Marianne Ophardt: Build soil in raised bed gardening

For the HeraldJanuary 24, 2013 

In early December, I talked about the first steps of raised bed gardening and promised to talk about building the bed or developing the soil in which the plants grow.

Some proponents of raised beds insist that success only is possible if you fill the beds with a soil-less mix of peat moss, vermiculite and compost. This mix can be expensive and discouraging for a gardener on a budget. It also provides a limited volume of soil in which plant roots can grow, making it much like growing your garden in a pot.

What’s an alternative to the soil-less mix for raised beds? Why not use the soil that’s already there? Start working up the soil by digging down 12 inches or more (if possible) and turning over the soil in the bed. (If the soil is dry, water it the day before to moisten the soil.) Deep digging for some will be impossible because of rocky soil or a thick layer of caliche. In these situations, the use of a soil-less mix or locally available clean topsoil are options. Note that the topsoil sold at many nurseries is not true topsoil scraped off the surface of the ground. It’s typically simply a mix of sand and compost.

This faux topsoil should work well in raised beds because it has large pores (spaces between soil particles), which provide good water drainage. Also, it already contains organic matter that helps retain moisture and nutrients. It should be noted that the soil-less mixes and the faux topsoil likely are to dry out more quickly than most native soils because of the large pores. This is an advantage in some areas of the state, but creates a challenge in our region because of our very hot, dry summer growing conditions.

Digging may be arduous, but most area gardeners will find that they can use their regular soil even if it’s a little rocky. When building more than one raised bed, many gardeners take some of the soil between the beds to add to the soil in the raised beds. However, this usually is not enough to completely fill the raised bed. Added faux topsoil will be needed.

Whether using a soil-less mix or faux topsoil, it’s important that you don’t create an interface of two distinctly different textures of soil. This results in poor water drainage. To avoid this, add a 3- to 4-inch layer of faux topsoil and mix it together with the native soil by digging and turning the soil. Add another layer of material and mix again, repeating until you have reached the top of your structure.

When finished you should have a soil that is at least 12 to 18 inches deep that gradually transitions to the native soil beneath the bed. How much fertilizer is needed? The only way you can tell for sure is to get your soil tested at a local lab. Once you have a soil test done, you’ll know how much nitrogen, phosphorus and potassium is needed. Then you can decide whether to use organic fertilizer or synthetic fertilizer. My personal fertilizer preference is rabbit manure. -- For more information on raised bed gardens, go the WSU publication on “Raised Beds” at -- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

UPCOMING WORKSHOP WSU Extension and the WSU Master Gardeners are sponsoring a How to Start a School or Community Garden workshop. It is from 4-6:30 p.m. Feb. 13 at the Kennewick Senior Center, 500 S. Auburn St.

Pat Munts, from the WSU Extension Small Farm Teams, will share her insight and experiences working with a number of successful community gardens in Spokane.

WSU Master Gardeners also will be available to discuss and answer questions about creating raised beds, preparing the soil and planting vegetables.

The event is free. For more information, call 531-5913 or email

TRI-CITY ROSE SOCIETY MEETING Alan Wicks will discuss “Feeding Roses for Best Bloom and Foliage” during the 7:30 p.m. Tri-City Rose Society meeting Jan. 28 at the Sandberg Center, just off Van Giesen Street in West Richland.

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