Garden Myth: Pine Needles Acidify the Soil

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Popular belief to the contrary, pine needles have almost no influence on soil acidity. Photo: sphere

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

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Pine needles used as a mulch. Photo: Charles Rondeau,

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?

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*The pH scale goes from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. Illustration:

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

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Soil tests easily show that pine needles have little to no effect on soil pH. Photo: Lynette, Flickr

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?

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Not many plants grow under pine trees, but it’s not because of the soil’s acidity. Photo: Hans Rohr, Wikimedia Commons

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


Bales of “pine straw.” Photo:

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

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Oak leaves have no more effect soil acidity than pine needles, but I’d suggest shredding them with a lawn mower before use, otherwise they are very slow to decompose. Photo: tracy, Wikimedia Commons

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!20171010A pxhere


Garden Myth: Oak Leaf Toxicity


Oak leaves are slow to decompose, but does that mean you should ban them from compost piles and mulch?

You often read that you shouldn’t put oak (Quercus spp.) leaves in the compost, because they’ll be toxic to micro-organisms … or is that rather that they’re too acid? (The proponents of this garden myth never seem to be able to agree on the explanation!) Nor, say these same authorities, should they be used as mulch for the same reason(s). But the whole idea is essentially false or at least, highly exaggerated … but it does contain a pinch of truth, as is often the case with garden myths.

The Truth About Oak Leaves

It’s true that oak leaves contain a lot of tannins, phenolic substances that would be toxic to humans if we ate too much of them … but nobody munches on oak leaves. Tannins in too high a concentration are also toxic to certain herbivores (horses, cows, etc.) … and they’ll avoid eating oak leaves if they have any other possibilities, largely because tannins make the leaves very bitter. There are many micro-organisms that will turn toxic tannins into harmless byproducts … but others that won’t touch leaves until the tannins have been broken down.

So, a bit of truth there, however…

The same tannins are present in most other tree leaves too, not only in those of oaks. Tannins are, in fact, very abundant in forests. It’s tannins, for example, that give the rivers that flow through forested areas their brown tea-like coloration. This happens even in northern forests well beyond where oaks grow. Water rich in tannins remains as drinkable as other similar water sources and harbors a wide range of fish species.

Also, humans regularly consume tannins without any harm. The clear brown color of tea comes from tannins, as does the somewhat astringent taste of red wine. And if distillers age whiskey in oak barrels, it’s so the tannins oak wood gives off enrich their taste.

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Fresh oak leaves are acidic, but by the time they decompose, most of that acidity is long gone. Photo: colin grice, Wikimedia Commons

As for acidity, true enough, the freshly fallen oak leaves are certainly acid, but they become less and less so as they decompose. At the end of the process, they end up being, depending on the species, slightly acid to even a bit alkaline! And “slightly acid” is actually the acidity most gardeners want for their garden soil.

As a result, the acidity of oak leaves does no harm to plants when they are used as mulch, nor does it make the soil more acidic than it originally was, much to the disappointment of rhododendron enthusiasts, who often mulch their favorite shrub with oak leaves under the mistaken belief that they will acidify their soil. (Rhododendrons and azaleas are, with blueberries, among the few plants that grow best in very acid soils, but mulching is not going to acidify their soil.)

In fact, if you analyze the soil under large oaks where their own leaves have been allowed to decompose for decades, you’ll find it to be … acidic, neutral or alkaline, depending on the pH (acidity level) of the underlying rock. Even after 100 years of superimposed layers of oak leaves, they will have had almost no influence on the acidity of the soil.

The simple truth is that mulch almost never modifies the pH of the soil underneath, no matter what it is made of.

But Oak Leaves Aren’t Perfect

So, when oak leaves are accused of poisoning soil or compost or being too acid, that’s essentially a myth … but that doesn’t mean oak leaves are necessarily a boon to gardeners.

First, oak leaves are very slow to decompose. Not only do they tend to be tough and leathery compared to most other leaves, but the presence of a lot of tannins does seriously slow down decomposition … and usually what you want if you add leaves to compost is fast decomposition.


Oak leaf mulch.

Also, when oak leaves are entire, they make a poor mulch, as they tend to clump together, forming an almost impenetrable layer that perennials and ground covers have trouble breaking through.

That’s why it’s always best to shred oak leaves before using them. Run them under the lawn mower, vacuum them up with a leaf blower (it will chop up the leaves as it picks them up), pour them into a garbage can and shred them with a string trimmer, or whatever. The method is up to you, but when you do break oak leaves into small fragments, tannins will be largely rinsed out the first time it rains, reducing the so-called toxicity to almost nothing, and bacteria will start to decompose the leaves in earnest.

It’s interesting to note that, in several of the world’s greatest gardens, shredded oak leaves are actually the preferred mulch, as they last a bit longer than other leaf mulches!

So, don’t be afraid to use oak leaves in your compost bin or as an ingredient of your mulch: they are essentially harmless and can even be most useful. Just make sure shred them first!20170515A