Do pine trees make alkaline soil more acidic, or is that a myth?

gruntle(Fremantle WAus)May 15, 2007

I've always thought they would, thinking that being pine needles and pine trees, they would acidify the soil.

It might have been mentioned on some gardening show here in Australia recently...any ideas?

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lou_midlothian_tx(z8 DFW, Tx)

Well kind of...

As long as you don't use fungicide, pesticide, herbicide, chemical fertilizer, etc..

The leaves of any trees will encourage fungal growth in the soil which tend to lower pH. Fungi is very fragile. That's why chemicals are to be avoided...

    Bookmark   May 15, 2007 at 1:44PM
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NO!. Research done by many people including Dr. Abigail Manynard at the UCONN Agricultural Research Station in New Haven, Conn has shown that there is no significant change in soil pH after years of adding oak leaves or pine needles to that soil.

    Bookmark   May 17, 2007 at 8:25AM
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reg_pnw7(WA 7, sunset 4)

Myth. pH has more to do with the mineral makeup of the soil, and the amount of rainfall, and only a little affected by the vegetation.

So much of gardening lore is just wistful thinking.

    Bookmark   May 20, 2007 at 12:39PM
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Terms are relative so we use them to our advantage.
What is "significant".....what is "most"....what is "many people"...what is "what chemicals should be avoided"....

A tree, such as pine, having been adapted to grow best in an acidic soil, then it follows it has to have "some" affect to the soil.

Oak leaves have been shown to be highly acidic....and therefore, if given to soil that has a pH reading of above what the leaves have, HAS to have some effect.

what is much is much.

The question was put if you put an acidic plant into soil that has some degree of alkalinity, would it change it any?

Yes, it would.

It is forever stated that mulching an acidic plant--such as rhododendron or azalea, with pine needles, it aids in maintaining the acidity of the plant.
Change the pH? If the acidic measure was already higher than what the needles are, then NO, it wouldn't change it.
If you add a drop of boiling water to a pot that is already boiling, does it change the temperature?

    Bookmark   May 20, 2007 at 6:26PM
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gruntle what were thinking of growing? Fremantle soil is highly alkaline so trying to make it acidic would be just about impossible for any length of time. there are a lot of plants that will grow in alkaline soil though.

    Bookmark   May 20, 2007 at 11:55PM
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"A tree, such as pine, having been adapted to grow best in an acidic soil, then it follows it has to have "some" affect to the soil."

That's a logical fallacy. If A then B doesn't equate to if B then A.

I know for a fact that planting something that needs acidic soil in my soil leads to chlorosis (and sometimes to the death of the plant), not to the plant changing the soil to suit it. I wish I could do something as simple as plant something that requires acidic soil to lower my soil's pH, but it doesn't work that way.

The idea that since the soil under pine trees is acidic, then the pine trees must be causing the acidity is somewhat akin to the belief in spontaneous generation in medieval times. Since mice were often found around moldy grain, moldy grain must turn into mice.

    Bookmark   May 21, 2007 at 12:03AM
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what is much is much.

Jeanie everything you say is true but it is differences without significance.

    Bookmark   May 21, 2007 at 1:07AM
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gruntle(Fremantle WAus)

Thanks for all your answers. I am an organic horticulturist and nursery person and think it's weird how long these kinds of misinformation can stick around.

Adding compost and manure changes the pH when mixed into a vege growing area, so I guess it's just presumed that pine needles or whatever on top can fix the pH under them.
What really does it is probably healthy soil critters making humic acids in the soil. Because they use mulch and decomposition materials as food it was seen to be mulch that changes pH it, rather than the soil bacteria.
Does that make any sense?
It's good to hear what folk reckon!

