which mixture will give a pH=5? 10/90?

No, what you are proposing cannot be done with any measure of accuracy.

First, distilled water equilibrated with our atmosphere has a pH of ~ 5.8 (+/- 0.2), not 7.

Second, distilled has almost no buffer capacity. Add distilled vinegar to distilled water and the pH will very quickly approach that of the vinegar. The pH will be dominated by the acetic acid/acetate equilibrium once you add even a couple of drops of vinegar to a quart of distilled water. You can calculate approximately what the pH would be if you accurately know (at about the ~10 mg level) the concentration of acetic acid in the vinegar and the amount of vinegar added to the water. Unfortunately, you won't know the concentration of acetic acid in distilled vinegar at the level desired.

If you want to calibrate a pH meter accurately you need a calibration buffer solution. To do what you propose would require at least two pH calibration buffers that surround the expected value to be measured. For soil that could be say pH 5 and pH 7. With those in hand you can do a simple two point calibration curve and probably be reasonable over that range by about +/- 0.4 pH units if the meter you want to use responds with at least a 1.6 pH unit difference.

Is that good enough for your needs? If so, narrow range pH test paper (+/- 0.2 pH units) and filter paper is probably more economical (pH calibration buffers aren't cheap), easier and reliable.

Before pH meters were invented, chemists were using dye indicator solutions to gain a visible pH measurement. These indicators are still available, and they can be used for soil pH testing. However, the measurement is not as precise as a pH reading done with a calibrated pH meter. I am using bromocresol green indicator solution for testing soil pH around our blueberry shrubs. The filtered sample will turn yellow if the pH is 3.8, or lower, and blue, if the pH is 5.4, or higher. A green sample indicates pH is 4.6, plus or minus 0.2. For example, if I take a soil sample, mix it with pure de-ionized water, and then use a filter to get a clear sample, I can add just enough indicator solution to see a color change. If the sample is green, then I know that soil pH is 4.4, 4.5, 4.6, 4.7, or 4.8.

Following up on what TXEB posted, the trick would be to find or develop a recipe for pH buffer solutions using simple readily available household materials. Of course you can buy calibration solutions, but if you only need them now and then, they sit on the shelf and leak, dry out or otherwise degrade. Better to use fresh ones.

Vinegar might work although its buffer pH is pretty low at 2. It would be nice to have something more like 5. Boric Acid, which is sold as roach powder, buffers at 5.2. For the upper end, baking soda would give you 8.3. This brackets the soil pH range pretty well. If I had a good lab pH meter it would be relatively simple to work out recipes in teaspoons to cups of water.
Someone around here developed a really low-tech and low-cost colorimetric method using red cabbage extract. If you searched 'red cabbage' on this forum you could find it.

For calibration buffers, a solution for the home enthusiast might be the dry powder buffer packs - mix them in water then calibrate. Something like this.

Ideally you would use calibration buffers that span either side of where you expect to measure, so pH 4 & 7 for the acidic side of neutral and 7 & 10 for the basic side.

This post was edited by TXEB on Wed, Sep 18, 13 at 12:23

What does you $20.00 pH meter tell you about the amount of Calcium and Magnesium and the balance between the two?
The soil test done here many years ago showed that this soils pH was 5.7 with adequate levels of Ca but deficient in Mg. Since plants need Ca to properly utilize Mg and Mg to properly utilize Ca having them in balance is necessary, and that is part of soil science. Now I am aware that there are some that will tell you that the balance of Ca and Mg are not of much concern but tell that to a plant that has Blossom End Rot.

Aside from the debate about Ca and Mg levels, I have yet to figure out what those cheap stick-in-the-soil pH testers are actually measuring. I have tried and it sure is cloaked in mystery. They may be responsive to changes in pH but I would bet they are not linear in pH over any broad range, and I would also bet that they are more responsive to moisture and conductivity/resistance/total ionic activity than they are pH. I can see no basis in any of those testers I've seen for any selectivity toward hydrogen ion activity, which is the essence of pH measurement.

A good quality glass membrane electrode (without the meter) will cost $200+, and can be expected to last 3-5 years if well cared for. A cheap pocket model glass electrode pH meter that will suffice for in-field soil analysis can be had for ~$40, and if well cared for might last ~ 2 years. If not cared for they won't last much more than a couple of months. The cost of use and proper care for a pH meter will be ~$20/ year. So if I buy a reasonable pocket model, and care for it properly it will cost me about $40/year. A basic soil test that includes pH along with the macronutrients cost about $10. Unless you have a lot of samples that require separate testing, sending samples to an accredited soil lab is a better bargain.

True.

I certainly wouldn't trust a dry probe, if I was going to do it myself with inexpensive eqpt. the 50/50 soil/distilled water slurry is the way to go.

Great idea on the buffer packets. I guess if one already knew the approximate pH of the soil, the calibration could bracket it more closely. We used to use 4 and 10 at the lab, but that was with good meters with glass electrodes that were linear across the whole range once you got them calibrated.

Well, I suspected that someone credible had actually tested and evaluated those inexpensive soil pH testers (probes). Sure enough, some sharp Ag Extension folks from MO (smile tox) did exactly that - published article linked below.

Bottom line - they don't work. Handheld glass electrodes and colorimetric test kits for soil pH do seem to work quite well provided that the user uses proper technique.

Here is a link that might be useful: A Comparison of Soil pH Test Kits

Thank you all very much. Special thanks to TXEB for his very informed info.

I learned quite a bit here. And have come to certain conclusions:

1- if you have a state or county Age Extension who will do a basic pH and N,P,K test for $10 to $15. then by all means take advantage of it and get an accurate analysis.

2-But, according to the MO Ag study, it is POSSIBLE to get a fairy practical pH measurement using both a hand held pH meter and/or a good color kit. Although , the costs may be even higher than if you had done it for $10 to $15.
You have to spend some time to prepare a good SAMPLE. Sticking the probe in the garden bed might not work.

While a good reliable soil test is an important tool in the gardeners kit, fixating on soil tests does not do much for anyone except maybe create anxiety. Available soil nutrients can vary depending on soil conditions, for example Nitrogen availability depends on soil temperature and moisture levels. Soil pH can take several months after application of lime or sulfur to show results.
So, an initial soil test for pH and nutrient levels gives a gardener a guide for taking corrective action and a follow up test 3 to 5 years later will give indications of how well that gardener is doing. But a test every month during the growing season is a waste of time and energy since change does not occur that fast.
Use the soil test as the tool it is but do not get overly fixated on them.

Agreed, it is definitely one tool and not the whole picture. The average gardener does not need to be checking pH constantly, once they get a good soil test. If it's way out of whack and you're trying to adjust it, an accurate method would be good to have for regular checking. If it's OK and you're just maintaining the garden with compost, it shouldn't be necessary.

Thanks TX for the article, I bookmarked that.