For Captan a pH of 5 is considered ideal and gives a half life of 32hrs. At a pH of 7 the half life is 8hrs. You want to measure the pH and adjust it if needed before adding the Captan. See the links in the post below for more details. For backyard use a range of 5-7 would be acceptable in most cases since you will spray right after mixing. It is possible to adjust the pH to be too low which can damage plant tissue. See the links in the post below for more details.
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If youâre small backyard grower, just use rain water.
Iâll offer a few more suggestions, as we seem to be back to discussing practical application. As mroot points out 5 is a good goal for Captan. I actually shoot for 6 pH because, as Rick points out, once the product dries, it no longer degrades from hydrolysis.
Iâve not used battery acid (sulfuric acid) to acidify water, but I imagine it would be fine. I would not use muratic (hydrochloric acid) as the foliage probably wouldnât like all the extra chlorine. It probably goes without saying that strong acids like sulfuric and hydrochloric are dangerous to handle and wouldnât take nearly as much to alter the pH.
As I mentioned earlier, everyone will have a different results with the amount of acid needed to reduce the pH. I previously used the term alkalinity, which is easy to confuse with alkaline, but alkalinity (aka buffering capacity) is the ability of the water to refused to be acidified. It has nothing to do with pH (or potential hydrogen).
Until one gets a baseline on their water, itâs best to either use test strips or a pH meter, if you want to be exact. If you use a pH meter, make sure the bulb doesnât dry out, which can alter the accuracy. I also bought some test packets with verified pH values to make sure my meter was calibrated correctly.
One doesnât need to be that OCD about it though. University extension specialists have general recommendations for adding citric acid without ever testing the water or spray solution. Thatâs because the foliage of most the temperate fruits grown commercially here are fairly forgiving of acidified water. Iâve âeyeballedâ the citric acid I add to the tank before, and added extra to make sure I have enough in the tank. Iâve never seen any negative effect on the foliage. As long as you arenât using something like Copper (which should never be acidified) it would take some pretty acidified water to hurt the foliage or fruit. You can google the recommendations for the amount of citric acid per hundred gallons of water. Just remember, if using vinegar, itâs going to take about 24 times as much vinegar as citric acid measured by volume.
Most pest control chemicals are not affected by calcium carbonate (i.e. dissolved limestone). It will bind some herbicides rendering them less effective. Most people add AMS (ammonium sulfate) to the water before adding a herbicide like glyphosate to bind up the cations before the glyphosate is added. This year Iâm using a Helena product called Request in place of AMS. But this issue really isnât a consideration, unless you are using herbicides.
If one enjoys reading about the effects of water quality on pesticide performance, here is a pretty good slideshow presentation from Purdue. I scanned it. It looks pretty good. It discusses even the use of pond water and well water as a spray carriers (i.e. water used in the spray tank).
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I like to use rainwater for as much as possible. I use a lot of water! It helps a lot.
It does both. Depends what you are using.
On this forum pH to use for pesticides has been well covered since inception and before that we covered it on Garden Web. Links to best pH for each major pesticide are posted in this forum. I lost most of my links when I switched computers, I know I posted at least three sites on specific pesticides and best pH for them.
One reason I can think of best pH is not mentioned is because the label is seen as the law. So if youâre not using the suggested pH you would be breaking the law. I guess stating use at all pHâs is allowed would solve that. It does point out how careful with wording you have to be on these labels.
I think itâs a good idea, but I donât write the labels.
