yea, u need to calibrate it to the 4.0 and the 7.1 also dont forget to put it in plain water between going back n forth to wash off solution. this sets the high and low of ur pen. if you live near a grow shop, u can take it to them and they can calibrate it for u also. once calibrated, it stays good for a while as long as the glass diode doesnt dry out
yes u will want to check the water u are going to water ur plants with. soil isnt as picky as hydro is with ph
"An ounce of prevention equals a pound of cure."
treat it before u get it