From what I have seen those pH measure instruments for soil are not worth it. I've read countless reviews and comments that all say similar things - they cannot be trusted. Some of those meters are an all in one measurement tool (humidity, temperature, pH, moistness). For awhile I wasn't doing anything for pH (I know noob move..). My plants were not growing very fast and a bit on the yellow side. I went to a grow mart and got some advice. They supplied me with a pH down solution (Phosphoric acid with 75% concentration i believe). You only need a very small amount. Any more importantly, supplied me with a pH liquid test. Essentially you fill up this tiny container half way, add one drop of the pH test solution and it give you a colour output (Its good for 800+ tests!). Before my pH was like 7-7.5. After using it I'm usually around 6.0 (yellowish colour). I test every time I get a new batch of water. I have a reservoir that I fill up, wait for a few days to ensure that the chlorine is gone (that kills good microbes). I'm still new but that's what I've learned about pH.