Vinegar is not even remotely dangerous to use for container plants when properly diluted

Yes, vinegar is used to kill vegetation, however it is used without dilution and many complain the standard 5% acedic acid vinegars found in groceries aren't strong enough to reliably kill vegetation.

To acidify water just add a tablespoon to a gallon of water. If you have the ability to test the pH of the water even better as you can be more precise as to how much you need to use to get the pH where you want it, but it really isn't all that important in my experience to be precise.

You will not harm your plants even if you use a cup per gallon of water.

I can absolutely understand your being apprehensive so why not try it out on a plant you don't much care about? Seeing is believing. If the problem is excessive alkalinity in your irrigation water you will come back here singing the praises of vinegar very shortly.

Hmmm - are you sure that acidification is your answer? Highly organic soils form many soluble complexes with iron (Fe) and make it readily available for uptake. Container soils are not often deficient in Fe. As soil pH goes down, iron (Fe) levels become more available and Fe toxicity a more common problem. Fe in soils also needs to be in balance with another (antagonistic) micro-nutrient, manganese (Mn) or it prevents Mn uptake. It just so happens that an Fe induced Mn deficiency looks remarkably like an Fe deficiency, so you tend to add additional Fe to correct. Saturated soils and lack or aeration in the rootzone also is a common cause of Fe deficiencies & chlorosis.

If you know your water needs acidification, vinegar (starting at 1 oz per gallon) or citric acid (buy at drugstore) are good acidifiers. If you have a friend with a RO water purification system, that would be very helpful too.

Agricultural sulfur (S), mixed with the soil will slowly lower pH, but it requires microorganism activity for it to be effective. It's often thought that due to its fungicidal properties, sulfur would reduce or toxify beneficial mycorrhiza on host roots. In calcareous soils or in situations where irrigation water is high in carbonates, S, through oxidation, will produce acid and improve rhizosphere conditions. This, in turn, leads to better establishment of mycorrhizal fungi on roots, increasing the beneficial symbiotic effects for host plants.

I have used agricultural S on acid loving containerized plants and sometimes noticed marked improvement in color and vigor.

Take care.

Al

Excellent info guys.

And yes Al, our water is pumped from a limestone acquifer. Can't easily avoid using a lot of that in the summer (when the rain barrel dries up from daily use).

But I now know how to proceed. My maples thank you both. :-)