Closed system nutrient replenishing
 

This might be a dumb question but how does nutrient replenishing work?
Do you just check the ec and bring it back up or down to the desired level that you began with? Or dow the plants use different amount of each nutrient which would mean that I would somehow have to test for each nutrient and adjust accordingly? how does this work?
thanks

Yes, that's exactly it, plants don't use nutrients in even ratios. They take what they need and leave the rest. EC/TDS/PPM meters can't tell you what is in the water, only the total combined. That's why I don't use them. You need to have the water Lab tested to be able to get a breakdown of the quantity of each individual mineral element in it. And that would be expensive, not to mention be outdated by the time you got the results back a week or two later. Then there's the question of how you plan to replenish only the mineral salts that are depleted withough't adding the ones that aren't.

To keep a balanced nutrient solution, you'll want to do regular nutrient solution changes. Dump out the used nutrient solution, and mix up a fresh batch regularly (so it's balanced). And in-between nutrient changes, you'll want to make sure you maintain the same water level. So as the plants drink the water, replace it with plain fresh water to keep the same water level as much as possible. The more the water level fluctuates, the more the strength of your nutrient solution will fluctuate.

okay thanks. And how do you dispose of the waste water (legally)?

It's not some type of toxic sludge, you don't need to be afraid of it, or getting it on you. It's just plant food. The same way I would get rid of any other soil plant food like miracle grow. I pour it on trees and other plants in the yard. Why waste it? The only real difference between miracle grow and hydroponic nutrients is hydroponic nutrients also have a small amount of trace elements in it, and pH buffers. Soil nutrients don't need the trace elements because their already found in soil in sufficient levels, but with hydroponics you don't use soil. And because of the wide range of pH levels in soil, pH buffers are useless in soil. If you live in an apartment and don't have a yard, pour it down the drain. There's nothing in it that even comes close to the hundreds of harsh household cleaners including bleach, and draino that goes down the drain daily. It's just plant food.