There is a very common and persistent garden myth that pine needles (and other conifer needles) acidify the soil and therefore should not be used as a mulch or added to the compost bin.
The belief behind this myth is that they are very acid and will make the soil too acidic for most plants. Some gardeners even mix pine needles into the soil of their acid-loving plants, such as rhododendrons, heathers, blueberries and blue hydrangeas, convinced they will make the soil more acidic. However, they’re wasting their time. The fact is that pine needles have almost no effect on soil acidity.
Not So Acidic
There are two main reasons why pine needles don’t acidify soil to any degree and the first is that they are simply not that acidic!
In fact, although fresh green pine needles are generally quite acidic, they’re already less so when they turn yellow (their condition when they fall off) and much less so when they finish decomposing. If you analyze the pH* (degree of acidity) of brown, fairly decomposed pine needles, it’s usually between 6.0 and 6.5 … more alkaline, in fact, than rainwater (it normally has a pH of about 5.6). And the ideal pH for most plants is between 6.0 and 7.0. In other words, by the time they finish decomposing, pine needles are about spot on perfect for 95% of all the plants you might want to grow. Where’s the problem?
Not Much Influence
The other factor is that soil pH is very stable. Several agents, including the soil’s microfauna, act as buffers to stabilize soil pH. Changing the pH of a soil is very difficult and requires significant applications of acidifying products, such as sulfur, or alkalinizing products, such as lime. In addition, the soil tends to return to its original pH if repeated applications are not made. Whether you like it or not, it’s largely the bedrock below that determines the pH of the soil in which you garden and changing it is never going to be easy.
The application of pine needles—or any other mulch or soil amendment resulting from the decomposition of plant material—will only have such a minor effect on the soil’s pH, even after years of repeated applications. In fact, in most cases, the effect will be so minor that most pH test kits won’t be able to detect it.
Easy Enough to Prove
It’s easy to prove that the pH of soil is not much influenced by the presence of pine needles. Do a soil test under a mature pine tree that has been showering the ground with needles for years and do another under a deciduous tree in the same area and the same soil type, one that doesn’t have the reputation of acidifying soil. The pH will be substantially the same and indeed probably identical.
But Why Don’t Plants Grow Well Under Pine Trees?
I know that many readers will object to the information above, insisting that pine needles must necessarily make the soil very acidic, otherwise how can you explain the fact that so few plants grow well under pine trees? But you have to remember that excessive acidity is only one factor that can stunt plant growth.
Try digging a hole under a pine tree and you will quickly understand the main reason why plants grow poorly there. Pines (and most other conifers) produce very dense, very superficial roots reaching out in all directions like the spokes of a wheel. These roots quickly absorb any rainwater that falls and any minerals available in the soil. The soil under a pine is therefore in a permanent state of drought and mineral deficiency. Few plants do well under such hostile conditions. This factor alone largely explains the paucity of vegetation under pine trees.
But there is another important factor: shade. Little light gets through the dense needles of most pines … and low light is simply not conducive to the growth of green plants.
A Popular Mulch
In areas where the “pine-needles-acidify-soil” myth has not taken hold, pine needles are sold as garden mulch. In fact, it’s often the most popular mulch, both effective and attractive. It’s sold in bales, often under the name “pine straw.” I never see pine straw sold in my area, where the “pine-needles-acidify-soil” myth is very strong, yet there are plenty of pine plantations that could yield a ready supply of inexpensive mulch.
Do note though that, in spite of other qualities, pine straw is highly inflammable and therefore should not be used as a mulch where forest fires are a concern.
Oak Leaves Too
The information above about pine needles largely applies to oak leaves, also often accused of being too acidic for gardening purposes.
Again, oak leaves are not all that acidic to start with and they too decompose into perfectly fine compost with a very reasonable pH. But in fact, that is of limited importance. What you really need to remember is that the natural pH of any soil is very difficult to change and that decayed or decaying vegetation of any type, whether left on the surface or worked into the soil, simply won’t have much influence on it.
So go ahead and use pine needles or oak leaves if they are available to you. Mother Nature put them there to be recycled … and you never go wrong by following her cues!