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Keeping soil in raised beds from settling prematurely

Q: I grow perennials and dwarf shrubs in raised beds filled with good soil. Unfortunately, the soil settles, and every year I’ve had to lift the plants to add more soil. Is there a way to avert this problem?

Ursula Chautems, Pierrefonds, Quebec, ca

A: Lee Reich, Ph.D., a soil scientist and Fine Gardening contributing editor, replies from New Paltz, New York: Your raised beds may have too much of a good thing. The soil you’re using is probably too rich in organic matter and is settling as the organic matter decomposes. If you’re using a product purchased as topsoil, it might be any kind of soil mixed with an equal volume of some organic material such as compost. In contrast, a naturally rich garden soil usually has only about five percent  organic material. The rest is primarily minerals, water, and air. 

You can avoid the problem of soil  settling by filling the bulk of the beds with any well-draining soil, rather than using topsoil or compost. This will give you a soil base that endures. Either dig up soil from your property or purchase soil that has a high percentage of sand or silt. Then top-dress that soil with a layer of compost or other organic material, such as leaves, straw, or wood chips. Base your specific recipe on the quality of the soil and the cultural needs of your plants. In some cases, you may need to use some type of fertilizer or amendment under the top-dressing.

Most of the feeder roots of perennials and shrubs are near the soil surface, and that’s where much of the beneficial biological activity takes place. Thus, it’s here that organic materials can best do their job of feeding plants and modulating the negative effects on the soil from pounding rain and hot sun. Settling will be minimal in the raised beds, and an annual replenishment of organic materials on the surface will not disturb your plants.

From Fine Gardening 73, pp. 96