You are asking some very basic questions....you have to do more research....it's all here....go read the stickies, FAQ's and grow logs......if you growing 50+ plants ....it's going to end badly....
I know it's not what you want to hear.........but growing dank is anything but easy....
good luck
Agreed years ago I finely broke down and got a meter a good one, and it really takes the guess work and the worry that comes with the uncertainty of not actually doing it right. Also a Truncheon is also a good ppm measuring device, I got mine for $%) I think blue lab owns it now, so they must have saw it a good competition.
Peace