Understanding pH for potting soil

I see a lot of recommendations for home-brew potting soil / growing medium. In some cases, amending commercially produced potting soil with things such as DE, pine bark chips or fines, compost, etc. I also understand that a pH around 6-7 is typically ideal for most plants, and think I’m hearing to target around 6.0 for most fruit trees.

If mixing your own or amending commercial, how do you know you are landing in an acceptable pH?

It would be good to know if I should be concerned.

It seems many of the home brew with pine bark falls a bit acidic. Is it recommended for containers to try to adjust this up with lime?

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First, invest in a really accurate pH meter, one that you can calibrate with pH 4 and pH 10 solutions. You want one that is accurate to tenths of a pH. I use the Aptera pH 20 and have been very pleased. It has allowed me to test composted horse manure (pH 8.3) and adjust with ammonium sulfate to reach 6.5 for fruit trees.

Thanks. I can see how that would work with finer or softer particles. How does one test something that, for instance, has 1/4" chunks of solid pine bark? does one first grind, then put in water solutions then test?

Yes, you put 1/2 cup of your coarse material in 1 cup distilled water. Shake and let set for 15 min. Then test top water portion.

Why? Why not get test strips? They work OK.

Ph in potting soil is a moving target that’s for sure. For me, they gradually get more acid for various reasons. OK at the beginning of last season doesn’t necessarily mean OK at the beginning of this season.

The el cheapo meters are useless.

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I found a digital meter 100% better than color- coded test strips, the colors of which left me guessing. You calibrate this Aptera meter with stock pH solutions before each use - accurate within .1 pH.

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The irrigation water also needs to be considered.Let’s say the target pH is 6 and at the tap,it’s 7.5.Eventually,the soil’s alkalinity is going to rise,if an acidifier isn’t used.
Ammonium Sulfate could be tried,but can be over applied,because of the high nitrogen.

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I too much prefer professional strips the colors could not be easier to read. I myself would never buy a Meter. Too much work to keep it accurate. Too much of an expense too. I like the 4 to7 range strips as anything out of that range isn’t important to me.
I mostly have to use city water to water so potting soils slowly become basic over time as the material composts. I myself do not use lime. I don’t have a need for it. I do add oyster or crab shells to add calcium or gypsum works too. I try to avoid lime as long term my soils become too basic.

You’re right re: ammonium sulfate, at 21-0-0, is very high in N. The amount needed to lower pH might put you way over the top in N.

Sulfur, on the other hand, is way safer but takes at least three seasons to show results. Recommendations are to apply in the fall and test the following spring.

In my post I meant to type aluminum sulfate.

Since I was looking for instant results so I took a chance with aluminum sulfate. I titrated the pH, repeatedly applying and testing a large wheelbarrow of composted horse manure. It took about 1/4 cup to lower pH from 8.3 to 6.5.

True, you could easily drastically lower the pH too much with aluminum sulfate …the reason that the label on Espoma Soil Acidifier ( sulfur and gypsum) reads: “safer than aluminum sulfate’”.

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Thanks - I guess I had not been aware that potting soil is on the move so much. It makes sense, given the little volume, but was not particularly thinking on that.

It probably has the ability to be more accurate and precise, given the specific meter. However, I think I have also read that the bulbs only last so long, and are therefore an ongoing expense. So probably makes sense to start with the narrow range paper for the few immediate tests and then if I find I am performing tests regularly, look to invest in the pH meter.

@Drew51, I think this is the best path currently, given I just have a few tests. Which brand/model of test strip have you had good luck with? I know there are some challenges surrounding measuring unbuffered solutions for pH with reliability. Is that specific to water (and not a mixture with water, such as the soil in water)?

@Bradybb That is a good callout. If the pH strips don’t work for water pH (at least I think that is what I heard around unbuffered solutions), how do you know your water pH?

OK, first what I do to measure soil ph. I soak the area with distilled water, so that the hydrogen ions in the soil can be measured easier. Others make s solution, which is fine too. Since the water adds no hydrogen ions being neutral as distilled water is. You can only measure ions in the soil. I then stick the strip in and wait five minutes.
Like once I lost a blueberry plant, so I did this and my 4-7 pH range strip showed it was 4.0 or lower. I determined I added to much sulfur and the low pH killed the plant. It must have been around 3.0 as it needs to be that low to kill a blueberry. It made me realize I need to monitor soil closer.
I use these strips. Wow! Price is about ten dollars higher. Look on Amazon. These have worked well, but I bet any commercial use strip would be fine. Others on Amazon might be cheaper.

Here is an example of testing blueberry soil. Better stop adding acid! The strip easily picked up the ions in the soil.

I tested my tap water which has a range of 7.5 to 8.2 (Water dept numbers) depending on the day…

I also tested my rainwater.

Maybe others won’t but the color range I think is easy to read. Like the above looks to be about 4.5.


The strips are meant for liquids, any liquids, but as you can see above, their are work arounds. Mixing soil with distilled water only shows ions in the soil since distilled water has no ions. I like putting in the soil because as I don’t know how much the water dilutes the ions present. Leave it in longer for a more accurate reading. 20 minutes is plenty of time. I usually only leave in 5 to 10 minutes.
In the soil I tested above the strip on the right shows it being about perfect. The one on the left is too close to 4.0. I decided to water the plant the next couple of times with tap water. Tested again and it came out at 5.0. You could also flush with tap water and measure again. Run tap into soil for 5 minutes.

