The Q&A Archives: Use pine needles as mulch?

Question: For years I've mulched with pine needles around my acid loving blueberries and rhododendrons to make the soil more acidic. Now I hear this doesn't work. What should I do? Ralph Henderson Brunswick, ME

Answer: Pine needles are not a particularly good way to lower soil pH. Neither are the leaves of oaks or any other trees. Before they rot, leaves of any sort have little effect on soil acidity. Last year I mulched my blueberries with a four inch layer of fresh and partially decomposed pine needles, and after a year the pH hadn't changed at all, said Eric Evans, lab director of the Woods End Soil Laboratory in Mount Vernon, Maine. After they've rotted, all leaves form a compost with a pH of about 6.5 only slightly acidic. Decomposed leaf or pine needle mulch, on the other hand, forms a substance similar to what you'd find on a forest floor. It should be quite good for your blueberries and rhododendrons. If plants have dark green leaves and are growing well, there is probably no pH problem. Keep doing what you've been doing, but you don't have to make special trips to hunt down pine needles. Almost any kind of tree leaves will be fine. If you are concerned about pH, send a soil sample from the root zone of your plants to a soil testing lab. If the pH needs to be lowered, use ammonium sulfate or try watering the plants with a liquid fertilizer formulated for azaleas and other acid loving plants.

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