That's what we're studying here Cajun! How do we know what to feed the plant when its brix is low? I just started a thread with references to alternative soil testing laboratories. A good place to start is with a soil test by a lab familiar with William Albrecht and Carey Reams work on soil fertility. Another possibility is to measure the pH of the plant sap, and then use Dr. Tanio's chart to see which group of nutrients may be missing. One can then do a test application of a weak solution of the nutrient and then re measure the brix. This may no be ideal but it would be a step towards learning to recognize what our plants are telling us.
BTW, I'm adding "BRIX:" as a prefix to the thread topic for any new threads related to this subject.
You'll need another apparatus or a roll of litmus paper. The litmus paper, aka pH Hydrion or pH papers are the less expensive method. I'm hoping that as I get more experience with the testing that I will also be observing some other signs that can be used to select the right nutrients. We've lost a lot of knowledge by moving to conventional chemical ag. Now we get to learn it all over again with the help of instruments. *sigh*