Several people have mentioned using rain water, since that is neutral pH. Others have mentioned more acidic soil for some plants. I found a site with recommended soil pH for different plants.

How do you measure pH in dirt? I have a little meter that I use for the water in the fish tank. I also have chemicals to raise the pH. Do you adjust bonsai (and other plant) pH with soil additives? Peat moss?

I do not measure my PH.
The use of rainwater is more to reduce the buildup of salts such as liesale, than to control the PH of the water.

From my previous life as a biologist I can only say, measuring soil PH in a reliable manner is not something done with a simple home-kit.