    Bookmark   May 21, 2007 at 5:43AM
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The pH of a soil is determined by the amount of free radical hydrogen ions which have little to nothing to do with what is planted in that soil. Some plants have adapted to growing in soils of certain pH's and their presence may indicate an "acidic" soil, but they are not responsible for that soils pH. The leaves of most trees will show a very low pH, Maples test out to 3.2 while Oaks will be 3.7, but that too has little to do with soil pH.

    Bookmark   May 21, 2007 at 7:15AM
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The answer is yes. If you had a stand of pine trees, the needles that they drop are going to have an influence on the soil pH local to the trees. It's not the actual plant changing the pH, it is the vegetation decomposing and adding to the soil that can indeed alter soil pH.

    Bookmark   October 26, 2007 at 3:23PM
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Tyler, as has been shown be real research, Dr. Abigail Maynard, pine needles will have such a negligible afect on soil pH that they do not change the soils pH. Adding organic matter to soils will buffer the adverse soil pH, maybe enough to allow plants to grow where they normally would not, but digesting vegetative material is not going to changge your soils pH. Over time, the Soil Food Web may alter the number of free hydrogen ions floating around in your soil which will cause a change in soil pH.

    Bookmark   October 27, 2007 at 7:23AM
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I'm not sure how these gardening myths get started and perpetuated, but they tend to be very persistant and hard to dispell. Pine needles are not inherently highly acidic - were you to crush and extract the oils, they would test out with a pH of around 6.0-6.5. Once decomposed, they become nearly neutral (as does any other composted material). There may be some slight lowering of pH on the surface of the soil but the use of pine needles (or oak leaves or sawdust or pine bark or other woody mulch) will NOT have a significant impact on the existing pH of the underlying soil.

Possibly this myth became established because of the lack of other vegetation one normally sees under stands of pines. This can be attributed to both the thickness of the accumulated layer of needles or pine straw that will occur naturally over time as well as the length of time it takes pine needles to decompose. Both the waxy cuticle of the needles and the high concentration of lignens and turpenes they contain slows decomposition. This is also why pine needles or straw make such a great and long lasting mulch.

FWIW, there are many species of pine that prefer neutral to alkaline soils rather than those that are more naturally acidic. Obviously if pine needles were by nature that low in pH and had that much effect in reducing soil pH, these trees would be making the soil inhospitable to themselves. Doesn't happen ;-)

    Bookmark   October 27, 2007 at 10:02AM
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The myth came about because most evergreens, pine trees to most people, tend to grow best in acidic soils, therefore pine trees, evergreens, make soil acidic. People often do not grasp that these pine trees are growing there because the soil was acidic, first, not that the trees made it so. You get the same thing with Oak trees, and almost any other tree which prefers to grow in soils that tend to be acidic.

    Bookmark   October 28, 2007 at 6:58AM
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barbaranh(z4 NH)


if things like pine needles, pine shavings, oak leaves, etc. don't make the soil more acidic, then what should we use to fertilize plants like blueberries? I just had a load of pine shavings laid down around my blueberry bushes, something we've been doing for a couple of years now, to help them produce.

    Bookmark   October 28, 2007 at 12:05PM
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The best would be to grow blueberries in a medium with 60% to 70% sphagnum peat moss and the remainder a good quality compost.

The mulch will be of benefit to your plants but added nitrogen may be required to compensate for what is being used to decompose the mulch.

To change the pH otherwise would mean the application of elemental sulphur to the soil or growing medium. You would have to determine how much can be safely added on an annual basis. What is the soil pH now?

An agricultural representative once mentioned that after 20 years or so of the application of a sulphur fertilizer, a farmer reduced the pH of a field by 1 unit - an astounding amount. Most prairie soils in the area are 8.0 to 8.5.

    Bookmark   October 28, 2007 at 6:25PM
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barbaranh, blueberries are not so picky about soil pH as most would like to think. Blueberries thrive here in slightly acidic soil (6.3-6.5) without any further amending to decrease pH. Most New England soils should be similarly acidic in nature, if not more so, and are generally sufficiently acidic to grow blueberries without concern (certain varieties are native to parts of New England). Depending on what your soil's natural pH may be, anything more than compost may be unnecessary. Certainly the sulfur referred to above can reduce pH to some degree; cottonseed meal is also a natural soil acidifier.

