Does Peat Moss Acidify Soil?

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Robert Pavlis

Some very desirable plants like rhododendrons, azaleas and blueberries demand acidic soil and many gardeners have alkaline soil which is not suitable to grow these plants. The most common solution I’ve seen is to mix peat moss with the soil to produce an acidic environment.

Peat moss is acidic so it makes sense that if you add some to your soil, the resulting soil will also be more acidic. But is this really true? How long does the acidity last? Can gardeners with alkaline soil use peat moss to grow rhododendrons, azaleas and blueberries?

Does peat moss change soil pH?
Does peat moss change soil pH?

Does Peat Moss Acidify Soil?

Many people claim peat moss acidifies soil but when I did this experiment I found no studies that reported the change in soil pH after adding peat moss. This sounded like a perfect experiment for Garden Myths.

More recently I have found some studies on this which are reported below in What Does Science Say section.

Experimental Setup

My soil is definitely alkaline with a pH of about 7.5, as reported recently by a local testing lab. My own testing a few years ago gave 7.4. The soil is about 40% clay and contains a lot of limestone which causes the alkaline condition.

Some purchased Canadian sphagnum peat moss was mixed with tap water and allowed to re-hydrate for 24 hours. The water was then removed, leaving behind moist peat moss. This wetting process was done because dry peat moss absorbs water very slowly, even when mixed with soil.

The peat moss and soil were mixed into various ratios in a wheel barrow and a sample of each was placed into a 6″ plastic flower pot. These were added to my outdoor collection of potted plants and received whatever water the plants got. During the initial 46 days, we had very little rain and the pots were frequently watered using my tap water which is fairly hard. After the 46 days they received only rain and snow, as nature provided.

Growing Great Tomaotes, by Robert Pavlis
Does peat moss acidify soil? Various mixtures of soil and peat moss were tested over time to monitor the pH, by Robert Pavlis
Does peat moss acidify soil? Various mixtures of soil and peat moss were tested over time to monitor the pH, by Robert Pavlis

Samples were taken at various intervals and analyzed for pH. Distilled water was added to the soil, mixed, allowed to sit for 20 minutes and then measured. Three pH readings were taken for each sample and averaged to get the final values shown in the above chart. The pH meter was a lab grade, Denver Instruments, model 220, which was calibrated using three buffers at pH 4, 7 and 10.

The measurements for day 0 were taken right after mixing the soil samples and before any watering. The pots were then watered.

Results of Adding Peat Moss to Soil

The chart above clearly shows that peat moss is acidic, with a pH of about 5.5. After only 1 day the pH was 6.5 and remained there for the duration of the test. I suspect that the sudden increase in pH is due to the addition of alkaline tap water and the leeching of acids out the bottom of the pot during watering.

The various mixtures of peat and soil resulted in a Day 0 pH that followed the ratio of peat in each pot; more peat moss – more acidic. After only 1 day the samples containing soil were already seeing a significant increase in pH. Within a week, the acidifying capability of peat moss was lost completely.

What Does Science Say?

One study looking at acidifying soil for growing blueberries found that peat moss did reduce soil pH. Unfortunately, there is no description of the type of soil or the type of water used.

The authors did conclude that, “when peat moss was mixed with the soil, the pH increased with time, but the rapid change was not observed. Therefore, it is necessary to continuously treat peat moss in the soil to maintain acidic soil”. Using peat moss to reduce pH is a short term solution. Using sulfur works much better.

 

graph showing pH of soil steady at 7.4. Soil mixed with various amounts of peat moss with pH between 5.5 and 7
pH of soil mixed with various amounts of peat moss, source: Lee et al

Another study planted blueberries in the field using peat moss added to the field soil and found that it lowered pH and that the pH stayed lower during the 2 year study. Unfortunately, they don’t report data for unamended soil, so it is difficult to reach conclusions about the effectiveness of peat moss changing pH.

Does Peat Moss Acidify Soil?

Rhododendrons need acid soil and peat moss is commonly recommended as an additive to acidify soil.
Rhododendrons need acid soil and peat moss is commonly recommended as an additive to acidify soil.

