Oh I like this thread. A refractometer measures total sap solids of which sugar is the main constituent and the 'other stuff' is just as important as sugars. When using an analog brix meter you'd like to see a more dispersed reading. That's the 'other stuff'. Sap analysis has been used by main stream agriculture for years and canna growers finally starting to catch on. You can check out info from Pike Agri Labs, Bruce Tainio and John Kempf for more details. A good book to start with is called Ask the Plant which is mainly about petiole testing. Any agronomist worth his salt is testing sap of crops he's consulting on. Sap analysis is so much more than just sugars. Sap analysis is the wave of the future for quality canna growing IMO. But yeah brix analysis to determine harvesting is unnecessary.
My interpretation is that sap sugar levels are that they are used as a measure of health, not so much ripeness, with the idea being the your goal is appropriate sugar content at the right times of day as well. Another instance of reading something in hardcopy that I cannot recall exactly where it was read, so I can't go lay my fingers on it and retype here for you. But in essence you don't want to see high sap sugar levels first thing in the morning (though hell if I can remember WHY, I think this was another article by Hugh Lovel I read and he lays so much on you that your brain had better be nicely folded before starting) as it indicates the wrong type of physiological activity, but you do want to see progressively higher sap Brix levels as the day goes on, as an indicator of health inasmuch as it can show how well the plant is making sugars. Or some shit like that.
Sap target pH is 6.4. Yes lower pH and brix indicates deficiency. As for comparing upper and lower growth on plants that would seem to work well for things that don't translocate and will show you how well you have things dialed in as your plant grows. You can always make up for inadequacies through foliars. Typical mid-day brix fluctuating between 12 and 16? Are you taking samples from the same portion of the plant at the same time of day and where does your fertigation and any foliar applications come into play here?
There's that, too (bolded). I was told I can still get accurate readings if I used petioles, would you agree with that? This is specific to Brix, though I do have pH strips that I've been able to get reading from because I just pinch & schmear. Again my own problem is how to manipulate to achieve the optimum numbers. I've chased numbers before and it wasn't pretty, killed a *lot* of fish. But I figure since you're throwing out these names that I'm somewhat familiar with, maybe we're speaking the same language? I'm still baby-talking at this point, but I'd like to grow it up.
I'm following some ideas from Graeme Sait @Nutri-tech and Bruce Taino who he refers to in that link JK posted. Not so sure about Reams though, lots of pseudo-science there. Comparing upper vs lower leaves was mainly to check K in mid to late flower, mobile and most likely to affect sap pH. Fuzzy demarcation on the refractometer indicates sufficient Ca and a crisp reading shows deficiency. I feed Ca in several forms to make certain it's not a limiting factor (amendment, fertigation, and weekly foliar). Another check is for B, deficient if mid-day brix isn't higher than morning (when sugars are still at the roots). I do try to take leaf samples from similar locations and around the same time of day, depending on the reasons for testing. Response to foliar sprays shows quickly via brix, so it's a good way to fine tune feeding without having to wait and watch.
This is interesting, did he discuss the mechanism of action? It's essentially a trace and micro mineral mix right? I couldn't find complete analysis, but there's a pdf brochure listing significant levels of selenium, iodine, and boron.
Will reply in this thread here so as not to clog things up.