And while they are great OM, pine needles, shavings or oak leaves are NOT fertilizers. If you need to fertilize your blueberries, any organic (or synthetic if you prefer) fertilizer formulated for acid loving plants will work. These are typically labeled ACR - azalea, camellia or rhododendron fertilizers.

    Bookmark   October 28, 2007 at 9:11PM
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The soil in New Hampshire is, because of the "normal" rainfall, naturally somewhat acidic, as is the soil around here and anywhere east of the Mississippi. Soils west of the Mississippi tend to be more alkaline because they get less rainfall and in the southwest even more so due to the lack of rain in those areas.
Adding lots of organic matter, peat moss, wood chips, Oak leaves, Pine needles, etc. will buffer what ever your soils pH is so things that maybe should not grow there will, if enough organic matter is in that soil. To drop your soils pH any significant amount requires something, such as sulfur, to do that, organic matter is not going to.
If you are in an area of the world that has an abundance of leaves that fall off trees every year there is no good reason to spend money on materials such as peat moss.

    Bookmark   October 29, 2007 at 6:57AM
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barbaranh(z4 NH)

Thanks. I always wanted to understand the difference between fertilizer and mulch. It sounds like compost is the answer to most of my questions on gardenweb! Seems you can never have enough of it.

    Bookmark   October 29, 2007 at 7:58AM
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michelle_co(z5 CO)

"A tree, such as pine, having been adapted to grow best in an acidic soil, then it follows it has to have "some" affect to the soil."

So why do pine trees predominate in Colorado's alkaline soil? Pine trees are one of the few things growing in the highly alkaline NM desert country south of me.

Is this another myth (or vastly over-generalized statement), that pine species 'grows best' in acidic soil?

From Pinion Juniper Country,

    Bookmark   October 29, 2007 at 10:55AM
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fertilizersalesman(z6 PA)

All organic decomposition that takes place in soils (or compost bins) is acidifying. That is, the decomposition process generates acid. If the material being decomposed is full of bases, it is possible that the end result is not acidic.

I am no pine needle expert, but I very seriously doubt that they are full of bases. Coniferous trees tend to suck their nutrients back into the tree before dropping old needles (unlike deciduous trees). Pine needles are very different from oak leaves for this reason.

So given the fact that decomposing needles generates acid, and that pine needles probably do not contain much in the way of bases, my guess would be that yes, they will acidify the soil. Whether it will be enough of a change to be noticeable is a different story. It would depend on the buffering capacity (the ability to resist change) of the soil.

Adding compost to the soil will help to buffer the soil as kimmsr says. However, if the soils pH is 5.5 and you want it to be 6.5 adding a lot of pine needle compost and increasing the buffering capacity is not helping. It is simply stabilizing the pH at 5.5. There will be other benefits from the compost, but it is not a magic cure-all. The only way to raise the pH is to add bases (lime).

One other thing, yes, pine trees do acidify soil to a certain extent. Plant roots interacting with soils give off acidic compounds.

    Bookmark   October 29, 2007 at 12:54PM
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michelle_co, Most all trees prefer growing in acidic soils, but they will grow in almost any soil, and some evergreens species actually do prefer alkaline soils. So it does, somewhat, depend on which plant species you are talking about whether that plant prefers growing in an acidic or alkaline soil.
Changing soil pH can be done with organic matter and no lime, apparently, since my soil 35 years ago tested with a pH of 5.7 and today after adding lots of compost and other organic matter (shredded leaves) and no lime, the soil pH today tests out at 7.2. 35 years ago the level of Magnesium was very low while the level of calcium was optimal and today they are in sync.

