How Do Conifer Needles Affect the Soil?

Although there are many types of conifers, pines are the most commonly used in the home landscape. Pine needle mulch helps retain soil moisture and suppress weed growth while having little effect on pH. When incorporated into the soil, pine needles can improve drainage, provide nutrients for plant growth and have a only a slight effect on pH according to This Old House.


Pine needles protect improve soil aeration, reduce weed growth, aerate soil and improve water retention. Decomposing pine needles add nutrients to soil. Pine needles do not significantly impact soil pH.

Soil Acidity and Drainage

While it is commonly believed that it is difficult to grow plants beneath conifer trees due to acidic soil, it is actually due to the tree roots being so numerous and growing so close to the surface that they outcompete other plants for nutrients and water according to University of New Hampshire Extension. Pine trees like Eastern white pine (Pinus strobus, US Department of Agriculture plant hardiness zones 3 through 8 according to Missouri Botanical Garden) grow best in well-drained acidic soils. This means the soil under the trees was already acidic rather than becoming acidic due to decomposing needles. In addition, many plants can't survive in the shade beneath pine trees.

Fresh-fallen pine needles have a pH between 3.2 to 3.8, indicating high acidity. When left on the surface, however, pine needles break down at such a slow rate that they have little effect on soil pH. Incorporated into the soil, pine needle mulch decomposes much faster, releasing their nutrients into the ground, but still only having a slight effect on lowering soil pH. Organisms living in the soil break down the needles into more simple elements, neutralizing the pH of the needles, so the net overall effect on soil pH is minimal according to University of California.

Pine Needle Mulch and Straw

Whether pine needles are lying on the surface or incorporated into the soil, as they break down, their nutrients are released and taken up by plant roots. However, it will take considerably longer for needles scattered on the surface to decompose and their nutrients to leach into the soil where they can be utilized by the roots of other plants. The advantage of their slow decay is that pine needle mulch doesn't need to be applied as often. Depending on the site, pine needle mulch may need to be replaced or added to every year to two.

Pine needle straw is commonly used as a mulch. It is effective in suppressing weeds when applied at the recommended 3-inch depth according to Lowe's. Pine straw mulch inhibits sunlight, retains soil moisture and lowers the frequency of watering, conserving resources and lowering water bills according to AgriLife Extension, Texas A&M University. However, pine needle straw settles faster than other types of mulch and needs replenishing more often, as often as twice per year.

Advantages of Pine Needle Mulch

University of Florida IFAS Extension showed decomposing pine needles can provide up to 50 pounds per acre per year of nitrogen and five pounds per acre per year of phosphorous. Additional nutrients from decomposing pine needles and straw include calcium (3 to 21 pounds per acre), magnesium (0.6 to 5 pounds per acre) and potassium (0.4 to 2.9 pounds per acre). Keep in mind that excess raking and removal of needles from beneath your pine trees also reduces the nutrients available for the trees, so consider setting a rotating schedule for raking under your trees.

Since pine needle mulch forms a loose interlocked mat, water is able to pass through to the soil while preventing excess runoff. In areas with compacted soil that retains water, working conifer needles into the ground will increase aeration, which improves drainage. For rapidly draining soils that cannot retain adequate amounts of water, adding conifer needles increases organic matter, giving moisture something to hold on to and allowing plants more time to absorb it.