Pine Needles, Oak Leaves, and Soil Acidity: What Does the Research Say?

In a recent Healthy Garden article, Paula Gross mentions research indicating that mulches of oak leaves and pine needles do not acidify the soil. In view of the tendencies of woodlands in my area to have a pH of 5 to 6, whereas lawns and fields (admittedly many of which have been limed) tend to be around 6.8, I would be interested in the specifics of the research.

—Will Ferrell, Kernersville, North Carolina

Paula Gross responds: In digging deeper for this article, I was reminded that soil pH is very stable and that it takes a lot to change it. When we want to change soil pH in either direction, we turn to inorganic materials—limestone to increase pH, and sulfur to lower it—because organic materials simply won’t have a strong enough effect in our lifetimes.

I think it is much more likely that the pH difference Mr. Ferrell observes is due to the liming of turf, as he suspects. North Carolina’s Piedmont soils tend to run acidic, but of course it all depends on the exact soil he is testing. Pockets of more and less acidic soils certainly exist.

The effect of oak leaves on soil pH has been studied extensively. A paper published by Nikolai, Rieke, and McVay of Michigan State University documented no change in soil pH after six years of mulching oak leaves into established turf grass. Abigail A. Maynard’s research at the Connecticut Agricultural Experiment Station also documented no difference in soil pH when uncomposted leaves were added to vegetable plots. (Editor’s note: You can find summaries of these studies online by searching for the authors’ names along with the keywords “leaves” and “soil pH.”)

Hard research on pine needles and soil pH is lacking, yet all university extension services I researched dispute the claim that pine needles acidify soil.

If Mr. Ferrell wants to try an experiment of his own over five to 10 years with pine needles, I can tell you he’d have an interested audience online!

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