Accurate method for testing soil pH

What you need:

  • Apera Instruments PH20 pH Tester ($50) - This is considered the “value” tester but it has served me very well since I purchased it in 2018 with no noticeable loss in accuracy. Keep an eye out on eBay and you might pick it up for $40.
  • Storage Solution for pH/ORP Electrode (~$12 / 230mL) - I believe you aren’t supposed to store the pH tester “dry”, this solution is specially designed to maintain accuracy. You might be able to also just use tap water but a $12 bottle has lasted me 3 years.
  • Ph 4.0 & Ph 7.0 Calibration Solution (~$17 / 16 floz) - This is optional as the PH20 kit comes with a small amount of calibration solution, but keep in mind each time you use it it gets a little bit more inaccurate. Good idea to refill with fresh solution after 10 or so calibrations.

How to test:

  1. Get 1/4 cup of soil from at least 2-3" below the surface*
  2. Mix with 1/2 cup distilled water, and stir / shake vigorously for at least 10 seconds
  3. Wait at least 10 minutes before taking your measurement
  4. Dip the probe of the pH tester into the soil sample water enough to just submerge the tip**
  5. Wait 15 seconds for the measurement to stabilize before taking a reading

* If measuring for a target plant you can either gather the 1/4 cup by collecting 1-2 Tbsp from 2-4 different areas in the area of the root zone, or just do more than one test
** Don’t let the glass ball of the probe touch the soil at the bottom of the cup as it can damage it

Validating my testing methods:

You can read about various methods of soil testing here: Methods of Greenhouse Media Testing and How They Differ

I wanted to test if my minimal effort method and budget pH meter could at least produce repeatable and reliable results within 0.2 pH. I took 4 samples (S1 to S4) within a 2x2’ area in my garden bed. Samples S3 and S4 were taken from within 1" of each other.

In addition to testing using the PH20 meter I tested using super cheap pH test strips for fun. The PH20 meter was calibrated with pH 4 and pH 7 solution before testing started and after testing to see if the dirty sample water caused any drift in the sensor (it didn’t).

Below is a chart showing the method of testing each sample, each samples pH and any observation. “12 min” stands for 12 minutes since all samples were mixed with distilled water.

Whats next:

I have read some research that said the 1:2 & filter method can result in a measurement that’s ~0.3 pH lower than actual soil pH. To double check my results I am planning on sending a soil sample to my extension office for testing and see what they say the pH is compared to my method above. I will update this post when I get the results.

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Thanks for posting this! I haven’t looked through the entire post yet but a general comment on pH calibration - using a digital probe, you can only get as accurate as the calibration fluid you are using. So if you have a soil pH below your calibration of 4, or above your calibration to 7, your numbers will likely be “close enough” but have not been accurately calibrated. At work I use a Hach probe that we calibrate using 4, 7, and 10 pH fluids. If the calibration fluid starts to grow white fluffy fungus looking stuff, time to throw it out, clean the container and get fresh fluid. It doesn’t take long for that to happen for us.

Good point. I think most soils should fall into the 4 to 7 pH range, and if they don’t then they need to be amended to get them into that range.

I was also thinking of trying to find some pH 4 to 7 (8?) test strips and seeing if I can make a method to read them accurately as a more budget-friendly alternative. The 1 to 14 test strips have a color guide every 2 pH steps (ie. pH 6 then pH 8, etc) which makes reading the pH more of a guessing game.

calibration fluid starts to grow white fluffy fungus looking stuff, time to throw it out

Yeah I get that in the meter’s storage solution after its been sitting around for a while, pretty gross but doesn’t seem to effect accuracy.

EDIT: Found some test strips that might work, might buy them to test them out for fun:

Got some strange results measuring the pH around my blueberries. I took samples from six different spots and then tested their pH using my method above.

This was similar to what happened last year which is why I was questioning the validity of my measuring methods and decided to do the testing in my original post.

I took a photo of where I took each sample from and then added their resulting pH so you guys could see what I’m talking about. The blueberry bed is a bit weedy right now so I added blue arrows indicating where the base of each blueberry plant is.

Looks like I have a tendancy to apply the soil acidifier on the right side of each plant or something, LOL!

Most of the linked test strips say for saliva or urine only. Many of the answers say that those strips are not good for soil.

One review for the strips linked below says it is for “buffered” solutions.
https://www.amazon.com/gp/product/B0045I6GU6

I have no idea what any of this means. I bought the ones linked above because the others seem to be intended to test water?

I settled on almost this exact procedure from my own research, with an apera ph60 instead, so, thumbs up. I also have high and low pH spots. my blueberries grow fine, I assume they get what they need from the “just right” areas

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I could be wrong but I don’t think the type of liquid being measured really matters. The soil might make the test strips dirty, making the strips slightly darker than what they should be for an accurate measurement with a “clean” sample. So with the strips you have the orange may appear more brown or the green more dark green.

The strips should be used as more of a ballpark figure to tell you if you are way off in your pH in one direction or another.