Quote Originally Posted by ryo
I read some are wondering how they can tell if a plant is lacking in a certain nute? I was also, so I ask him, he said about every couple of months he sends a leaf into some place and they do a tissue sample from it. Somehow they can tell what nutes at what levels are enough and what are lacking. He then adjust his nute solution to the correct level. The tomato plants were amazing, about 16ft long each, well maintain and bearing a bunch of tomatoes.
You can tell most problems by viewing the leaf structure and color.

A Brix meter will tell you how well the plant is up taking the nutrients too.