Do Pine Needles Acidify Soil

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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.

Growing Great Tomaotes, 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.

Soil Science for Gardeners book by Robert Pavlis

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
  5. Before I increased(slightly) my limited knowledge of blueberries. I planted 3 blueberries in my yard alkaline soil Cal, AB. One died second year, one died 4 year and grew terribly. The third one tiny as it is, is alive about 7 yrs later, flowers OK no fruit. Growing nearby are many bleeding hearts which crept from next door and a rose which flowers great in mostly shaded area. Also nearby are 3 large pine trees next door. As they were touching my house I have been pruning them and chopping them up and mulch the BB. All of these plant will enjoy a slightly acidic soil I believe. I do not fully doubt your facts, but do slightly. NTL, I plan this year to pot up the BB, and PH the soil in diff. spots in my yard. THX.

    Reply
    • Bleeding hearts and roses grow just fine in my soil – pH 7.3. They might prefer a lower pH but they don’t seem to need it.

      Reply
  6. Thank you for this insightful information.
    I have a vegetable garden close to some pine trees.
    I found that for some reason the same vegetables grow very well in one spot and very badly in another nearby spot.
    Naturally I heard the pine tree myth story, started looking into this and then came across this article.
    Any ideas or suggestions of what could be the cause of this ?

    Reply
  7. I live in southern California and have a 60 year old pine tree in my front yard. The trunk is about 5′ in diameter, the canopy about 70′ in diameter and the height about 30′. When I bought the house the tree was already over 25 years old and had never been trimmed. A tree trimming company did a hack job and took off 2 of the lower branches that were 18-24″ in diameter. About 15 years ago I raised the grade around the base of the trunk about. 12″ for flower bed. The last 2 trimmings, 10 and 5 years ago were done by an aborist. California is in draught period and the tree is not getting as much water as in the past. I do not think that is in danger, but it does not seem to be thriving as much, i.e. dropping more needles and a little less new growth. The arborist suggested taking out the flower bed and lower grade around the trunk back to the original elevation.
    1. Your thoughts?
    2. Life expectancy of the tree?
    3. Impact of the draught?
    Appreciate your advice.

    Reply
    • If you raised the grade 15 years ago the roots have now grown into the new bed. Removing it will only harm the tree. You might try removing the soil from around the trunk to make sure there is no rot on the trunk.

      I can’t comment about your local drought conditions. At 60 years old it might be on its way out. Trees do not last as long in our artificial gardens as in the wild.

      Reply
    • I had seen that study before. Its finding about evergreens contradicts another study I saw that measure pH under old evergreens and found the pH the same as where they did not grow. I think a lot has to do with the soil type. In your study the soil was a sandy loam, which is much easier to change pH than clay. The soil mentioned in the post is alkaline and even acidic rain is not making it acidic.

      For the gardener – collecting pine needles is not going to change the soil pH.

      Reply
  8. I ran a test to see what pine needles produce when they break down. With two buckets and a pond water pump from Harbor Freight, I circulated water through a bed of needles for about 4 months. The water percolated through a bucket and drained back into the other where it was pumped back to the upper bucket. I watched the pH with indicator papers. The circulating water started at 6.8 and after 4 months of decay with mold growth and who knows what else, the pH was still 6.8. The volume of the needles decreased so there was decay, just not much. I would say that pine needles do not have enough of anything to change soil pH to change it either up or down.

    Reply
    • That is a really smart design for an experiment. Leaching acids should certainly accumulate in the water and lower the pH. The design eliminates the buffering action of the soil which is good – you see just what comes out of the needles. Good job.

      Reply

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