Garden Myths - Learn the truth about gardening

Do Pine Needles Acidify Soil

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.

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

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 a pine needle gets old and are ready to drop off the tree they are barely acidic. After a few days on the ground, they loose 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

Robert Pavlis
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writting 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

19 Responses to 'Do Pine Needles Acidify Soil'

  1. Ray says:

    Good article thanks, but you need to learn more about pH. It’s an even bigger myth that certain plants prefer acidic soil. I grow incredible blueberries at a pH of 7.5, for example. I’m not going to explain it here as it’s a rather long explanation, but you’ll have to dig past most conventional horticulture textbooks to get to the bottom of the pervasive pH myth.

    • So blueberries grow in soil with pH 7.5? News to me, but I wanted to check this out.

      I looked at several university and cooperative extension papers on growing blueberries. All of the recommend a pH of below 5.3. Ok – maybe all of these folks are also wrong?

      So I posted a question on the Garden Professors Group: https://www.facebook.com/groups/GardenProfessors/permalink/10153794222966490/ There seems to consensus ” some plants certainly require acidic soil”

      So if you really do grow blueberries in soil with a pH of 7.5 – provide some references to support this position. If you had a pH of 7.5 and modified the soil, for example by adding sulfur, then your soil is no longer 7.5.

      This just came into my mail box. Here is Lee Reich’s recommendation for blueberries. He recommends acidity (pH 4-5.5).

      http://www.leereich.com/2015/04/blueberries-outside-aphids-inside.html?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+InLeesGardenNow+%28In+Lee%27s+Garden+Now%29

    • Ines Radman says:

      I agree with you Ray. I live on a small island in the Adriatic it’s a rock literally. Our ancients had to remove huge rocks and pile them into fences in order to get a trace of dirt. Over the centuries, a pine tree seed landed here and in the last 50 years 60% of the island is now pine trees. I have collected the needles at times being 8 to 10 inches in depth and underneath that even better dark and rich compost. 100% of my raised beds are filled with both materials. The dark compost is for the roots and needles cover the top. For summer they serve as protection from the sun and in winter they compost leaving me with a new layer in the spring. I have not yet had one plant that has NOT developed and I don’t use anything such as fertilizers. I also collect my own compost and collect it under trees because we don’t have large enough trees on the plot for leaves yes.
      Can we say that maybe because I live in the Mediterreanean that acidity has no value? I don’t know. Can I say that it will work for you in Florida? I don’t know.
      One thing is clear though, the deeper the pine needles are collected, the less acidic content they have. In fact, I would even go as far to say that once you take a handful of pine needles and crunch them easily, they are dry and contain no acid.

  2. Cory Fox says:

    What is it about a plants biochemistry that makes some plants like acidic soil but not others?

    • Good question and something that would be worth researching. Acidity of the soil affects how certain nutrients bind to soil and to each other. Some nutrients are not readily available at pH above 7. So if a plant needs that nutrient it is better able to get it a a lower pH. The same thing happens at low pH–say below 5. So I suspect that certain plants who use a lot of a certain nutrient find it easier to grow at certain pH values. But I suspect the complete story is more complex than this and has to do with the way plants actually absorb nutrients through the roots.

      • Ines Radman says:

        Interesting stuff. I live on a small island in the Adriatic Sea, pine trees are not indigenous to this island but they are now widespread all over the place and in some places the needles are a foot deep and another foot of compost, I take the pine needles on top and scoop out the compost and use them differently.
        In the winter I cover the top with the pine needles so that by spring they have started to break down and compost, the compost itself I fill beds up with it or add to already pine composted beds, and so far lettuce and green peppers don’t like that soil much, tomatoes are not too fussy but I add some ashes to them to balance out, basically my raised bed garden with 17 containers are 60% pine needle compost and mixed with olive pits as we have an olive mill so after 6 months they are great for adding oxygen to the soil.
        Our island is very clay based soil and full of rocks so I am the only one on this island that uses this method and now in spring time people stop by to take a walk through it. I wish I had a place here when I can add some photos, but overall, because of lack of good soil, I use mostly composted pine needles and never any fertilizer as I have a compost bin as well.

        • Sounds like a good system, making good use of local organic matter. If you post pictures on one of the gardening Facebook sites, you should be able to post a link here. It may not be a hot link, but people could cut and past the link.

