If you can find it deionized water should have a pH of 7.0, until you expose it to air and it starts to absorb carbon dioxide.

Keep the lid on distilled water...then it is 7.

Your distilled water should have almost no buffer capacity, hence the soil test should reflect the pH and buffers in the soil solution.

If your strip shows household vinegar with pH of 4, then it is worthless.If you dilute vinegar 50 times(1 part ving. 49 parts water) still its pH will be under 4. I would use the test kit that come as liquid. The you check the color of the solution against the color chart.

Another thing:

Your garden soil is wetted by city water and creates a given pH. You do not water your garden with distilled water. And even then pH of water, added to soil has insignificant effect on altering the soil pH. For example: If you get a completely dry soil sample and make two solutions: one with a true distilled water with pH of 7 and another with tap water wit pH of 6, and measure their pH, you cannot detect any measurable difference between them, especially if your actual soil pH was in 6 to 7 range.

seysonn, that's a good point you make. If neutral distilled water won't make a noticeable difference compared with tap water, then the only problem I have is finding a reliable way to test for pH. I thought my cheapo test strips might be off by a little bit, but the vinegar test does show them to be worthless.

So what is the liquid test kit you're referring to. What brand is it?.

I've been looking at better quality test strips, ColorpHast is a good brand (used by my scientist friend) , and also Phinex (4.5-9 pH range). I'd like to find some with a range of 2-9 pH so I can use vinegar as a low end standard. ColorpHast makes that range, but seems to be out of it every time I check. And does it matter if a test kit is labelled for saliva and urine...is that just because of the range, and can any pH test kit/method be used to test a soil sample?

Any opinions on favorite pH testing methods? I think I'll start a new thread to ask that questions, or has this been done many times already?

I use bromocresol green solution for soil pH testing here in Madison. This particular dye indicator is appropriate for growing blueberries, so it might not be the one you are looking for. The filtered sample will turn yellow if the pH is 3.8, or lower, & blue, if the pH is 5.4, or higher. A green sample indicates pH around 4.6, which is very close to optimum for blueberries. I use DI(reverse osmosis) water, available locally, for sample preparation. When I tested a fresh jug of commercial distilled water a few years ago, it had pH = 7.1. Indicator dye solutions are available from HMS Beagle, online. In the past, I have used a pH meter for this purpose, and the results were in agreement with results that I got with dye indicators. When the pH meter finally died, I began to use dye indicators exclusively.

I use a cheapo soil test meter. Works perfectly and no measuring or strips. Just shove it in the soil and get on with my day. :)

There are two kinds of pH test kits, I think.

One comes with litmus paper strips. After you prepare your solution(per directions) , insert the strip in the solution. From the color of the strip(as gauged against a master) you can tell the pH.

Another kink of kit comes with either some power or liquid. Again, after you prepare you sample , add that powder to the sampling tube, shake it, let it sit. Once again, compare the color to sample chart and figure out your pH.

El cheapo pH testers. can/may work fairly ok for a while. But I think the litmus paper test is more reliable the those testers. you can get them from pet shops that sell fish and fish tanks, cheaper than the nurseries. But you got to get the ones with a range say 5 to 7.

albert_135 answered correctly. DI water at equilibrium with the atmosphere will have a pH around 5.65, assuming CO2 partial pressure of 0.000355. However, the water has very little buffering capacity. When you add your soil to the water it will control the solution pH. You aren't making lab grade reagents, no need to be so precise.

Buffering is not intuitively obvious to the non-scientist and people often ask that same question (why is distilled water not pH 7?). It has to do with the capacity of the solution to resist changes in pH, to absorb added acid or base without changing pH very much. It's like hitting a football player: there is some resistance if he's been working out. Distilled water, having nothing in it, is like a football player stuffed with feathers. Look at it funny and it's all over the place. I did experiments back in college (Los Alamos National Lab, imagine how cool that was for a college kid to play with plutonium) with completely unbuffered distilled water solutions. We had to measure the pH in a glove box full of nitrogen or the air would drive it all over.