One advantage to having your soil tested by a cooperative exension rather than testing yourself is that the extension will give reccomendations and other information to help interperet the results. If you want to do it yourself (a good way to learn about soil), I have used this kit and it does come with 2 informative booklets to help you interperet the results: http://www.benmeadows.com/search/soil+test+kits/1852/?s_kwcid=soil+test+kits%7C2586111268
Did you try? Just curious to know if it worked. I would guess that it might not work because the soil is not a liquid, and I’m not sure that just adding water would give an accurate idea of the pH; plus you would need to know the pH of the water to begin with… Or could you use distilled water?