  3. Ines Radman says:

    I use pine needle compost all the time, except for green beans, this is compost that is years old, I clear the top of the brown needles and set them aside and use for winter covering, the soil underneath I use to fill my raised beds and everything grows great. I think the acidity is when the needles are not composted enough. My garden flourishes with the compost and living on a small island in Croatia, nobody uses it because they too believe it’s acidic. They ask : How come nothing grows under the pine trees? Because the needles are so thick that seeds can’t get to the soil DOH, so, every spring I go out and collect a few bags and top my beds, so far no problems, I have even grown potatoes into the needles.

  4. I have pencil pines dropping pine needles on my lawn in Australia, and the lawn is not growing very well underneath the overhanging pine branches. I have added some sheep manure and lots of water to the lawn. I am also trying to establish vegetables patches underneath and next to the pines trees in three small patches. I’ve mixed old, dried sheep manure to the existing soil, and placed a thick layer of Lucerne hay over the top, and watered heavily. I am trying to create a richer soil, encourage the worms, etc. I don’t know the PH level. Any advice, fellas?

    • Evergreens get blamed for making the soil too acidic under themselves–that is not true. http://www.gardenmyths.com/pine-needles-acidify-your-soil/#more-356

      Pines shade the soil, and use a lot of water from the soil. The soil under them is basically a very dry, shady area. Few things from in such conditions. Using a grass that likes shade, and watering more will help. Manure can’t hurt since fines are also good feeders. The rest of what you are doing should help the vegetable gardens. Vegetables need lots of direct sun.

  5. Ron Beique says:

    BTW the soil under years of pine needle debris is rich in nutrients and pretty PH balanced. Not highly acidic at all. If you want a great soil additive go scoop you up some bucketfuls,

    • Robert Pavlis says:

      I agree with the comment about rich soil, but the soil is not “pH balanced” assuming you mean a pH of 7. The soil pH under evergreens does not change much from the natural soil pH. If you naturally have acidic soil, it will be acidic, and if you have alkaline soil it will be alkaline.

  6. Ron Beique says:

    From the Forest Industry Council site.

    “Although pine humus (organic
    constituent of soil formed by
    decomposition of plant materials) is
    acidic, it does not increase soil
    acidity. The humus is acidic due to
    organic acid production, but this
    does not significantly impact on the soil”

  7. Ines Radman says:

    I live on a small island in the Adriatic Sea, the island is 70% rock and my yard had very little usable soil. I went into the forest of pine trees and collected pine compost, it’s so old that it’s not even compost anymore but a rich light dark mixture, has the texture of top soil.
    It’s unbelievable, all my plants grow and thrive in it, spinach sprouts in 3 days, beans in 7 days, people that pass by say they have never seen such green and lush vegetables. I takes me about 1 hr to fill 4 50 lbs bags, there is so much of it because the locals think that pine is “Acidic”. Just wanted to share that. For the last 3 years I have not used any booster or fertilizer, and each year now, I just top the raised beds with more compost.

  8. is cow manure organic, when the grass ,corn and grain they eat aren’t.

    • Robert Pavlis says:

      That all depends on what you consider to be organic. This post will help: http://www.gardenmyths.com/organic-definition/#more-1

      When you are growing “organic food” and you want to ‘certify’ it as organic, then you do need to be careful of using manure from non-organic sources, but there are a number of exceptions, and not all countries use the same rules. If you are an organic farmer, you will know the rules so I can assume you are a gardener.

      From a gardeners point of view, manure is organic even if the cow eat corn that was not grown organically (defined as ‘organic food’).

      I suggest we should care less about the definition and instead try to understand the under lying facts. Let’s say a commercial pesticide was sprayed on the corn that will be fed to the cow. First of all it is very likely that this pesticide is less dangerous to your health and the health of the cow, than an organic pesticide, provided the corn was grown in Europe or North America.

      By the time the corn is harvested there is virtually none of the pesticide left in the corn plant. You see reports of pesticides in our food all the time, but the important fact is the concentration of the pesticide, not its presence. I am not saying a lab count not detect it. What I am saying is that the amount in the corn is so small that it does not harm us or the cow.

      Now the cow eats the corn and digests the corn. All of these processes further decrease the amount of pesticide, so that by the time you get the manure it is extremely low. Now you put it in the garden, and microbes digest the pesticide molecules further. You plants may absorb a very tinny amount of pesticide, but probably they don’t. Even if they do, plants also have the ability to degrade pesticides, and so by the time you eat the vegetable, there is essentially nothing left. Certainly there is a lot less than you will find in a cup of coffee–and we consider that safe to drink.

Trackbacks/Pingbacks
  1. […] Pine needles, particularly the small quantity from an XMas tree, are perhaps not as effective an Acid mulch as we have been brought up to assume Pine Needles Acidify Your Soil – Garden Myths […]

Please let me know what you think - Leave a Reply