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.
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 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.
1) Photo Source: Iowa State University