Sunday, April 03, 2011

Determining Soil pH

Here within the luxurious Catican Compound, we have raised beds for flowers and vegetables and a wide array of potted plants, capped off, of course, by the radiant Momma Daisy. Wishing to be more scientific about our agriculture, we've recently decided to begin measuring soil chemistry and comparing the soil we provide our plants with the optimum soil characteristics recommended by experts. First up is soil pH. From answers.yahoo comes this set of instructions (modified a bit for grammar and spelling) for measuring pH:
  1. Select soil, scoop it out and dry it under shade

  2. After sieving, take 5 grams of soil and add 100 ml distilled water

  3. Stir vigorously and filter through cotton wool or filter paper

  4. Measure the pH of this liquid with test strips or a test meter
Perusing Amazon, we found that high quality pH test meters cost around $100. Unwilling to spend that much for this project, we discovered test strips which will measure pH in 0.5 increments. We figured we could eyeball interpolate or use Photoshop's pixel analysis capabilities on test strip digital photos to get a bit more accuracy on these strips and that they would do just fine.



An order has been placed and we expect to receive our test strips forthwith. Experimentation will follow.

Update: By comparison, the Oaktron pH2 handheld meter is accurate to 0.1, but costs much more and requires calibration. After writing the post above and heading to the checkout at Amazon with the test strips, I wondered if it was silly to not spend an extra $40 to get the handheld meter. The problem with the meter is that you also need calibration solutions - liquids of known pH that allow you to make sure the meter is accurate. That adds another $20 or so to the cost.


Maybe I should have gone this way, but the cost doesn't seem to warrant it.

11 comments:

Kelly the little black dog said...

Nice tip! So how often do need to check the soil?

Foxfier said...

Did you try a local garden shop? I seem to remember some meters that look like they're straight out of the 70s, for about $20 bucks, that you use year after year....

tim eisele said...

I think you made the right choice going with the test strips. I've used those little handheld pH meters before, and my opinion of them is not something that should be written on a family-friendly site. Their claims to be accurate to 0.1 pH units is, to put it bluntly, a lie. Sure, they *register* to that accuracy, but the decimal place is effectively a random number. Plus they only last for maybe a year before the glass bulb sensing element goes kerflooey (and shorter if it isn't stored under exactly the right conditions).

Basically, if you aren't paying at least $100 just for the *electrode*, with another $300 or so for the meter proper, then you will get better accuracy sticking with the test strips.

I've never used the ones Foxfier mentions, but I think I know which ones they are. They are effectively conductivity probes, so they don't *quite* measure pH, but they measure something that is a reasonable proxy for pH, so it wouldn't hurt to try one.

K T Cat said...

Kelly, I'm not sure you need to check it unless the plants seem to be in distress. I doubt the soil chemistry changes other than when you add fertilizers to it. I'm just doing it to be fanatical.

K T Cat said...

Foxie, part of this is to play with chemistry again. I've seen those meters and Amazon had several on sale. I've used them in the past, but was concerned that they would go out of calibration.

Foxfier said...

I wasn't sure if there were some that could be calibrated with, say, tap water.

Secular Apostate said...

Many aquarium pH test kits are very accurate and inexpensive. The creme de la creme is LaMotte, but Red Sea and others make excellent kits.

You can order them online from Marine Depot.

tim eisele said...

I think you made the right choice going with the test strips.

I've worked with those little handheld pH meters. All the ones I have seen are essentially worthless (that reading past the decimal place is pretty much just a random number). Plus the electrode only lasts about a year under ideal conditions, and only a few months under realistic conditions.

If you aren't spending at least $100 just on the *electrode* (electronics are extra), then you get just as good of accuracy from the paper at much less cost.

Whittlin' Man - formerly "Lawman" said...

I usually just liberally apply a generous amount of "barnyard dirt" to my vegetable garden. The plants don't really seem to care what the pH is at that point as long as they get adequate water.

The more the price of fertilizers increase, the more I love my horse.

Good luck with the experiments. Anything to get the children interested in science is a good thing.

K T Cat said...

Thanks for all the great comments! The aquarium test strips would probably have been a better idea and I could have picked them up at our local Petco. I might get some anyway and do a comparison.

Foxie, I was concerned that the device would be nonlinear. That is, you'd need to calibrate it at several points and not just one. I would also need some assurance that the pH of our water was relatively consistent.

K T Cat said...

Tim, I thought the same thing. I never really trusted the store-bought water meters all that much, either, and they're considerably simpler in design.