Soil Drainage – an overview

Importance of soil drainage

Most plants demand good drainage. If the soil doesn’t drain well, the root zone becomes saturated with water, leaving no room for air and causing plants to suffocate. Fortunately, the most common drainage problems are fairly easy to identify and correct. Wait to make any decisions about drainage until you have observed the drainage patterns in your garden over the course of a year.

Water might puddle and drain slowly for any of three basic reasons: the soil might be too heavy for water to pass easily, something might be obstructing the water’s flow, or there might be too much water.

Drainage properties of different soil types

Drainage in Heavy (Clay) Soil

Soil is called “heavy” when it is dense and sticky. This occurs when the soil is rich in clay, and the clay is not structured well. Clay that is structured into clumps has spaces between the clumps that allow quick drainage, but clay in which the structure has been damaged drains very slowly.

In some cases, especially in areas with salinity problems, soil structure is damaged by sodium. Sodium is so damaging to soil structure that it can make it impervious to water. This is what causes alkali salt flats.

In most cases, however, clay soil has some structure, but it needs improving.

Drainage in Compacted Soil

Clay soil can have its structure damaged by compaction as well as chemical means. Compaction occurs when people or vehicles repeatedly press the soil, squeezing the air from it. When this happens, the compacted area drains poorly.

Drainage in Layered Soil

Water moves most easily through soil with a continuous texture. Sudden changes of soil texture—from fine to coarse or coarse to fine—impede the flow of water. This type of layering can happen artificially if somebody spreads a layer of sandy soil on a clay native soil, or it can happen naturally when a creek deposits different materials in different flood seasons. In any case, the water is impeded by a sharp change in texture.

Drainage in Hardpan and Plowpan

Special categories of layered soil are hardpan and plowpan. Hardpan is a layer of hard, rocklike cemented soil. It is formed by centuries of chemical deposits. In many cases, iron or calcium compounds wash a short distance into the soil, then precipitate and deposit at the same depth, cementing the soil into a hard mass that blocks the flow of water. Hardpan is usually a foot or more under the surface, but erosion can expose it, making it more shallow.

Plowpan is caused by the soles of moldboard plows sliding over the soil about 7 inches under the surface. After years of plowing, especially when the soil is wet, a compacted layer about 2 inches thick develops at this depth.

Drainage in Shallow Soil

Especially in hilly country, the layer of soil on top of bedrock may be too shallow to grow plants well. Drainage is impeded because there is no place for the water to drain to.

Underground Water

In some areas ground water may rise to the surface. Gardeners may experience this water as a high water table or a spring. High water tables are often found near ponds, lakes, or rivers, and in low areas. Water tables rise and fall with the season and precipitation. A water table may be high in early spring but drop to manageable levels later. One solution to a high water table is to put in a pond or bog garden. Another is to lower the water table with drain lines.

Springs are places where underground creeks, called aquifers, erupt on the surface. They are often experienced as a persistent wet spot, usually on a hillside. If you can’t put in a small pond or use the water somehow, the spring can be diverted.