Do Pine Needles Acidify Soil

Robert Pavlis

This is an old gardening myth that just won’t rot away!

This common, incorrect, advice goes as follows: if your soil is alkaline (ie has a pH above 7) and you want to make it more acidic, add pine needles to the soil. Since pine needles are acidic they will acidify your soil. This advice is very prevalent especially for growing acid loving plants such as azaleas and rhododendrons.

Pine Needles Acidify Soil
Do Pine Needles Acidify Soil?

There are two important questions to ask. Firstly, are pine needles acidic? Secondly, do they acidify the soil? Let’s have a closer look at both questions.

Soil Acidity

Your soil has a certain pH level which is expressed as a number between 1 and 14. A value of 1 is extremely acidic, a value of 14 is extremely alkaline (or basic) and a value of 7 is consider neutral – neither acidic or alkaline. Most plants prefer a value of around 6.8. Most plants will grow just find with a pH in the range of 6.4 to 7.5. Acid loving plants like rhododendrons like a pH of 4.5 to 6.0.

Let’s say your soil is more alkaline than your plants want. The solution seems obvious – add something that is acidic. When you add acid to soil it should reduce the pH making it more acidic. Anyone who has taken basic chemistry in school has probably seen this take place in a test tube. You start with a blue basic solution, add some acid and the color changes to red showing that it is now acidic.

Plant Science for Gardeners by Robert Pavlis

For more on soil pH see the post Soil pH Testers–Are They Accurate?

Dr. Abigail Maynard’s Study

While researching this topic I came across numerous comments referring to a study done by Dr. Abigail Maynard on pine needles, but I could not find a link to the actual study. So I contacted her and she was kind enough to provide this reply;

For some reason, someone got the idea that I have worked with pine needles. Unfortunately, I have not. I have done extensive work with oak and maple leaves and their effect on soil and vegetable yields but nothing with pine needles. I get so many inquiries about pine needles that I am actually thinking of conducting some research with them!

Clearly there is no such study.

Are Pine Needles Acidic?

Let’s have a look at the first question; are pine needles acidic? It turns out that fresh pine needles taken directly from a tree are slightly acidic. By the time pine needles gets old and are ready to drop off the tree they are barely acidic. After a few days on the ground, they lose their acidity completely. The brown pine needles, also called pine straw, are not acidic.

There are two important points here. Since your source for pine needles is probably not green, they are NOT acidic. Collecting old pine needles is pointless if you are trying to acidify your soil.

The second point is that even when fresh, pine needles are only slightly acidic and therefore can have limited effect on changing the pH of the soil.

But, but , but, you say – surely over many years, the acidity must build up. This seems very reasonable and so some scientists tested this theory. They collected soil samples from underneath 50 year old pines. They also collected nearby soil samples where no pines had been growing during the same time period. They found that the pH of both soil samples were the same. The growing pines did NOT acidify the soil even after 50 years.

How can we explain these findings? They don’t agree with what we saw in the test tube!

Why Does Acid Rain Not Acidify the Soil?

Southern Ontario can be considered to be a large limestone rock. Our soil has been created over millions of years from this limestone. Limestone is alkaline and so our soil is also alkaline. Mine has a pH of about 7.4.

Consider this. Rain that has no pollution in it has a pH of 5.6. You might expect it to have a pH of 7.0 since that is the pH of pure water. However, as rain falls, it absorbs CO2 from the air. When you add CO2 to water you create a weak acid (carbonic acid) and that acid has a pH of about 5.6. Keep in mind that this is taking place without pollution. Add in the pollution and we get acid rain. The rain falling in central Ontario is about 4.5.

For millions of years, Ontario has had rain fall with a pH of at 5.6. In all that time this amount of acid has not been enough to neutralize the alkalinity of our limestone rock. As the acidic rain hits the ground, it neutralizes (dissolves) a bit of limestone, but the amount is extremely small. It will take another billion or so years before it changes the soil pH.

I have used Ontario as an example, because I know it best. The same principle applies to most soils. It takes huge amounts of acid to change the pH of alkaline soil. The exception might be very sandy soil.

Even with acidic rain mother nature can’t acidify the soil. Do you really think you will make a difference with a handful of pine needles???

Before I close, let me say that adding pine needles to your garden is a good thing. They are organic and will help enrich your soil. They just won’t make it acidic.

