Pine Needles Acidify Your Soil

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

This common, incorrect, advice goes as follows: if you 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.

White Pine ready to drop its fall needles, Source: Iowa State University

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.

Your soil has a certain pH level. pH 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, ie make 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.

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 from a tree are “acidic”. By the time a pine needle gets old and is ready to drop off the tree it is barely acidic. After a few days on the ground, it looses its acidity completely. The brown pine needles you will likely collect from the forest are not acidic.

There are two important points here. Since your source for pine needles is probably not fresh 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 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 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.

 

 

7 Responses to 'Pine Needles Acidify Your Soil'

  1. 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.

  2. 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.

  3. 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”

  4. 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.

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