I tentatively tried to get a good soil sample, but digging down into a 3 gallon bag, I encounter quite a thick patchwork of roots that I’d need to basically tear through with a spoon or so. Maybe I should, in the name of “science”. But I’ve chickened out for the time being.
In the spirit, though, of actively trying to see what’s going on, I did give the suffering, yellowing, curling plant enough plain water to get the tiniest of run-off samples. 
Here’s what I found:
PH 6.27 (in)
PH 7.11 (out)
PPM (out) was 1020 and EC (out) was 2.02. Not terrible numbers? In fact, based on them, I struggle to see what the problem is…
I respect that doing this reading from a tiny amount of run off (I was careful to water very slowly so the liquid didn’t just run down side and into the tray) is not super accurate, but as an indication, these numbers don’t seem so bad to me?
For sake of comparison, I fed the healthy looking-Titan as normal (which I’ve also been doing for the affected plant up until this watering) after what I took be the prevailing advice from this discussion board, and here is what came out:
PH 6.37 (in)
PH 6.51 (out)
PPM was 2020 (out) and EC was a massive 4.02. Those numbers look, on face of them, more concerning to me, but weirdly this plant is doing much better.
Which begs the question: if the struggling plant shows both PPM and EC to be almost exactly half of the healthier plant’s readings are (PPM1020 + EC 2.02 v. PPM2020 + EC 4.02), then does it make sense to assume that the struggling plant is actually deficient, not in excess?
I’m naive enough to hope this might be true, but experienced enough already to suspect the answer will not be so straightforward. But maybe? I guess if, after giving the struggling plant “plain” water, it continues to get worse, I’ll change tact and add more nutrients?
