Soil Acidity
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.
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.
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?
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.
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!
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
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.
Not sure what you mean by bark of perennials, but I doubt they acidify soil. Most plant material is close to neutral.
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.
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.
No. But shade will kill grass. Shade from the pint tree, and shade from too much mulch – the needles.
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!
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.
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.
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 ?
Not really since I know nothing about the two spots. did you take soil tests?
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.
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.
Lots of views posted…here’s another:
http://www.telegraph.co.uk/gardening/plants/trees/10633984/Do-conifers-make-soil-more-acid.html
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.
Are there any household products I can use to increase soil acidity for black hills spruce trees?
Thank you.
No. If you need a lower pH, then use sulfur.
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.
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.
Hmm. So why does the lawn under my pine trees, at my previous home (cedars) and now at the new one (spruce) both with branches removed up to a height of 8-10 feet, , grow sparse, beat up looking grass and an abundance of surly weeds? Is it the reduced light levels plus a shedding of rainfall? I was going to scatter some baking soda, but I guess this wouldn’t be helpful.
The main reason is that evergreen has fine roots near the surface of the soil, and they suck up all the nutrients and water. The soil under them is a difficult place for plants to grow. the reduced light is also a factor. Best thing to do is to plant tough plants. Hostas will grow there, but slowly. Adding extra fertilizer and frequent watering also help.
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).
pH value is just like inflation rate. It is a summary of the relative abundance H+ and OH- ions in the solution that “may” have some effects on certain chemical reactions. Using just pH as an absolute measure of how your soil will benefit or damage your plant is not at all meaningful, not to mention unscientific. BTW, plants need organic matter, not fertilizer. The more organic matter your have, the better the soil environment for any plant (and microorganisms) to grow. Decaying pine needles provide good organic matter that will certainly benefit any plant regardless of its pH.
pH is actually quite a critical parameter that affects the availability of nutrients in soil – there is no maybe about it and it is quite meaningful in selecting plants.
Plants do NOT need organic matter – all of your hydroponically grown vegetables are grown just on fertilizer.
The statement “The more organic matter your have, the better the soil environment for any plant” is also not correct. Having about 5% organic matter is good for most plants (too much for many rock garden plants for example), but you can have too much organic matter, which can then become toxic to plants.
Plants can’t use organic matter. Read “Organic Fertilizer – What is its Real Value” for more on this.
Pine needles can benefit plants – I never said they don’t.
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.
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.
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.
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.
Pine needles make good compost. Even fresh pine needles have very little acidity–much less acidic than rain.
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. https://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.
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,
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.
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”
That is the myth, but it is not true that pine humus is acidic.
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.
is cow manure organic, when the grass ,corn and grain they eat aren’t.
That all depends on what you consider to be organic. This post will help: https://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.
While this is slightly off topic, recently we have been seeing issues with aminopyralid residue contamination in vegetable gardens which has arisen through the use of manures as well as straw.
Aminopyralid is a selective herbicide that is used to kill broad leaf weeds. If sheep feed on the wheat stubble the aminopyralid appears to pass through them reasonably unaltered and the use of the resulting manure can damage many plants. Straw made from wheat stubble contains high levels of aminopyralid and is causing similar problems. Being residual once the aminopyralid is in the soil it is difficult to remove so one is left with limited growing options (corn and other monocots seem to be unaffected by aminopyralid)..
[…] 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 […]
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Soil Acidity
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