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 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.
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.
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 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.
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.
Hi Robert, hope you are well. It seems every few years when I get the garden bug I find one of your articles like this one and I always find them helpful, so firstly I wanted to say thank you for your time and efforts in helping to educate.
Regarding the experiment, not only is it helpful, but also the comments that follow always seem to be just as interesting, just as educational, and a pleasure to read as well. It’s always nice to see when people of different backgrounds can have issues with each other’s opinions, illustrations, etc. and still be quite friendly. I chalk this up at least partly to the environment that you have fostered here on your website, so again I thank you for such efforts, and extend that thanks to the other contributors who indeed offer great insight as well.
Now, if I may offer a bit of my own, starting with an observation which, unless I’m mistaken, has yet to be made, I think the main problem some may have with both the article and the experiment/results, is simply a precision of wording i.e. “peat moss will not acidify this kind of clay”
But before I address that particular statement and other similar ones, I want to start by talking about the strengths of your experiment as I see them. I understand where some might say “all soil is different, so this basically only applies to your local area. or even just your garden”, but I believe we can gather quite a bit more than that from this experiment. For instance, you say you purchased peat moss from Canada, well, from my experience, everywhere I’ve been from New York, to the Mississippi and down into the Keys, if you go into any big box store, or even your local hardware, you are probably buying peat moss from Canada. That stuff is sold everywhere in middle America and I’m yet to find even a small pouch of peat moss from anywhere else, so yes, in my mind your peat moss is just as good as mine, and just as good as Joe’s etc. And please, correct me if I’m off my rocker, in those regards.
But even if your peat moss is a bit different from mine, this brings me to another point where I see validity in this experiment, because in my mind, from my own experience, experiments, and research, we could use just about any plant that was once green, then has turned brown and been mulched to a consistent state, like that of peat moss; grass clippings, forest leaves, pine needles, bark, you name it and soak that in water- even if it’s hard water- then that water will at least within a few hours show itself to be acidic, and from my limited experience and equipment, the acidity will be very close to the range of wet peat moss. Which leads me to my next point, that peat moss may, again from my understanding, have some other debris, such as sticks from trees, pollen, etc. and so 100% Sphagnum isn’t guaranteed.
And if true, then this, I believe, would lend credibility to the notion that just about any plant material that has been through a process of dying, then drying and turning brown, will be acidic when wet, or more accurately worded- will turn water more acidic. Perhaps another critical component is indeed that drying process as without it, we may not get the same acidic effects from the decaying matter (more tests need to be run and made available on this).
And sorry if I’m being too windy here, but if the above notion is true that all dried brown decaying matter will make water more acidic, then this indeed lends yet more credibility to your experiment, as we can now make safe generalizations about our own brown compost source, once we understand a bit more about the products we are using.
The same goes for your “limestone” soil, and the clay, which I presume is in there as well. Just like the peat moss that comes in the green and white packages all being pretty much the same, we can quickly make some generalizations about the soils of many geographic regions simply by asking a few questions. What does the soil look like? Can you shape it like pottery? Are there caves, lakes, streams nearby? Do you have limestone, or shale in your area? Again, throughout middle America, we know much of the ground, and hillside terrain is rich in lime. We know most valleys in most states with a stream nearby will be full of clay, and probably saturated with lime- ish type of stone dust, leached into the soils for who knows how long.
Another point you make, which for some reason seems to go unnoticed, is that you are 100% correct when you say that even highly acidic rain will be neutralized quickly once it hits soil that is heavy in lime, which is much of the eastern half of the US that I’m aware, and probably in many other places around the world as well.
So just knowing that you used hard water tells us something, again making this a useful experiment. You added high PH water to peat moss and what happened? It turn that water acidic- or low PH- at least for a time. And regarding that use of hard water, it immediately made sense to me why the PH of the 100% peat pot raised from what ever it was to it’s current state at time of results readings. Now of course, in my mind it would have have risen eventually even if you had used neutral or acidic rain water, because this is probably, again the biggest issue I think people are having, is the precision of words or their, perhaps loose definitions.
To the others who may think this experiment was useless, or at least not very helpful, I would challenge you to ask Robert, who I’m sure would be open to reason, if he thought his greatest talent was in writing titles, or defining terms. On the contrary I think he might agree that his background as a chemist is what lead him to conduct the experiment in the first place, and that he might have worded this or that in a better way, but the point still remains, his experiment has validity and we can all learn something from it, especially when we continue to share insights about his experiment that perhaps even he himself may have overlooked.
So, to sum it up, which I’m sure, if you are still reading you would like me to do hopefully at some point, I would say to anyone listening that the results to Robert’s experiment are quite conclusive- Will adding peat moss to soil lower the PH (of the water that’s in it)? And that answer is a resounding a definitive YES- BUT, ONLY FOR A TIME, which his chart and experiment clearly show, without him having to say it.
And indeed, from my experience, this is and always will be the case. ALL organic matter that is decaying and is acidic will eventually be neutralized or become alkaline over time, especially as it comes in contact with the immense amounts of alkaline materials found in the ground. This presumably has been the case for a very long time, and I don’t have any reason to think that process will change anytime soon.
So the question really is, and perhaps the title of the article might be more accurate if simply asked, ‘How Acidic Do You Want The Water To Be In Your Soil, And For How Long?’
It’s like the old question, ‘Do you want coffee with your sugar? I would ask my blueberries the same question, only, “Do you want clay and limestone with your peat moss?” 🙂
-Rick
Any Canadian sparghum peat moss I have ever purchased (and I have purchased many) it has always had a ph of 7.
Some sources add lime to bring the pH up. Peat moss on its own is acidic.