[quote="CaptainCompostAL"]Like Dave Hall said early, I still don't fully understand why acetic acid (vinegar) is so useful in an aerated compost tea recipe as a regular fertilizer or biostimulant for most of my plants. It makes a lot of sense as a universal herbicide or as an acidic fertilizer.
Well, I suppose the specific answer depends a little on when you add the vinegar. The simplest answer is the scenario where you add the vinegar to the foliar spray just before applying it so that it goes into/onto the plant mostly intact. Whether it's applied that way or is allowed to stew in the compost tea for awhile before applying it, the answer is pretty simple. The theory behind it isn't quite as easy, but the value of applying acetic acid will become obvious when you review the general metabolic cycles that are important in plants and microbes, particularly the citric acid cycle and the glyoxylate cycle, and especially the light-independent processes in plants. This is an overly-general explanation, but the general oxidative metabolic conversion in the glyoxylate cycle of acetate (CH3COO-) --> --> acetyl-CoA --> --> oxaloacetate --> +GTP --> phosphoenolpyruvate + GDP + CO2
probably tells enough of the story for the casual observer. In the metabolism context (not in a conventional acid dissociation in water sense -- see below), acetic acid (CH3COOH) can deprotonate fairly easily to form its conjugate base, acetate, although that probably isn't a formal part of that pathway. There isn't room here to show the entire pathways, even if I could remember all of them, but the acetate ion is a fairly important basic component in oxidative metabolism.
I doubt if the mere topical application of vinegar/acetic acid sets off something like a signal cascade in plants like maybe a hormone or a messenger protein might, but I can't say that is impossible. Maybe LFH can shed some light on that question.
If you delve into the oxidative pathways, you also might be interested in how citric acid/citrate may fit into the mix. I suppose we use more vinegar and less citric acid in foliar spray programs because vinegar is so readily available and inexpensive compared with citric acid/citrate. I doubt if citric acid would work any better than acetic acid, but I could be wrong; it apparently would fit metabolically.
Contrary to the way I emphasized it in my earlier message, the pH "probably" isn't the more important part of using acetic acid unless the foliar spray otherwise is quite alkaline, but one should not entirely dismiss the role that pH might play in plant uptake of foliar spray ingredients. The acetic acid in vinegar is a very weak acid (it has a very low disssociation constant in water) when compared with the common mineral acids, so it doesn't affect the pH of the foliar solution as much as, say, hydrochloric acid would. There's a lot more to the biochemistry than I've writren, but vinegar fits very well into a foliar spray formula for a variety of reasons. We didn't pick it at random. 
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