October 18th, 2011

Acidifying Soil for Blueberries

My poor blueberry plants look yellow, stunted, and totally unhappy despite my efforts to acidify the soil with garden sulfur. Should I keep adding sulfur until they green up?

blog-dougBlueberries grow best in soil with a pH between 4.5 and 5.5. (On the pH scale, numbers lower than 7 indicate acidity; over 7 means alkaline, and 7 is neutral.) In the regions of eastern North America where blueberries are native, soils that are this acidic are not unusual. Elsewhere, gardeners try to accommodate acid-lovers such as blueberries by lowering the pH, using garden sulfur or aluminum sulfate.

Sometimes it works. But some soils have another trait that foils the gardeners’ attempts: high cation exchange capacity (CEC). Because CEC represents the ability of soil particles to stockpile mineral nutrients for later use by plants, moderately high CEC is usually considered a good thing. But soil particles can also cling to the types of minerals that increase alkalinity and make it more difficult for the gardener to acidify soil; high CEC soils are “buffered,” in effect, and resistant to pH change. Soils with plenty of clay or humus tend to have high CECs.

If you get a professional soil test, be sure to ask for an analysis of CEC. A measurement less than 20 means it will be fairly easy for you to lower the soil pH with sulfur. Between 20 and 40, pH change is possible but will require repeated applications and monitoring. Above 40? Grow your blueberries in pots.  —Doug Hall

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