Pine needles have a pH between 3.2 to 3.8, indicating high acidity. When left on the surface, pine needles break down at such a slow rate that they have little affect on soil pH.
Although there are many types of conifers, pines are the most commonly used in gardens. When used as a mulch, pine needles help retain soil moisture, suppress weed growth and have little affect on pH. When mixed into the soil, they can improve drainage, provide nutrients for plant growth and have a only a slight affect on pH. While it is commonly believed that it is difficult to grow plants beneath conifer and pine 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.