Do Pine Needles Acidify Soil?

When a myth dies hard, you can bet that it is either because people want to believe it (for example, when it makes their lives easier) or because there is a kernel of truth in it just big enough to make it convincing. The long-lived gardening myth that pine needles lower soil pH falls into the latter category. It is a twofold myth, erroneously claiming that pine needles should be used as a mulch only around acid-loving plants, and that they will acidify compost if added to a compost bin.

But using pine needles in the garden does not, in fact, appreciably lower the pH and acidify your soil. So you don't have to worry about them having a negative impact on plants that don't like acidic soil. The converse is also true: You can't use pine needles to make the soil more acidic for plants that do like acidic soil. Let's find out why, and, having dispelled the myth, learn the benefits of pine needles for the garden.

The pH of Pine Needles

The aforementioned kernel of truth to the myth is that green pine needles, themselves do have an acidic pH. This means that, if you were to conduct a chemical test on fresh pine needles that had just been actively growing on a tree and that have just fallen off the tree, the reading would be acidic. The temptation is strong to extrapolate from this that, when applied as a mulch around plants, for example, this acidity would "leak" into the soil, thereby acidifying it. But it's just not true (at least not appreciably). To understand why this is a myth, it is critical to distinguish between the substance, itself (the pine needles) and the effect, or lack thereof, that they have on the soil (which is the real issue).

Why Not to Worry About Pine Needles Acidifying Soil

What do we mean by saying that pine needles do not "appreciably" lower soil pH? Here is why the qualification is necessary, technically:

If you were to rake up green pine needles (before they had started to turn brown or decompose) that had just fallen, move them to the garden, turn them under, and test the soil pH in that spot shortly after, you might see a minimal drop in soil pH.

But here's why that's not important:

• The change is so slight that any plants growing in that area would not even notice it.
• Gardeners rarely have occasion to use many green pine needles; when they speak of using pine needles as mulch in the garden, it is mainly to brown needles that they refer. Brown needles are not as acidic as green needles.
• Composted pine needles also lose much of their inherent acidity. So if you are thinking of adding pine needles to your compost pile but worry that the resulting compost would be too acidic, you can put your fears to rest.
• If you were using the green pine needles as a mulch, you would not be turning them under, anyway. Unlike compost, mulch is meant to rest on top of the soil. The needles will eventually decompose and work their way down into the soil naturally. Once down there, the microbes in the soil neutralize them, removing whatever acidity they may have started out with.
  

Perhaps lending further credibility to the myth is that gardeners often observe that it is difficult to grow most plants under large pine trees. They attach an assumption to this observation: It must be that the pine needles shed by the tree cause the soil there to become acidic. However, this is a case of drawing an incorrect conclusion from the facts. The reasons most plants perform poorly when grown under large pine trees are that the trees cast too much shade, or the root system of the trees blankets the soil surface, forming a barrier and sucking up most of the available water and nutrients.

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So Are Pine Needles Useful in the Garden?

Yes. They work well as a soil amendment: Treat them as an ingredient to be added to your compost bin. Shred them first so that they will break down more quickly in the compost bin. Most often you will be using them once they have browned because it is at this stage that they most commonly fall off the tree. Brown pine needles are one of the carbon-rich ingredients needed for compost, along with other "brown" materials such as dried leaves. They are complemented by nitrogen-rich ingredients: the "green" materials such as vegetable scraps.

Pine needles are even more popular as a mulch. As a commercial product, they are often sold as "pine straw." Don't be confused by the terminology: Pine straw is pine needles, not straw. But it is even better if you can get your own pine needles because this allows you to save money. People with large pine trees on their property are rarely lacking in pine needles that have to be raked up.

Once you reject the myth of pine needles acidifying the soil, you can use them as a mulch, confident in the knowledge that they offer several benefits and have few drawbacks. For example, when compared to a redwood bark mulch, pine needles:

• Offer a nice reddish-brown color (although less vivid than that of the redwood bark)
• Offer good insulation from summer's heat and winter's cold
• Provide fair nourishment and aeration, but are excellent at allowing water to penetrate into the soil
• Are easy to apply (because they are so light-weight) and maintain
• Do not break down too quickly, thereby providing you with a long-lasting mulch