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Does Peat Moss Acidify Soil?

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 I have yet to find anyone who provides some data to show what really happens. This sounded like a perfect experiment for Garden Myths.

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.

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.

The experiment was started Sept 13, 2017. Final readings will be taken in spring of 2018 and added to this report.

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.

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 this 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 will not be acidified by adding peat moss. Very sandy soil, which contains few minerals, may be acidified and slightly acidic soils may be acidified more with peat moss.

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.

If it does work it is not because the peat moss acidifies the soil.

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

Acidifying Soil

What about other recommendations for acidifying soil? I have discussed some of these previously:

Compost Creates Acidic Soil

Coffee Ground Acidify Soil

Increasing Soil Acidity

Do Pine Needles Acidify Soil?

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

15 Responses to 'Does Peat Moss Acidify Soil?'

  1. Paul Alaback says:

    This post makes several important points, but important one is the imprecision in making suggestions like “add peat moss for growing acid loving plants”. No matter how you do it, if plant is not growing well you should first measure pH and not assume that peat moss or sulfur did what you wanted it to do. If it is an alkaline soil it will require additions of peat moss and/or sulfur over time. I have had great results with peat moss in my soils, but that does not necessarily mean it will work in other soils or climates. I think in most instances organic matter is a key part of the ecology of acid loving plants and how they uptake nutrients, so this is an additional factor to consider in growing these kinds of plants in neutral or alkaline soils, and also explains why simply adding sulfur will not always work. Part of the art and science of gardening.

  2. Paul Alaback says:

    Peat moss for growing acid loving plants is recommended by ag extension services as well as by most nurseries in the Rocky Mountain region which tends to have alkaline soils. This is mostly based on experience from growing these plants using different amendments. I can’t find good studies that explain the chemistry of why this is so. I agree with others that the main effect is probably not pH, because as you found its effect on pH is either small or transient. We grow blueberries in a buried bale of peat moss, as recommended by local fruit growers, with great yields. Blueberries in native soil without amendments rarely survive for more than one growing season here. Some people have been able to get good yields for 10 years or more with one bale, so it can last a long time. In this case we use normal irrigation water but do use fertilizers that are recommended for acid loving plants, so that might help maintain acidity or whatever it is that peat moss provides. I also mulch them with conifer needles (old Christmas tree boughs) over the winter, which might help too. A better experiment would be to grow blueberries in a variety of soil amendments and see what works best. I found several experiments like this for restoration studies, but again they were not able to explain why peat usually works so well compared to other amendments. A good test of comparative soils chemistry with such an experiment might be illuminating.

  3. Michael says:

    Thank you for the article and experiment. So many articles and YouTube videos recommend Peat Moss to acidifying soil. Nice to see a science based answer.

  4. rogerbrook says:

    You make good sense Robert and I suspect the main advantage of the peat is physical rather than chemical.
    I would expect raised beds of peat where they might receive less alkaline drainage water to retain their acidity longer.
    Some of our Somerset peats are of alkaline nature

  5. Joe says:

    Great article. We use to use peat moss 20 years ago as was recommended by nurseries. Since then we have used Agricultural Sulphur with good results on our acid loving plants (azaeleas etc). My questions is,is Agricultural Sulphur the correct Sulphur to use? Or is all sulphurs the same?

  6. John Law says:

    Rhodies, azaleas and other “acid loving” plants grow just fine in in soils below pH 8.2 in CA. Geologically young, high CEC soil is probably why. Rhizosphere pH is probably much lower than bulk soil pH.

    • They won’t grow in my 7.4 soil – nobody around here grows them successfully. Rhizosphere pH can be quite different.

      • Kishka says:

        Yes, you are right, the rhizosphere pH can be quite different in Wellington county (Guelph, Cambridge, Kitchener, Elora). As a matter of fact, the latest agriculture Canada report states that the A horizon fluctuates between 6.0 and 7.3 and the B horizon between 6.8 to 7,5 (most roots in the Ah). (http://sis.agr.gc.ca/cansis/publications/surveys/on/on35/on35_report.pdf). So basically, the rhizosphere pH ranges from 6.0 to 7.5, slightly acid to neutral to slightly alkaline.

        Now, as far as the Rhodies, azaleas and other “acid loving” plants that nobody around here grows successfully, maybe a trip to the University’s Arboretum ‘Leslie Hancock Memorial Rhododendron Collection’ should be in the agenda for next spring. Last time I was there, those plants were performing very well.

  7. Kishka says:

    Peat Moss is somewhat acidic if you test it right out of the bag, but the acidity comes from tannic and humic acids, which are weak acids that easily bind with other items in the soil, furthermore both acids with very short “half life”.
    While all organic material will go through an acidic phase while decomposing, it does not mean that they are long-term contributors to overall soil acidity. I do not know of any reputable garden centre or website that advertises long term pH acidity gains from peat moss use.

    • Lots of people recommend peat moss for acidifying soil.

    • janet says:

      It may be dated information, but the Dallas Chapter of the Azalea Society recommended a blend of half moistened peat and half finely shredded pine bark mulch for the raised beds that contain their acid loving plants. This seemed to do the trick for the Dallas Arboretum which designed their beds on their advice and has thousands of Azaleas in an area of alkaline soil.
      This advice is what I used to grow my azaleas as well. They have lovely green leaf color and our water is supposedly from an area west of town that is alkaline.
      Thanks, Robert for giving us more details on why this works.

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