Garden Tips: You don't need pricy soil for raised-bed gardens

Marianne C. Ophardt, WSU Benton County ExtensionJuly 3, 2014 

Today's homeowners have smaller yards and less space for vegetable gardening. Many have opted to use raised beds as a way to maximize space and minimize garden maintenance.

"Square-foot" gardening is a raised-bed system for optimizing garden production promoted by Mel Bartholemew. His system includes using a potting mix that he calls Mel's Mix. It contains compost, peat moss and coarse vermiculite. It can be pricy, especially when filling numerous beds.

Gardeners wanting to grow vegetables in raised beds do not need to invest in an expensive soil or potting mix. They can use their own soil in low-sided raised beds.

First, after setting up the beds, take the native soil in the pathways around the beds and mix it with good quality compost (no more than 10 percent by volume).

If there is not enough soil to fill the beds, bring in topsoil. True topsoil is natural surface soil scraped up and transported to a site. Topsoil in many regions is more desirable than the subsoil (the soil layer beneath the topsoil) because natural processes have created a crumbly soil structure that is conducive to good plant growth. However, digging and transporting topsoil generally destroys this crumbly structure and nullifies its benefits.

Dr. Craig Cogger, Washington State University Extension soil specialist, recommends sandy landscaping fill as a compromise, but notes that it will hold little water and dry quickly. (True topsoil in our region generally lacks the crumbly structure found in areas with more rainfall.)

Sandy landscaping fill is sandy soil mixed with organic matter. Probably much of what is sold commercially as topsoil in our region is sandy landscaping fill. If you decide to buy this kind, ask where the soil came from and what it contains.

Buy soil or fill from a reputable company. Not all soils sold as topsoil should be used in raised-bed gardens. It can contain broken glass, too many rocks, wood waste and other debris. Inspect the topsoil before you buy and before accepting delivery. You also do not want soil that might have come from an area that was treated with long-term residual herbicides or other chemicals.

Ask if the company has had the topsoil tested or knows how much organic matter it contains. If the topsoil or landscape fill already contains 10 percent or more organic matter by volume (5 percent by weight), you do not need to add compost or other organic matter.

Dr. Linda Chalker-Scott, WSU Extension horticulturist, says that 10 percent organic matter is adequate from a fertility perspective. Adding substantially more of it to soil contributes to high nutrient levels that can lead to plant health problems. Chalker-Scott recommends having the soil tested to determine the organic matter content and nutrient levels.

Finally, if the soil is distinctly different from the native soil beneath, it can impede drainage. Cogger recommends mixing the introduced soil with native soil as the bed is built to create a textural gradient that will allow for better drainage.

For more information, read the WSU Extension Fact Sheet on raised beds at http://tinyurl.com/WSU-raised-beds.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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