If you need a bigger range, they make a multitude of ranges for these strips. See link in above post.

When measuring pH,I used the liquid drops,like what aquarium owners have.There are ranges available,similar to the strips.

@Drew51 - that is quite the explanation and demonstration! I really do appreciate all the detailed information. Makes sense about using the distilled water to avoid affecting the pH. As I recall, pH is a measurement of concentration of the ions, so wouldn’t how much water is added (dilution) affect the result?

I think what I’m gathering from this is that there are a lot of influencing factors, including the variability of conditions from day to day potentially affecting the pH, and to not get too hung up on it.

Now, if measuring water directly… isn’t there something about it needing to be buffered to get an accurate reading?

@Bradybb - you’re saying that you use the liquid method for all your pH testing, or you are saying in particular, that is a good way to measure water’s ph as an unbuffered solution?

I may have tried the strips,but found the drops were easier.Now that the pH of my tap water is known,I can approximate what my goal is,adjusting for different plant needs.

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Yes sort of. Ph can only be tested in solution and all soil pH tests put soil in distilled water. If you send your soil to be tested, that is how they do the ph test.
Think of it like if you plug something into your wall socket are you lowering voltage? As electrons are diluted in the copper wire. No you are not. As long as mixture does not have an overabundance of water, the reading should be very accurate.

No, You would get a false reading if buffered. A buffer is either an acid and it’s salt or a base and it’s salt. So tap water is buffered with carbonates to make it basic. Distilled water has no buffers. So one would never want to start with a buffered solution, use distilled water.

Text book definition
A buffer is an aqueous solution that consists of a mixture of a weak acid and its salt (acid buffer) or a weak base with its salt (basic buffer).

The labs say that ph is independent of dilution, but a huge amount of water will change results. As long as 50% is soil, it should read the same if at 60 or 70%.
I feel in ground if best as soil is always kept moist anyway, it gives the true ph of the soil most of the time. Now if ground is saturated day after day after day ph would likely be higher or lower depending on how water is buffered. I acid buffer (rain) my water for acid loving plants. I base buffer (tap) my water for regular plants.
In some places long term exposure to acid rain has lowered the ph of the soil and is why many lime grass etc. But since containers I have mostly get tap water, the opposite happens and they become basic. Which is not much of a problem. Most plants grow fine in basic. So my soil mix is about 6.0 and I will change it in 3 years or sooner, so I do not have to adjust. Composting is a basic activity. Even peat or pine will become basic as it breaks down, so I do have to watch the ph for acid loving plants. If you use lime in your mixes, I would check that ph does not get too high, say over 11. As it may very well become too basic. I myself right now use oyster shells to add calcium to soils, it breaks down super slowly. Lobster shells are best as it has chitin and attracts chitin eating bacteria. These bacteria also love the chitin in grubs. It is used to form the outer coat of the larvae. I used to get thousands of Japanese beetles, now I see 4 or 5 the whole year.
Gypsum is neutral yet makes calcium available to plants. An excellent additive over lime.

Drew - thanks for all the education there. Very good information and explanation. I think I might have seen you mention a science based background at some point, and it fits!

Out of curiosity, in your mix, about what pH is the compost you use? I like the idea of the DE here in colorado where it is dry and containers get hot. I guess i just have to be careful I don’t add too much if I end up using plastic and not fabric pots.

Very cool on the japanese beetle control. Interesting Pine can become basic on break-down… I have lots of pine trees, and hence needles, and was always curious why the soil wasn’t more acidic.

Yes pine does increase in pH as it breaks down so years and years of Pine needles does not make the soil more acidic. Although compost can range from 6 to 8. Pine bark is about 5.5 when it breaks down it is usually around 6.5 still slightly acidic. But too high for blueberries. Any anaerobic areas will be acidic. Why you turn compost to keep it from smelling and becoming acidic.
Soil is dynamic in pH. It changes with conditions. Pine would not grow in areas not acidic already and the mulch from fallen trees and needles do not really change it. Often our acidic rains do slowly over decades drive areas lower. So if an area is lower it most likely was the acid rain that did it. I use whatever I can get. One can assume it’s about 8.0 but compost can be 6.0 like pine and peat. I like to use manure in peat moss as often the pH is in the 6.5 to 7 range depending on the amount of manure. Manure is the highest so mixed with composting peat helps knock it down.

DE in plastic containers is fine too. DE will not add too much water to the soil to make it wet and saturated. The stored water inside actually increases drainage of excess. It acts just like perlite once saturated. So soils even though may be holding up to a gallon more of water will drain very well if additional water is added. If a container is staying saturated it’s because it needs more DE not less. DE increases drainage just like perlite. In the studies I have seen 30% DE is ideal. I only use about 10% as I have so many containers and not enough money😊 I would estimate I have about 150 containers. Looking in the backyard I see about 35 fabric containers I leave out all year. I have about 70 in the garage right now and about 20 in the house. Fabric can be left out all year if the plant is hardy enough. Plastic containers would crack. Fabric has many disadvantages but winter storage outside is a hugh plus for me.

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