Mr. Clean wrote:
I had hoped you would chime in on this post. My experiments were all conducted within the window of 9-11 am. The mornings were sunny and warm. When using 20% dilution during the same window, results were rather dramatic in comparison. Oddly enough, I have experienced better results using the 20% dilution on an overcast day, than I did with any of the 10% solutions.
I have thought that perhaps I could dilute the 20% vinegar with apple cider vinegar (5%). If this solution were to work, it would result in at least a minor cost savings.
Most of the vinegar/weed threads are general in nature, so maybe we can get a little more specific here. First, for the chemists on the board, we use the term "acid" loosely, but generally are thinking of Bronsted acids when we use the term. Second, how well/whether vinegar works to kill weeds in a particular situation "probably" depends on what kind of weeds they are and how old they are. In some field tests, straight vinegar (5% acetic acid) has been shown to work at least on young/tender shoots/sprouts. Generally speaking, it probably does not work as well on older plants, which may need a higher concentration of acid, but it seems like that varies some among different species also. (Disregard what I wrote earlier about the non-effeciveness of 5%; I was fixated on mature weeds. It apparently can be somewhat effective, depending on the situation. Adding soap and orange oil "should" make it more effective.)
In one study, I forget the species of weed involved (thistles maybe), a first application of ordinary kitchen vinegar killed the weed crown, but not the roots. It seems to me that, in a robust organic program, even that might be enough to allow competition to overcome the weeds. For the tough ones where root death was needed, I believe they used a 10 or 20% acetice acid. I gather that acetic acid kills the plant by disrupting the cell wall. RE: the comment about the effect on cloudy days makes me wonder if there might have been some entry through the stoma. That in turm makes me want to investigate a bit more about the time of day application aspect. I guess the theory behind applying it in the hot part of the day is to dessicate the ruptured cells and tissue as quickly as possible, but maybe there is more to it than that.
As for diluting 20% acetic acid with kitchen vinegar I would use water instead merely as a cost factor. I had a thought that, if you want to cut the 20% acetic acid with another kitchen-type acid, use lemon juice instead of vinegar. Then, I ran across at least one commercially available product that uses the acetic acid/lemon juice combination with a cactus juice surfactant (I "assume" the cactus juice is ~ the same as the yucca juice that was discussed in the foliar spray surfactant thread). As such, there may be something to adding lemon juice to dilute the 20% acetic acid. The thing is, I don't know how economical that would be. I know that if I had bottled lemon juice that I thought was too old to use for whatever I otherwise would use it for, I'd filter it and use it to clean or spray or use it in the compost pile rather than toss it. There probably is a fair amount to learn once we expand from straight acetic acid to other organic acids or mixtures, and I have a few ideas that I'd like to test.
If you mix acids, be careful. I limit this discussion to acetic acid, so don't go trying to use strong mineral acids in the same way we are discussing acetic acid. Acids are strong oxidants, and mixing strong acids with other acids, bases, solvents, or water can cause a dangerous boiling/explosion, depending on what is mixed. Also, general acid-base chemistry may affect what species can be mixed effectively. It is a good practice to wear eye protection and generally to protect against splashing on clothes or skin. Also, if you mix concentrated acids with water, always add the acid to the water, not the other way around. I would think that, in mixing 30%+ acetic acid with water or lemon juice, one would add the acetic acid to the other. There shouldn't be any problem with mixing lemon juice and 20% acetic. Acetic is not as reactive as the strong acids, but good practice applies regardless of the specie. If anyone reading this sprays the acid through a sprayer with metal parts, rinse/flush it well with water or with a mild baking soda solution right after using it.
I wrote on another forum about the relative unavailability of fruit or grain-sourced strong acetic acid, but I know that 12, 20, and 30% acetic acid derived from apples and/or grain are available. The federal "organic" standards have increased interest in those products, so the availability and cost may be changing. Because this board is or will be read by persons in various parts of the country or internationally, they may have access to products that aren't easily available in the Dirt Doctor's home area (and vice versa). I suppose I should say that, at least until recently, the higher molarity, typically available acetic acids were petro-based. Maybe the increasing interest in organic farming will change the economies of scale and pricing on the plant-based high molarity acetic acids.
There probably is a lot more to say about this topic, and there probably is a lot that I don't know about it. I usually don't worry about weeds very much, but I know some people get pretty excited about them.