I always use my hydro nutes to feed my soil plants. I use it at about half strength and every other watering. PPM means parts per million of nutrients. You don't need a meter if you start low and slowly increase until you see the first signs of nute burn, which is yellowing and browning of leaf tips first, then edges. Once that happens, back it off a bit of course.

I'd trust whatever rhizome says, but my understanding is that if your water has chlorine in it, it needs to be neutralized with drops or by letting it sit out. Am I (and has everyone else been) wrong about this? Maybe that's only for hydro, not sure since I use well water.