Q: Do ponderosa pine needles make the soil more acidic?
A: It’s one of the most common questions I get asked every year. This along with, “Can I use pine needles as mulch?”
The notion that pine needles change the soil pH so that nothing will grow or that it will damage plants has been out there for years. So is it fact or fiction? The truth is pine needles do not make the soil more acidic in an urban landscape environment, at least not in the way that most people fear. It is true that pine needles have a pH of 3.2 to 3.8 when they drop from a tree. This is acidic on a pH scale of one through 14, with one being the most acidic and seven being neutral. If you were to take the freshly fallen needles (before the needles decompose) and turn them into the soil right away, you may see a slight drop in the soil pH, but the change would not be significant enough to damage surrounding plants. This is quite different from what occurs in a large forested setting where the change in soil pH would be more dramatic than that of a home landscape with just a few pine trees. Our native soils generally have a pH of 7.0, and most conifers prefer to grow in a pH of 5.5 to 6.5. Deciduous trees prefer to grow in a pH range of 6.0 to 7.0.
For those of you who leave the needles on the ground below the tree, they will begin to break down naturally, and the microbes (decomposers) in the soil will neutralize them. So you can leave needles there for a little while (if you’re not in a wildfire prone area). Those living in the wildland urban interface should be removing pine needles to help maintain a defensible space. In the urban environment, needles can be used beneath the trees to create a 2- to 4-inch mulch layer. This should be done in areas that have consistent irrigation to prevent the buildup of dry fuels. They are a good mulching material that will keep the moisture in, suppress weeds and eventually add nutrients back to the soil. Do not allow them to build up beyond 4 inches as they will prevent the flow of water through them. You can also add them to a compost pile; they will slowly break down over time. If you run them through a shredder they will break down faster. A general rule of thumb is not to add more than 10 percent of pine needles to your compost pile.
If you are having difficulty growing other plants under your pine trees it is likely due to the fact that evergreen roots are numerous and shallow and compete for water and nutrients. The shady conditions under a tree cans also make growing other plants a challenge.
— Amy Jo Detweiler is an associate professor of horticulture at Oregon State University Extension Service.