    Bookmark   October 30, 2007 at 7:08AM
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fertilizersalesman(z6 PA)

This is very interesting kimmsr, assuming your 1972 pH test and your current pH test are accurate, you manged to lime your soil without lime. I did some reading and it turns out you are correct (who would have thought?); some composts are alkaline enough to raise soil pH over time. Others lowered soil pH. It depended on if there were enough bases in the original material to neutralize the acids that formed during the decomposition process. What do you compost? Also, just out of curiosity, what is the CEC of your soil? And what is your water like?

    Bookmark   October 30, 2007 at 12:41PM
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Well, studies have already been done, one being a 30 year study on particular question.. the results are of course the soil becomes more acidic with configers, and more alkaline with broad leaf trees,, the key being the amount of calcium in the leaves,, link is here

    Bookmark   May 17, 2014 at 4:11AM
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If you were to crush various tree leaves and evergreen needles and then put them into some distilled water and allow them to steep for 24 hours and then test that water with a good quality pH meter you would find that the water that started out at a pH of 7.0, neutral, was now registering in the 3.0 to 4.0 range. Adding compost to my soils that most often test at a pH of 5.7 with little to no organic matter in that soil results in my soil testing at a pH of 7.2, whether that compost started out as mostly Oak leaves, Maple leaves, or the needles from different evergreens. The soil under the various evergreens planted around will be 5.7 pH as will the soil out and away from those evergreens that has not been amended with compost, or other forms of vegetative waste. So after a number of years of mulching with Oak, Maple, Birch, Beech, Chestnut, Chokecherry, and many other deciduous tree leaves the soil so covered with that mulch has a pH change of from 5.7 to 7.2, as long as the humus level (residual organic matter) is in the 5 to 8 percent range.
Soil pH is affected by a great many more things then just which trees may be growing there, although different trees do prefer to grow in soils with different pH's. Some evergreens (pines to many) do prefer soil with lower pH's and some deciduous trees do prefer growing in soils closer to neutral.

    Bookmark   May 17, 2014 at 6:59AM
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tropical_thought(San Francisco)

This is a myth, sure pine trees drop needles and over time the soil will improve a bit, but not enough. I knew this dumb person who wanted to add lime, she believed her pine tree had made her alkaline soil acid and she needed to add lime. When the soil is already alkaline it won't turn acid due to pine tress, I looked this up, but what is going is the soil in pine forest was already acid to the rocks surrounding the area had broken down creating acid soil, that and not the little bit of pine needles. Even with much coffee composting, my soil won't become more acid, because it is so alkaline all changes are off set by the type of soil. It is very hard to change soil ph by even one degree. I also add other things to create more acid like sulfur, but my soil is always alkaline unfortunately.

    Bookmark   May 18, 2014 at 1:00AM
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Let's get to the meat of this discussion once and for all. Soil pH is NOT determined by the type of plant material growing there. Acidic soil is dependent primarily on the underlying mineral content of the soil and the amount of rainfall. Soils with a mineral base composed of mostly igneous rock tend towards an acidic pH as do those with higher rainfalls - like much of the eastern seaboard and the Pacific Northwest. Areas with more sedimentary zeolitic rocks as a base - like limestone or sandstone - tend towards more basic or even alkaline pH. And areas with very xeric climates - like much of the southwest - are also more in the alkaline pH range.

The use of synthetic fertilizers can also alter pH to some degree (usually more acidic) but the effect tends to be temporary - soils will tend to return to their natural pH. Accumulation of plant debris - both coniferous and broadleaf - may have an impact on the pH at the soil surface only but it will have no significant change on the chemistry of the underlying soil.

Documentation of this position is very well established. Plants only grow in acidic or alkaline soils because they prefer those growing conditions, NOT because they create them.

