Oak Mulch?

Oak Mulch?
Photo credit: Creative Commons

Do Oak leaves make good mulch? I have heard they are too high in acid or something.

- Washington County, Oregon

Good day, and thank you for your inquiry. “Mulching Woody Ornamentals with Organic Materials” (page 4), under the heading “Do mulches affect the pH of the soil?” states, in part: "It is often assumed that … acidic mulches will lower soil pH. However, in all but the sandiest soils, soil pH is strongly buffered (resists change), so soil pH changes very little after applying most mulches. Some changes in soil pH may occur in the top 2 inches of soil, especially if mulch is applied repeatedly. In general, though, mulches do not change pH enough to be a major concern for woody plants." Although oak leaves have an acid pH (4.5 to 4.7) when they are fresh, the breakdown products are neutral to slightly alkaline. To be very frank, I wonder about the source of the oak leaves. If they’re from an oak woodland, you’re removing fertilizer elements the oak trees require. But if the leaves are from your landscape, using them as mulch is certainly preferred to discarding them. Shred the leaves so they are less likely to blow around.

Jean N.
Master Gardener Diagnostician
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