Keep in mind that my experiment looked at only one type of soil; an alkaline soil containing lots of limestone. When rain falls to earth, the acid in the rain is quickly neutralized by the limestone. Rain has a natural pH of about 5.5 and pollutants can lower this number. This has been happening for million of years now and our soil is still alkaline.

Adding peat moss to the soil is really no different than having rain fall. Both are acidic, and both get neutralized by the limestone. The soil remains alkaline.

Soil that contains minerals similar to limestone are not likely to be acidified by adding peat moss, at least not in the long term. Very sandy soil, which contains few minerals, may be acidified and slightly acidic soils may be acidified more with peat moss.

Plant Science for Gardeners by Robert Pavlis

If you have alkaline soil and feel the need to acidify, sulfur is the best option.

Does Peat Moss Help Acid Loving Plants Grow in Alkaline Soil?

I don’t know the answer to this. Many people claim that this does work, but these people have not run controls nor do they usually report the pH of their soil. Peat moss does make the soil very loose and airy – something Rhododendrons like, so maybe this extra aeration is the reason peat moss works.

Peat moss may also reduce the pH in the short term, but other techniques, such as sulfur, are required to keep the pH low. Gardeners claim that pH in soil can be changed by a variety of things including compost, coffee grounds and pine needles. None of these work.

To find out more about peat moss, have a look at Peat and Peat Moss – The True Story.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing 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!

53 thoughts on “Does Peat Moss Acidify Soil?”

  1. Great experiment. A limitation is that you have a very limestone soil and all that calcium carbonate may be too much for peat to acidify. It would be interesting to see how this does for a different soils.

    Reply
  2. Hi, Robert
    Still out there I hope?
    I thought I had followed this rabbit to ground, but here I am again. I have serially killed blueberries for decades. My next neighbour has about 100 of them shooting up over his head, covered in luscious berries. Pictures of vigour and health. Never loses a one. So why don’t I do what he does? He does nothing. He has a peat bog.
    I heard, and observed, that blueberries grow naturally in sand or in peat. Odd. Don’t seem the same at all. Also, the Northern Shield, of granite, was mentioned. Aha! I need me some of that!
    But it turns out not to be acidifying, especially. Hmm? Too bad. An weird.
    Finally, I think I have come to enlightenment, the key to understanding. It isn’t what you add. It’s what you don’t add. The granite isn’t especially good for blueberries. I have granite. Even the peat may not be(tried that, you bet), or the (presumably) granitic or silica sand where they thrive. Add all you like. Hardly helps a bit, mixed in.
    But what those materials do do right, is they aren’t limestone. At all. And so my blueberries survive better in their pots(full of peat) than they do if I plant them out in soil. They would do better in a peat bog(which of course they are probably also adapted for) but for sake of this argument, because there, they are insulated from the limestone rocks. People are growing them in sawdust or bark mulch(with extra fertiliser). They would do well in a peat bale. In sand of certain kinds. Possibly in fibreglass insulation, or plastic balls, almond shells, I don’t know. But, please, please, don’t mix or add even a little bit of that awful limestone soil or rock. Use peat or compost or whatever, but use it to insulate the plants from the alkaline world out there. That’s my theory. That appears to explain everything.
    Where I live, rain forest by winter, desert by summer, water is also an issue. Winter flooding is fine. Summer drying is not. I sure like the buried bale idea, leaving the plastic on I take it? Make your own mini bog. I sure like it! Hope they do too! Thanks for all the ideas, everyone!
    Cheers!

    Reply
  3. @Matthew You’re right the acidity / pH of the water would matter in the general case.

    In this particular case, he’s said it’s*limestone* soil.
    Limestone is calcium carbonate. You know what else is calcium carbonate? Tums. Antacid.

    He was literally trying to acidify 30,000 pounds of *antacid*. The pH of the water hardly matters when he’s pouring it on a pile of Tums aka limestone.

    Aside from the fact he first soaked it and *threw out the water*. Ph is defined as the hydroxide balance *in water*. So he literally threw out the alkalinity as the first step of the process.

