Some very desirable plants, like rhododendrons, azaleas, and blueberries, demand acidic soil, and many gardeners have alkaline soil, which is not suitable for growing 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 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 the 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 rehydrate 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 wheelbarrow, and a sample of each was placed into a 6″ plastic flowerpot. 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.
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 leaching 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.
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?
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 millions 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 is 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.
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.
We have applied Agricultural Sulphur once in the spring here in Woodstock Ontario for years as our soil after a soil test showed a PH 7.2. Our issue Robert is how often and how much should we apply to acid loving plants ,evergreens ,azaleas, rodo’s etc. without an expensive soil test every year? Just wondering what a general guideline would be? Excellent work you perform by the way.
Thank you
The only way you can know is to do a soil test. Next time you do a test, you could ask them for a recommendation for the amount of sulfur you should be using.
Okay, shall do. Thank you Robert.
Joe
Any chance you can share your ratios with us since the article is rather useless without! 🙂
The ratios are on the graph. 0, 25, 50,75,100 peatmoss.
Unfortunately the data does not mean much here because you did not use water with a neutral ph water. You used hard water that is a ph of 7.4 out of the tap, and has probably at least 200 ppm of Calcium carbonate thus having an extreme alkaline buffer effect. Thus, it is just as possible that the changes to PH over time is due to this buffer from your water and nothing to do with long term peat moss acidity effects.
Many people have soft water, or RO water with a mucher lower ph and lower amount of calcium carbonate which is the same as dolomite lime, which is actually put into peat moss to raise the ph!
I suggest try this again and remove the water variable.
I don’t disagree with your comments. But it does mean something, because I water with my water, and so it shows the effect in my garden.
If peat actually acidified the soil, then there would be other test results on line – I could not find any. And remember – even my acidic rain at pH around 5 can’t acidify my soil.
I have a better suggestion – why not test it yourself and report your findings.
Commercial growers use peat for acid loving plants but they also feed them a nutrient solution at pH 5.5. The advantage of using peat is that they can skip a feeding cycle on a cloudy day without risking too much pH drift.
It’s possible this idea about peat for lowering pH is copied from commercial hydroponic growers wich doesn’t really translate to soil growing at all.
Another thing, in my native language there’s more than 20 words for peat. I grew up in a town that roughly translates to old peat. I wouldn’t be surprised if there’s a big variance among peat sourced from different places. It’s gonna vary in organic matter percentage, humidity, ph, nutrient density etc. It’s a natural product after all so it’s gonna vary year by year, source by source.
We use what we call sphagnum peat moss hear – the only peat I have used. Last year in Ireland I found their peat and it is nothing like I expected. It is quite hard and dense. Lots of different peats.
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.
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.
I’ll have to look for some studies that show peat moss does work for acid loving plants.
Conifer needles will not help acidify the soil either, https://www.gardenmyths.com/pine-needles-acidify-soil/
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.
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
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?
Any powdered sulphur is the same, as far as I know.
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.
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.
I have seen the ones at the arboretum – they don’t look very healthy to me. There are two groups – one bunch is almost dead.
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.
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.
Do you know what your pH is? How alkaline is it?
That native soil at the Dallas arboretum, the northern part of the “Houston Black” clay deposit that runs along I-35, is around 7.8, but that’s not actually the key measurement. The key measurement is *alkanity*, which is related to ph, but not the same thing.
Baking soda has a ph of only 8.4, but you’ll notice it’s pretty darn effective at neutralizing acid. Why? Notice the bubbles that form when you mix baking soda with acid? Those bubbles are carrying away oxygen, thereby increasing the ratio of hydrogen. More hydrogen means higher Potential Hydrogen (PH).
peat moss will not acidify this kind of clay.