It looks like some of the good info is already there, and the best advice given was to get a ph meter. If you want to know how much you're plants are really eating and how much you really need to feed them, buy a ph meter that has tds/ec built in. Then you will know if the nute levels are high or low and also be able to ph your water and check you're run off water to make sure you're soil has an acceptable ph level. Nutes can't be absorbed when the ph is off, simple as that. I'm no expert by any means, as I just bought my ph/tdc/ec meter and havn't even recalabrated it yet but i can already tell a ph/tdc/ec tester is an essential item in a hydro grow and is also extremely useful in a soil based grow.