    Reply
  4. This test does nothing. It shows your tap water is alkaline. And Obviously not suitable for your garden soil!! If this is the case. You should be using a fertilizer to offset the alkalinity, something high in ammonium which will help acidify the soil and offset your tap water. Talk to promix theylll help you out lll

    Reply
    • How do you know it was the water and not the alkaline soil? You don’t. The point of the post is to show that claims about peat moss acidifying soil are not true in cases where the soil is alkaline for a reason.

      Also, the change in pH was not due to the water because the change happens differently in each pot, and each pot had varying amounts of peat moss in it. The change was in relation to the amount of alkaline soil.

      It is a valid test, because I do use this soil and this water in my garden.

      Reply
  5. Major, you make several good points.
    I see you saved the main one for last, then wrote it in all capitals – you can’t acidify limestone. 🙂
    The closest one could get would be to dissolve all the limestone, the acidify what’s left after all the limestone is gone.

    An additional issue is that he soaked the peat, then threw out the water. Ph is of course only defined for aqueous solutions (water). His first step was to get rid of all the low pH water, the acid. When you’re trying to apply acid to something, throwing away the acid isn’t a great first step.

    I know Robert has put a lot of effort into trying all different ways to acidify “akaline soil”, so at this point it’s gotta be tough to accept the fact it could never work – because he’s been trying to acidify antacid. (Limestone and Tums are the same compound).

    Reply
    • “you can’t acidify limestone.” – absolutely correct.

      If that first bit of water was the only acid that comes out of peat moss then the first rain would end its ability to acidy soil. It does not change the outcome of the experiment – besides we don’t know how acidic that water was.

      “Robert has put a lot of effort into trying all different ways to acidify “alkaline soil”,” – not really.

      You missed the main point of the post. I knew peat moss would not acidify my soil – it is basic chemistry.

      But people all over the internet and gardening blogs keep saying it will acidy alkaline soil. The chemistry is quite clear on this – it won’t. But when I searched for data to prove it would not work, I found none. So I designed this simple experiment to show people that peat moss does not acidify alkaline soil.

      Reply
  6. @alex unfortunately this experiment doesn’t tell you anything about what will happen with your clay soil. This test used limestone soil. Limestone is not only alkaline, but it’s what’s called a “buffer”. Nothing is going to turn *limestone* acidic, unless you add so much acid that you’ve dissolved all of the limestone. Not peat, not sulfur, not battery acid.

    Trying to make acidic limestone soil is like trying to make dry water. Not gonna happen.

    Your clay soil is probably not composed of limestone, so peat may work very well for you.

    Reply
    • You seemed to have missed the point here.

      This experiment was done with my clay soil.

      Correct, peat moss won’t acidify alkaline soil due to the limestone or other alkaline minerals – that is the point I am making.

      Reply
      • My response was to Alex. Hence starting it with @alex.

        Your limestone doesn’t tell Alex anything about his soil.
        The fact that limestone (aka Tums antacid) cannot be made acidic (without converting it to CO2 and water and having it drift away) doesn’t tell us anything about whether the clay *Alex* has can be made acidic.

        You actually didn’t test whether peat moss can acidify soil.
        What your test showed is that nothing can acidify calcium carbonate aka Tums aka limestone. Alex is mistaken in thinking that means it can’t acidify their soil.

        Limestone + acid = CO2 + water + calcium chloride (road salt).
        *Alex* doesn’t learn anything useful from that because *Alex* isn’t working with limestone.

        Reply
        • Any alkaline clay will have the same issue. Something makes it alkaline, and the acidic rain that has been falling for thousands of years has not made it acid – a bit of peat moss won’t either.

          Reply
          • No air, not all alkaline compounds are carbonates.
            A simple way to tell is to put a little vinegar or other acid on it.
            If it bubbles, it’s a carbonate. Those bubbles are the oxygen (basically the acidity) literally drifting off into the air. That’s called a “buffer”. It’s not a matter of ph5 + ph8 averages out to ph6 when you have a buffer because molecules actually LEAVE the system. Most commonly leaving as CO2, because the buffer is a carbonate.

          • Sorry, but there are a few problems with your test.