    Bookmark   May 18, 2014 at 12:47PM
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madtripper(5/6 Guelph)

Gardengal48 is correct. pH is most dependent on the rock type that makes up the soil.

consider this; regular unpolluted rain is quite acidic pH=5.6. it has been falling on alkaline soil for millions of years. Yet the soil is still acidic. the link below has more details on this.

Here is a link that might be useful: Do pine needles acidify soil?

    Bookmark   January 22, 2015 at 11:14AM
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toxcrusadr(Zone 6a - MO)

I think you mean "the soil is still alkaline". I agree with your point though, and gardengal's. Rain, for what it's worth, is nearly distilled water which has no buffering capacity. The slightly acid pH is due to CO2 dissolving in the water. There is so little acidity there that it has little hope of changing soil pH. Instead it will dissolve a teeny tiny amount of alkaline soil minerals and carry them downward into the ground.

    Bookmark   January 22, 2015 at 12:02PM
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I understand that maple leaves are alkaline. Is Canadian soil also alkaline? I doubt it.

    Bookmark   January 23, 2015 at 8:30PM
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Maple leaves, freshly fallen, have tested at a pH of 3.2, quite acidic. Oak leaves tested at a pH of 3.7, and other trees leaves have tested in that same pH range, 3.0 to 4.0.

    Bookmark   January 24, 2015 at 6:59AM
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tropical_thought(San Francisco)

I am coming back to this statement from above
"An agricultural representative once mentioned that after 20 years or so of the application of a sulphur fertilizer, a farmer reduced the pH of a field by 1 unit - an astounding amount. Most prairie soils in the area are 8.0 to 8.5. "

I want to know what the optimal amount of sulfur to add to the soil to make it more acid and how often should one add it? My soil is made of all sand and needs to be more acid. I don't want to over do it, so I have only added it a couple of times.

    Bookmark   January 25, 2015 at 10:35AM
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There are many things that determine what a soils pH will be, and all have been clearly laid out here by several contributors. What the soil is composed of, rainfall, and amount of organic matter all play a role in how many free Hydrogen ions are in that soil and soil pH is a measure of that. What is needed to change a soils pH depends on where it is and what is meant to be done. Adding lime (Calcium Carbonate) will reduce the free Hydrogen ions and raise the soils pH while Sulfur adds more of them and lowers the soils pH.
A good reliable soil test from a good soil testing lab is a good source of information and they should be able to tell you how much Sulfur needs to be added to make the needed changes. Adding 1 pound per 1,000 square feet when 10 pounds is needed will not do much of anything. Throwing a handful or three of something is more a waste of time, money, and energy than any help.

Here is a link that might be useful: understanding soil pH

    Bookmark   January 26, 2015 at 6:27AM
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toxcrusadr(Zone 6a - MO)

And if you don't get recommendations from a local lab for whatever reason, it's a fairly simple matter to Google 'soil sulfur application rate', and get links like this one to a Soil Sulfur Garden Calculator. It's probably not precise since soils vary so much, but you'd want to home in on it over several seasons anyway so you have an opportunity to adjust.

Here is a link that might be useful: Sulfur Calculator

    Bookmark   January 26, 2015 at 11:38AM
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tropical_thought(San Francisco)

Thanks that was really cool, now if I just knew how many square feet my planting areas are. I don't need it all over my garden, as some areas and just walking areas with no lawn and no plants.

    Bookmark   January 26, 2015 at 7:31PM
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It is quite simple to calculate the square footage of any area. Multiply the width times the length and that will equal the square feet. If you have several planting beds calculate each bed and add them together to get the total.
For example, if the planting bed is 4 feet wide and 20 feet long you would multiply 4 x 20 to get 80 square feet. If you have 2 beds of that size adding them gives you a total of 160 square feet.

    Bookmark   January 27, 2015 at 6:49AM
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toxcrusadr(Zone 6a - MO)

Want us to come out there and sprinkle it on for you? LOL, just kiddin.

    Bookmark   January 27, 2015 at 1:50PM
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