Yes, sometimes I can't finish the solution I made and leave it around for a while. Dyna-Gro website says something like "Citric acid and vinegar introduce organic compounds which can support
the growth of pathogens and should be products of last resort as pH
buffers." So probably that is the fungi growing in it.

The weird thing is that I have read so many people here using FP + vinegar. One person was even keeping undiluted FP and vinegar in a soap dispense. Nobody mentions anything like this so far. I couldn't be the first person to have pathogen growing in a FP + vinegar solution, could I? Maybe the undiluted FP and vinegar was strong enough to kill pathogens but a solution wasn't.

As to your last question, yeah, you are probably right that I may not need lowering the pH since I'm using pure water. I guess I should leave the vinegar alone before I can find a better way to lower the pH, like sulfuric acid.

Anytime you add an acid to a basic or less acid solution, you create salt; and that is what you are seeing as a sediment. I believe the FP is already slightly acid; and if your water is RO you probably don't need the vinegar.

If you don't know if you need it I wound't use anything to lower PH, it always surprises me that people are willing to add things without knowing if they are needed/will help or harm beforehand. PH test strips are super cheap and easily found so I suggest getting some to make sure your on the right track, which you may be but its always nice to know. What are you growing that you aren't using a gallon of water at a time?

Have you checked the pH of your water like Sammers suggested?

It's usually very helpful from tap hard water but as for the water you are using, I would certainly test it..You may not need it at all.

John, thanks for your comment. I agree with you on the latter point. Just for the sake of chemistry discussion, FP already contains soluble "salts" before I add acid, and I don't think adding a weak acid to a salt solution creates sediments. For instance, adding CO2 to CaCl2 would not create CaCO3 sediment. There's actually no (visible) sediment even one day after adding the acid. The white substance only starts to appear after a week or so. Considering it's protein-like texture, I think it is some sort of organic thing.

My tap water has pH around 7.5 and a 280 ppm reading for total dissolved salt (kinda hard). After adding the 2 tbsp vinegar to a gallon of tap water, it has a pH around 5.5. So I use pure water whenever I can, but also sometimes I use tap water to reduce cost. After seeing the white substance "growing" in the solution, I tried it again using pure water to rule out if it was caused by salts in the tap water. It still grew.

Sammers, you are definitely right that we shouldn't be adding things without knowing if it would harm the plants. But I also have a strong scientific curiosity. After reading about many people using FP and vinegar, I want to give it a try too.

It's not like I can't finish a gallon of solution a time. Sometime I need 1.5 or 2.5 gallons, and there will always be some leftover.

Elliot,

Creation of salts is a given; precipitation of salts is mostly a matter of concentration. What you may well be seeing/experiencing is the growth of "mother"; you can google "vinegar mother" to know more about it; but in your case, I would just discard it.

Good point, John! I should've thought about that because I drink kombucha every day. LOL!

I have read about vinegar and citrus, but I am too chicken to try it. Is there a reason why people don't use Miracle Gro's Miracid? I have not used either at this point.

MG's Miracid does not have the micronutrients, especially Mg and Ca that citrus need.

Denise, many people use some sort of way to lower the pH. Using vinegar is relatively safe (I believe). I've actually tried it on all my plants (hundreds, of at least 50+ species) for months without seeing any positive or negative effects so far. I need to set up a control group but I probably will never get time to do that. For now I'm more like doing cargo cult science. I'm just doing it because I "think" it is good. I don't know if there will be enough improvements to justify the effort.

Denise, why are you so afraid of vinegar? I have been using it for years and never had an issue. In fact it has saved all of my trees, Hoya, tropical plants, and anything in between...When diluted at such a weak rate, it causes harm to none of my plants, even if were water was already acidic...It does not last long enough to stay in the soil, but just long enough to make the nutrients in your fertilizer solution readily available..Thank God for it, or my plants would all look like crap. It's as natural as you can get.

Let me just say, if anyone here has ever seen how green the good looking the trees Four Winds grows, just know that it was the grower of those trees that saved my tree and turned them a rich green which I could of never attained without vinegar, as he put it for us hobbyist, "the key to success".

Elliot has got it.)

I use Osmocote Plus for the micronutrients, but am considering using a weak dose of Miracid to see if it makes a difference. I have a box that has been sitting on my garage shelf for my azaleas, gardenias, and camillias so I may as well try it.

Denise,Take a close look at the label of your Miracid; and if it does not include 2% Mg and 2% Calcium, save it for things that are not citrus.

I use diluted apple cider vinegar w/o ferts. After a few days, it grows black stuff. Which I find disconcerting since I am using it for fungicide. So now I am careful to use it right away.