References:

1) Photo Source: Iowa State University

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

46 thoughts on “Do Pine Needles Acidify Soil”

  1. Thank you for all your work. My wife and I are new to gardening and I have found your books very helpful in understanding the basics of the science of soil. I am studying biophysics and I work at a biotechnology research instrumentation company, so I always appreciate when someone can understand and interpret the data and science.

    We live in a small one bedroom apartment in Boise ID. We rented a garden box from the apartment leasing office a few days ago and started working the soil today. I found that there was at least 1 1/2 in of good loose top soil but I quickly found out that the soil underneath was cemented together, almost as hard as rock. So we spent some time breaking that layer up and removing it. Then putting the good topsoil in its place. We plan on doing this throughout the entire garden box.

    Once the cemented soil has been removed and replaced with the good topsoil, our plan is to gather fallen leaves and cover the garden box with at least 6in of autumn leaves.

    After applying the leaves, I have been thinking about applying pine needs as the top most layer of the garden box. Then just let it sit over winter.

    Today I did some simple qualitative testing on the soil using the rapitest soil test kit. The results were around adequate for N and P, K was higher at sufficient/surplus. The pH was around 6.5.

    My hope is that the autumn leaves will break down relatively quickly and provide a good humus layer above the top soil by next spring. Then the layer of pine needles (pine straw) will settle down as the mulch for the coming growing season.

    The insightful knowledge you provide in this post was very helpful and corroborates with other research articles I have read.

    I would love to read any suggestions you have or things you would change about my methodology.

    Reply
    • Leaves do not break down very quickly. You can speed that up by adding nitrogen, but why? Just let nature do her thing. Sounds like a good start. Just keep the garden mulched with organic material and it will continue to get better.

      Reply
  2. I have trimmed my hedge tree and was planning to use the leaves to mulch between my veggies rows but was discouraged by local gardeners telling me it will drop my soil pH and damage my crop. Thoughts?

    Reply
  3. Hi Robert. It’s awesome all your work. I’m a subscriber of your YouTube channel and this site it’s on my favourites. I have a question about this topic, it’s clear that the pine needles doesn’t turn the soil acidic, but, why the bark of perennials is the main source to acidify the blueberries in almost all cases? I can’t find reliable information about that. Greetings from Argentina.

    Reply
    • Not sure what you mean by bark of perennials, but I doubt they acidify soil. Most plant material is close to neutral.

      Reply
  4. I totally agree with the conclusion that pine needles will not acidify soil, the same goes for spruce needles by the way. I too believed the myth, and tried adding it to a vermicomposting system to lower pH. No effect so I did a more thorough, long term experiment, monitoring the pH in the following months. No acidification effect at all. The worms thrived, by the way, and it decomposed rather slowly.

    I was eager to find the source of the myth, and I think I did: in areas with wet, temperate climates and well-drained soils with igneous base rock (not too calcium-rich rocks), there the soils in areas dominated by pines and spruce become significantly more acidic than areas with broad leaf trees. This is explained by the leafs of pines and spruce are less rich in calcium, which is then enriched in the top soil by decaying needles. At least not in amounts large enough to counteract the effect of the acidic rain dissolving the calcium and carry it away into the subsoils or even to streams.

    So, it is not acidic per se, but it will not help normalize pH in acidic-prone, wet climate soils as effectively as broad leaf trees will. To people living in those areas, it can certainly seem like it are the pine and spruce trees that make the soil acidic.

    Reply
    • Its not so much Im interested in the acidity of soil, but how true is it that pine needles kill grass around the tree ? I have a 80 ft. pine tree in my back yard neighbors yard, and alot of the branches are hanging on my side. I’ve been here for 22 yrs now and I’m finding a ton of needles in my driveway, and all around my back yard. My back yard is like totally dead with so many needles on the ground. So what I’m wondering is if there’s any facts backing up that pine needles are acidic, and will kill grass. Thanks.

      Reply
      • Hi Frank,
        I have a neighbor that has Pine Trees all around his property line and needles are everywhere on my property as well on my patio, stuck in the framework of my house.
        I have trimmed any overhanging branches, but they are SO tall and I can not reach the tops.
        I believe the choice of some of my flowers in beds are affected by the needles besides making my beds unsightly!
        I wish the choices of neighbors to keep trees that impact on their neighbors was outlawed!

        Reply

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