Clay soil is soil with a high percentage of clay. If poorly structured, clay soil can be almost impervious to water, or admit it only very slowly. Clay soil particles are the size of bacteria. In addition, they are composed of flat plates that fit tightly together when compacted. The pore spaces between the particles, where water moves, are microscopic, slowing the passage of water simply through friction.
However, clay particles have the ability to clump together into crumbs, or aggregates, with larger pores between the crumbs that readily admit water. Clay with this ability is said to have a strong structure.
Poor structure in clay soil can be caused by a deficiency of humus, or it may be due to the presence of sodium. Humus is well-decomposed organic matter. It is a relatively stable mass of sticky, spongy organic material that bonds with the clay particles to improve their structure.
Adding Organic Matter
Almost any clay soil can be improved by adding organic matter. The organic matter helps instantly by separating the soil particles and creating pores in the soil. Then, as it decomposes, it bonds with the clay to improve its structure. The effect of organic matter is long-lasting, and has been found to last as long as ten years. The effect is also accumulative, improving the soil more each time organic matter is added.
Fine-textured organic matter decomposes most readily. Compost and manure may be the most effective materials to add. Composted sawdust, straw, leaf mold, and many other types of organic matter will work. It may be necessary to add nitrogen fertilizer at the same time as the organic matter to avoid Nitrogen Draft, a nitrogen deficiency. Avoid material with a coarse texture or large chunks, which decompose slowly. See Adding Organic Matter to Soil for more information.
Soils with large amounts of sodium are called sodic, or alkali soils. Sodium destroys the structure of clay soils, making it impervious to water. To improve drainage in sodic soils, the sodium must be displaced with another chemical—usually calcium—then leached out of the soil. See Sodic Soils for more information.
Other Methods of Improving Drainage in Clay Soil
In cases where it isn’t possible to work large amounts of organic matter into the soil, other methods may be employed. Anything that increases the surface area of the soil to absorb water speeds drainage.
In planted areas, use a soil auger to drill holes between the plants every couple of feet. Add organic matter to the soil before backfilling the hole, or fill the hole with gravel or a fast-draining soil. The sides of the holes greatly increase the surface area to absorb water, speeding drainage.
In lawns, use a chain saw with an old chain to cut rows of parallel slots in the grass. Fill these slots with coarse sand. The grass will grow over the slots, hiding them.
French drains are ditches filled with drain rock. They are usually used as drainage ditches to carry excess water away, but they may also be placed in the landscape to increase the surface area of the soil to absorb water faster. Used this way, they don’t need to empty into a lower area, as drainage ditches do.
To make a French drain, dig a ditch a foot or so deep and any width and fill it with gravel to the surface. The surface of the drain may be used as a path, mowing strip, or border. To make the drain last longer, install header boards at its edges that protrude an inch or so above the soil. The header boards keep soil from washing into the drain and plugging it up.