GDub51answered grow question 4 years ago The closer the two measurements are the better. You want the two to be about the same which indicates the feed is balanced. If the runoff gets too out of balance, one way or the other, you adjust the feed water in the opposite direction to bring it back inline. TDS should also be measured in and out to see how much the plant is eating or if it's "full" and can't eat that much. This balance is the key to quality if other conditions are equally met.