients to fuel plant growth. Organic matter can be in the form of compost, humus, composted manure, or even green manure. Add approximately three to four inches (or 25 to 50 percent of soil volume) of organic matter to the clay being treated. Turn the area with a tiller until the compost and soil have been well mixed.

Tip: Adding fine sand directly to heavy clay soil will create the unwanted result of a denser soil. Instead, the application of well composted organic matter every year will improve the structure of clay soil over time.

5. Benefits of Clay Soil

Clay soil is unique in that it can retain moisture, making conditioned clay excellent for use in dry climates. It also adds density to the soil, which allows plants to grow stronger and taller. Pure clay inhibits root growth, but conditioned clay soils offer an almost optimal support for bushes, shrubs, and trees. Clay soils have a high cation exchange capacity (CEC) and can hold more nutrients than a sandy soil.