            1. By measuring in H2O you disregarded reserve acidity. You should have measured pH in 1M KCl instead.
            2. Ratio of KCl (or H2O for that matter) should be constant, in Europe standard is 10g of soil per 25ml of KCl solution. It should rest for at least an hour, even better a day before taking measurement (shaken from time to time).
            3. Your peat is of mediocre quality. I’m using light coloured Latvian sphagnum peat and it has pH of 3,5/4,5 (KCl/H2O). I’m pretty sure that you can source material of comparable quality in Northern America.
            4. Looking at your chart one can notice that after one day pH of your peat rose by one. This is bonkers. Once again, poor quality of your peat and/or really hard water. In order to cut down variables, you should have watered your mix with distilled water (or at least rainwater).

            The title should have said “Does Peat Moss Acidify Soil CONTAINING A LOT OF FREAKING LIMESTONE?”. This is only a small subset of alkaline soils, if not the most challenging of them. I have successfully lowered pH of heavy clay soils with sphagnum peat, although (from 7 to about 6, tested after a month) 50-50 mix – although admittedly the main reason for mixing with peat was improvement of the texture.

            So can you lower pH by using (quality) sphagnum peat? Sure.
            Is it feasible? Depends, however it can be nice bonus when using peat to improve the texture.
            And if you need really low pH you can just put peat in trenches as mentioned earlier. Just mix with pine/fir coir for drainage or use coarser grades of peat. I like mixing 20-40 mm with 7-20 mm grades (no “dusty” – 0-5mm fraction), lots of air in the substrate for good root growth.

          • 1) Measuring soil pH using water is a very standard way to measure it. https://extension.oregonstate.edu/measuring-soil-ph
            2) This is Canadian sphagnum peat – that is not “mediocre” peat moss – it is actually considered one of the best types and it is the used all over North America.

            It is good to see you did your own testing. Please provide the link to the documented procedure and results. We would love to see them.

  7. Under your circumstances, with limestone-rich sol and alkaline water, you can use sphagnum peat for growing Rhododendron, but in a special way. You dig trenches, about two feet deep, full these with peat and plant them directly in that peat. You can add a small amount of fertiliser. Then collect rain water from the roof and give the plants that.
    If the ground is dry and/or very alkaline you can put plastic foil in the bottom before you add the peat, then you get wetbeds that might work even better especially if summers are dry and hot.

    Reply
  8. I am adding a lot of peat to my basically clay soil and was wondering what the effect would be on the PH. I was worried that it would contribute too much acidity. As you are scientifically trained I have confidence in your experiment. Somethings just get passed on without any questioning, for instance planting your asparagus three feet deep. Well, the wild asparagus growing along the road is not that deep. Thanks for the info.

    Reply
    • Peat can certainly acidify many types of clay. Nothing can acidify limestone. Limestone + acid turns to mostly CO2 and water. That doesn’t apply to your soil, unless your soil is limestone.

      Reply
  9. If this experiment is specific to YOUR water, why would you waste your time doing an entire write-up that is essentially useless to anyone else.

    Instead you should of documented the process so people could test their own soil.

    The information you present can’t be duplicated by anyone without getting different results.

    It’s just disappointing to finish reading the article and not actually obtain any new information. It would be CLEARLY labeled as specific to YOUR garden.

    Reply
    • It is not specific to my water. If it was, all ratios of peat moss to soil would give the same results. And the peat only container would also show the same result as the peat/soil mixes.

      So it is clearly the soil, not the water.

      Reply
  10. Your hard water ruined your experiment. You’re adding a strong alkalizer with *massive* buffering capacity (as evidenced by the raise in 100%-peat’s pH in your data) at the same time as you claim to be testing if peat is an acidifier. Can’t you see you’re adding 2 variables working against each other? That’s ridiculous. You said up above that this is relevant to your garden because you use your water… true, but that soil/water combo are rare, and this does NOT support your broad sweeping conclusions in the article or answer your questions at the start. Myth busting busted.

    Reply
    • That may be a valid point. However, our tap water is hard because it washes through out soil, which is full of limestone. The test mixture will get limestone either from the soil or from the water.

      When acidic rain hits our soil, it is also made alkaline by this soil. So even rain falling here will be alkaline once it is in the soil.

      Reply

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