Sustainable Urban Design and Soil pH

By Ellen Vincent, Clemson University Horticulture Lecturer

Fall Field Day

More people are moving to urban environments in the United States than ever before. Walkable access to work, food, and recreation plus plenty of opportunities for social exchange are some of the benefits associated with urban living. The need for walkable greenspace is identified by most major health organizations such as CDC (Center for Disease Control) and WHO (World Health Organization) as essential for health and well-being.

One green industry challenge surrounding new development and urban spaces is high soil pH levels. Soil pH refers to the relative acidity of the soil. These numbers are important because the pH level determines nutrient availability to plants. Nutrient deficiencies (yellowing leaves is one indicator) are caused when the soil pH levels are incompatible with specific plant needs. Soil testing by a laboratory is a reliable way to collect soil pH levels prior to selecting plants for a landscape. Soil samples can be submitted to your local Extension Service or mailed directly to Ag Services Laboratory in Pendleton, SC. A visit to the Website will provide soil sampling and mailing instructions.

Results of a soil sample submission commonly take between 7-14 days. The pH scale is from 0-14 and 7 is considered neutral. Numbers less than 7 are considered acidic and numbers higher than 7 are alkaline (http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1652.html). Plants prefer different pH ranges with azaleas and blueberries requiring an acidic pH range of 5.0-5.5; while many ornamentals prefer 6.0-6.5. When soils are too acidic for the plant species being installed lime may be used to raise the soil pH to a desired level. When soil pH results indicate numbers above 7.0, aluminum sulfate or sulfur may be applied to lower the soil pH. A fact sheet with instructions on changing soil pH is available from the Clemson Extension Home and Garden Information Center at http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html.

If you prefer to not alter the soil pH one sustainable design approach may be to select the right plant for the right place, which means that you seek plants that tolerate high pH soils.

One place to look for a list of plants that tolerate alkaline (high pH) soils is the Carolina Yards Plant Database located on line at http://www.clemson.edu/extension/carolinayards/plant-database/index.html. You can select the section of South Carolina you are working in, whether you want native or non-native plants, and select cultural requirements pertaining to sun, soil, pH, and more. On a quick search of plants tolerating alkaline pH levels 177 fabulous plants appeared. Sweet viburnum (Viburnum odoratissimum), Virginia sweetspire (Itea virginica) and Pittosporum (Pittosporum tobira) are three commonly used shrubs on the list. American holly (Ilex opaca), Bur oak (Quercus macrocarpa), and Redbud (Cercis canadensis) are glorious trees to choose from. Choice perennials include Rosemary (Rosmarius officinalis), Pink muhlygrass (Muhlenbergia capillaris), and Coneflower (Echinacea purpurea). Most of these plants tolerate a wide range of soil pH levels-which makes them particularly satisfying for meeting landscape design needs.

Having a list that is made in South Carolina for South Carolinians, makes choosing the right plant for the right place quite a bit easier than it used to be. Performing annual or biannual soil tests and keeping hold of the records will also help professionals track soil pH changes and plant health for specific sites.

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