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American Society of Landscape Architects


May 2008 Issue

Strategies for Clay Soils
Whether you choose to amend it or embrace it, you need a plan to plant in clay soil. Here are some tips.

By Barrett L. Kays, FASLA

Strategies for Clay Soils C/O Barrett L. Kays, FASLA

You know the “sandy loam” plants love? That’s just the little bit of topsoil that’s probably not even there when you start working on the design for your site. If the site’s been graded over, you’ll often find that the surface soil is subsoil. There’s a good chance that subsoil will be clayey, and clayey soil comes in many mineralogical and chemical compositions.

Clayey subsoils are normally highly infertile and may have plant-toxic levels of metals. Clayey subsoils can pose both significant chemical and physical problems to establishing successful landscapes.

Fertilizing recommendations by soil testing labs often don’t work for clayey soils. Soil fertility analysis as we know it was established primarily for cultivated crops. Soil sciences use crop yield studies to determine the amount of fertilizer to annually add to the topsoil. Landscape architects have only one shot to do it right. We don’t plow the fields every year. We’re dealing with highly disturbed soils. We need a whole new soil testing procedure, but for now we’re trying to throw darts in a room with our eyes closed.

Too often landscape architects solve their soil problems by hauling in topsoil to hide the underlying problems. The techniques discussed in this article are less expensive and more sustainable: Test the soil. Amend it as needed. Break up compaction and incorporate amendments to rooting depth with tilling. Protect that soil with fencing, with your design, or by limiting the grading. Use pH-adaptable plants and those able to withstand the drainage conditions on your site.

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