Pine Trees & Acid Soil
Most gardeners find it difficult to grow garden plants under pine trees. Indeed, it is often impossible to maintain grass under pines. This problem has traditionally been blamed on acid soil, since gardeners believe that pine trees create acid soil. As a result, many gardeners refuse to use pine needles as mulch or include it in their compost piles. However, the belief is not accurate. Pine needles make an excellent mulch and can be freely used in making compost.
What Is Acid Soil?
Acidity or alkalinity of a substance is measured by the pH factor and described by a number on a scale that runs from 1.0 to 14.0 pH, with neutral represented as 7.0 pH. Lower numbers are more acidic; higher numbers are more alkaline. Most garden soil is neutral, an acid-alkaline balance that makes a larger amount of the minerals in the soil available to plants. Most garden plants tolerate a range of pH values, though some prefer soils that are more acidic or alkaline.
Good Soil for Pine Trees
Pine trees thrive in soil that is mildly acidic, about 5.5 pH. This frequently observed pairing may be part of the reason that pine trees are said to create acid soil. Gardeners often find it difficult to grow other plants under pine trees, and blame this on acid soil. However, it is probably because pines, like other fast-growing trees, absorb large amounts of the nutrients available in soil. The heavy shade cast by pine trees also inhibits other plant growth.
Acidity of Pine Needles
Pines, like other conifers, replace their needles annually. Since the shed is gradual, conifers do not have a leafless period, as deciduous trees do. Over years, a layer of pine needles accumulates under pine trees. These needles are very slightly acidic, about 6.0 to 6.5 pH.
Affect of Pine Needles on Soil pH
Pine needles are so slightly acidic that they do not cause soil to become acidic quickly. When needles have been present for decades from large numbers of pine trees, as in a mature pine forest, the soil is likely to become a little more acidic, but the change is small even under those conditions.
Pine Needles as Mulch
Pine needles, often called pine straw, make an excellent mulch. They decompose slowly, so garden beds will not have to be mulched as frequently. They are lightweight, making the job easier. They are also a sustainable resource. Most pine straw is gathered from the natural shed of pine needles in large pine plantations, so no trees are destroyed in commercial sale of pine straw.
- “The Garden Primer”; Barbara Damrosch; 2008
- SUNY College of Environmental Science and Forestry: Soil pH