Soil Preparation for Optimum Drainage

How and why soil drainage is important.

Santa Fe Greenhouses Greensand
Item # 99736 		Soil Mender Jersey Greensand

each $15.95
  • Topic: Soil
  • Author: Mary Ann Walz
  • Keywords: soil, drainage, texture, mulch, Soil
  • Date: March 2003

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One of the cultural requirements of xeriscape plants is the soil has excellent drainage. In some cases this is easily accomplished while in others it will take a considerable amount of work to improve the drainage. The effort it takes, though, will be worth it if you want healthy plants, trees and lawns. Having a loose soil that is well drained will not only retain the appropriate amount of moisture but will also provide sufficient pore space so plant roots get enough air. Pore space is the space between solid particles.

How do you tell if you have good drainage? A simple test is to dig a hole about 12 inches deep and 6 inches wide. Fill the hole with water and allow it to completely drain. Then, fill it a second time. Observe how long it takes to drain. If it takes more than 8 hours, you have a drainage problem.

Many drainage problems occur due to a high percentage of clay particles in the soil. However, there are other possibilities including soil that has been compacted because of foot or vehicular traffic. Springs in the vicinity or a high water table will also slow drainage. In the Southwest, another likelihood for poor drainage is the presence of caliche, a white or light colored layer of soil in which the soil particles have been cemented together by lime (calcium carbonate, CaCO3). When planting in soils with a caliche layer, it is necessary to “punch through” this water impermeable layer by digging deep holes into the non-caliche soil below.

The first step in creating well-drained soil is determining the texture and composition of your soil. Texture refers to the proportion of sand, silt and clay in the soil. Sandy soil drains rapidly but also dries out quickly. Clay soil drains poorly and sometimes remains waterlogged. One remedy that improves the texture of most soils is the addition of organic matter, with compost being the most commonly used amendment. Compost is typically incorporated in a ratio of 1 part compost to 2 parts existing garden soil. Any organic matter will continually decompose so more must be added on a regular basis.

If clay soil is the problem, the addition of compost may not be enough. You may need to add generous quantities of coarse sand or crusher fines, the end result of screened crushed rock. Do not add fine sand as that will result in a concrete-like consistency. Gypsum is also sometimes added to improve the texture of clay soil. As it breaks down, gypsum helps the clay particles lump together, improving the circulation of air and water. Greensand is another additive that helps improve the texture of clay soil. But you need to consider that the use of gypsum and Greensand will take time to effect a change.

For native plants that demand great drainage, immediate fixes like berming (mounding) the soil with the addition of coarse sand or creating a raised bed is a good solution. As the soil in beds with added compost or gypsum become more water permeable, drainage sensitive plants can then be added.

As you continue to add organic matter to your soil, beneficial organisms such as earthworms will appear and gradually become more numerous. These creatures are Nature’s best soil aerators and as a bonus, their castings are an excellent fertilizer. Earthworms prefer soils that are moist, not too wet or too dry.

Don’t forget to use mulch in hot arid areas. The mulch will moderate the soil temperature and will help retain water in sandy and loamy soils. It will help prevent cracking that occurs in clay soils and minimize the chance of tiny roots being exposed to the hot sun.

Always be alert for areas where drainage problems still occur. If a plant dies, examine it to see if the roots have rotted. After a rain, observe standing water in any garden spot. If drainage problems persist in some areas and defy correction, you might consider using containers or building a raised bed.