Will Pine Needles Make My Soil pH Too Acidic? (No, It’s a Myth)

Popular belief suggests pine needles will acidify soil, but, off the tree, the pH level normalizes. Just like leaves, pine needles are an excellent organic mulch option.

This is part of a series where we examine common garden advice to separate the myths from facts.

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Do Pine Needles Change Soil pH?

Have you heard you should avoid growing plants near pine trees due to acidity from the pine needles?

Or, on the flip side, that we should grow plants like blueberries or rhododendrons that like a lower soil pH near pine trees to take advantage of that acidity?

Either way, it’s misleading advice.

Here’s why.

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Contents

• About Pine Needles
  • Acidity
  • Competition
  • The pH of Pine Needles
• Changing Soil pH Levels
• Summary
• Resources

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About Pine Needles

Pine Needles Are Acidic—But Only on the Tree

Pine needles are acidic (pH of 3.2 to 3.8 and 7 is neutral) when they are living, green, and growing on the tree. This much is true. And that’s probably how the whole things started.

Related: What is Soil pH and Why it Matters

Between that and noticing that plants do not grow well under evergreens, or, more specifically, near conifers such as pine trees, it is assumed the dropped needles negatively affect the soil below.

It is also assumed that unless you specifically want to lower your soil’s alkalinity or grow ‘acid-loving’ plants, you should avoid them.

Again, a whole bunch of broad, misleading assumptions.

Here’s what we know.

Plants and Trees Compete for Resources

The fact that other plants fail to thrive under conifers is not hard to explain.

Between the dense roots—many of the them near the surface—and the tree canopy blocking sunlight, it’s a challenging growing environment for other plants.

This is most likely due to a lack of the basics like adequate sun, water, root space, nutrients, and air circulation—the things we know most plants need to thrive.

The problem is not the pine needles but competition for space and resources—just as it is when you try planting near any big trees. If basic growing conditions are sufficient, it can work, if not, it doesn’t.

Related: The Black Walnut Tree Juglone Myth

The pH of Pine Needles

What about soil acidity? Do pine needles alter the soil pH?

When pine needles fall to the ground, they have left their life source (the tree) and begin to decompose. And that’s when the pH increases (acidity decreases) and they turn yellow and then brown.

It’s the same for pine straw (pine needles) you can buy as mulch. Add this mulch to your garden and any affect on soil pH is so minimal, temporary, and at surface level that it’s insignificant.

They decompose so slowly that their influence is scant and may be countered if any other neutralizers or alkaline substances like lime are present [1].

So, no, pine needles acidity should not be a concern for gardeners.

Blame the trees for being resource hogs, not the pine needles for any soil issues.

And this brings us to another big misconception about soil pH: the idea that you can permanently alter your soil pH.

Related: Biochar as a Soil Amendment (Pros and Cons)

Changing Soil pH Levels

Changing your soil pH level long-term is nearly impossible.

Let’s say you have had your soil tested in multiple locations in your garden (to be sure) and the pH levels are outside optimum range (5 to 8, depending on what you grow).

It’s not common but let’s say yours is.

Creating a lasting change to your soil pH levels is very difficult.

Just think about the vastness of your soil and what it would require to alter it.

It’s like trying to change the color of the ocean with drops of paint.

And, without constant inputs, it would continually revert back to its natural state.

A layer of pine straw mulch—even a 4-inch layer (which is too deep for adequate water penetration but an example to make this point)—is not going to do much. The needles offer little in the vastness of the soil.

That’s why the best advice is to work with what you have instead of against it. Or, if you are set on growing plants that will not love your soil, grow in containers and control the soil and compost there.

Related: Comparing Home Soil Test Kit Versus Lab Test

Long-Term Study

A study a few years ago at the University of Wisconsin tested soil under different conifers and non-coniferous trees [2].

These were established trees that had been dropping needles and leaves for 25 years.

So how did the soil pH levels compare?

No difference between the different types of trees.

This said, we do know trees can alter soil pH, and, very generally, evergreens do so more than others, but these changes—if they happen—occur over many decades[3], not from the presence of fallen pine needles or mulch.

Summary

• The presence of decomposing pine needles or pine straw mulch in a home garden is not going to change your soil pH, harm your plants, or boost the ‘acid-loving’ ones.
• Stick to the basics when growing plants: provide adequate sun, water, root space, nutrients, and air circulation.
• If you have pine needles, go ahead and use them like any other good organic mulch.

Resources

Listen

Read More

1. Do Conifers Make Soil More Acid?
   All trees can affect the pH of your soil – but it’s not as simple as it sounds, it takes years [Source]
2. Examining the Effect of Established Conifer and Deciduous Trees on Soil pH [Source]
3. The Influence of Tree Species on Forest Soils: Processes and Patterns (PDF) [Source]
4. Linking Litter Calcium, Earthworms and Soil Properties: a Common Garden Test with 14 tree species [Source]
   

And that’s it for pine needles: definitely not harmful and often useful.

Also see the truth about Epsom salt and why eggshells are not really plant problem solvers for more myth busting.

~Melissa the Empress of Dirt ♛