Are you refering to those $6 soil ph probes? If so, they are notoriously inaccurate.

You can't make a sow's ear from a silk purse. (or whatever the saying is...) Get the best potting soil you can afford for your growroom. Most commercial brands are ready for use, trouble-free, and buffered to maintain healthy growth. Cheap mixes are hit-n-miss for both content and quality. Most need ammending, which makes them more expensive than the commercial mixes.