test soils with pH meter?

17 years ago

I have a pH meter at work, used for chemical analysis. In other words, it typically tests solutions. It's not a soil probe. Is there a methodology for using a pH meter to test soil? I would have to add distilled water to my soil in order to test, but the amount of water added will affect the results. I would like to hear some thoughts on this.

thanks

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Comments (4)

gargwarb
17 years ago
You're meter will work just fine since soil pH is just a measurement of the pH in the soil solution anyway.
What you do is make a saturated paste. You put your soil into a container and slowly add D.I. water and mix it in until it is fully, but not overly saturated. Saturate the soil until (I know this sounds goofy) it has a consistency pretty close to a Frosty from Wendy's. Then add a few more drops and mix until you get a tiny meniscus (for lack of a better term) of water along the bottom edge of your soil blob when you tip the container at an angle.
Then you need to let it sit for a minimum of 8 hours. 24 is better. Cover the container with aluminum foil to keep water from evaporating. When you take off the foil, there will be a little bit of free water pooling on top. Don't sweat it, that's normal, just mix it back in.
If you're in a lab, I'm assuming you have a Buchner funnel and an Erlenmeyer flask you can hook up to suction. If you do, set it up and throw some filter paper in the funnel and suck out the moisture. Then run your pH on the leachate.
Michael
17 years ago
Gargwarb's method is the standard but often a 1:1 soil:water mix will do just fine though not quite as good. The USDA Handbook #18 describes the 1:1 method as follows. Weigh 5g of soil into a 50 ml beaker and add 5ml of distilled water then mix thoroughly. Let sit for 30 minutes, stir vigourously and then immerse the probe into the mix. Of course, follow all calibration procedures for your pH apparatus first. For organic soils, a 1:5 soil:water mix is used with a 2 hr setting time.

Of course, do all of this while your boss and co-workers aren't looking. And also, above all, TAKE A PROPER, REPRESENTATIVE SOIL SAMPLE. Otherwise the results you get may be misleading to useless. Garbage in, garbage out.

Michael
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gargwarb
17 years ago
Of course, do all of this while your boss and co-workers aren't looking

I like the cut of your jib. ;)
alphonse
17 years ago
"Of course, do all of this while your boss and co-workers aren't looking."

Reminds me of an old joke. A husband wants to know why his wife never gets mad at him.
"I just work it off"
"Oh? What do you do?"
"I clean the toilet"
" How does that help?"
" I use your toothbrush"