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Real Solutions: Spring Cleaning

We’re trying to use organic gardening around our yard, and for the past few years I’ve been adding lots of compost to the soil. My rhododendrons, azaleas, and blueberries are looking weak and have yellowish leaves. I’m wondering whether I’m doing something wrong. Can you advise us? — S.P., Salem, NH

Many plants such as those you describe need acidic (low pH) conditions to grow their best. Most composts, despite the addition of organic content, tend to have a higher pH than ideal for acid-loving plants. Testing your soil is easy to do at home; most garden centers sell kits for simple acidity testing.

When you plant acid-loving species, it’s always a good idea to add peat moss to the soil before you backfill around the roots, and then to mulch with pine needles or composted oak leaves, all of which are acidic. If your soil pH tests above 6.0, you can also increase its acidity (reduce the pH) by about a half-point by mixing in about a pound of ground sulfur (flowers of sulfur) and a half-pound of ferrous sulfate per 100 square feet.

Note that sandy soils require less sulfur, and heavy clay soils more. Depending on conditions, soil pH may take months to adjust, so be patient, measure again before reapplying, and don’t overdo it!

— R. Wayne Mezitt, Chairman, Weston Nurseries, Hopkinton, MA

Updated Wednesday, February 20th, 2008
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