Pine Needles – Excessive Acidity

Q: After reading your answer to the question about leaving pine needles on the lawn last week, I’m still confused. I understand about not letting them make a mat on the grass but what about the acidity they produce?

A: You’re confusing causation with association. It’s a common misconception that pine needles excessively acidify the soil. Because pine trees can tolerate poor growing conditions, folks assume that their needles made the soil bad in the first place. I’ve known gardeners who fear that using pine straw as mulch under shrubs and trees will make the soil too acid. Neither pine trees nor pine straw make soil unduly acid. It’s true that decomposing leaves from any tree naturally release a small amount of organic acid. But soil chemistry is such that their effect on soil pH is not noticeable except over many, many years.

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