I can think of two general chemistry things. First, uniform mixing is important. I assume you are adding the (well shaken) Pro-TeKt, mixing thoroughly, and then adding increments of vinegar, again mixing thoroughly between pH measurements .. but I have to mention it.

I think the second thing, given your precise goals, is that teaspoons are very hard to measure with home tools. A small graduated cylinder or medium sized pipette will let you hit your measures much more reliably.

Given your accuracy goals, and small volumes, you should probably go metric.

brain...don't get lost in the trees. Your forest sounds fine. You definitely have a good grasp on what you are doing. Don't sweat the small stuff, the buffering capacity of nature and each individual pot and plant will override your imperfect fertigation. I'm nowhere near as accurate as you with my measurements. If there are little differences in the amounts of vinegar or fertilizer in my batches, I just don't worry about it. It will all equal out in the end. Just curious, how are you measuring the pH of the liquid?

Remove the Pro-tekt from the equation for a while and see if you get the same variability.

Potassium silicate will react with acetic acid, but the reaction is relatively slow - meaning the pH will slowly change over time. Dyna-gro says that its compatible with any fertilizer, but this is only because it won't react immediately. You can't mix it and let it sit.

Try using a syringe to measure things to increase precision.

When metals go into solution in water, they form complexes with the water and/or other things that are in solution. Some metals grab the negative (oxygen) end of the water molecule so strongly that the water releases a hydrogen, which then bonds to another water molecule, creating HâÂÂO⺠and effectively making the solution more acidic.

So, if you didn't turn on the faucet for a long time and the water was sitting in metal pipes, it potentially could have a lower pH because of dissolved metal. Then you go back to fill a second bucket and you're using water which hasn't been sitting in the metal pipes for so long.

But I don't really know what's going on in your case. Try checking pH before you add anything to the water.

I have had these kind of things happen too. Sometimes you need to let the mix sit for a bit to get a better reading.
I use a mechanical meter, you have to clean the probe often. I use sulfuric acid to clean probe.

so i guess i should go with a syringe or pipette type instrument then, and make sure im shaking up the nutrients in the bottle before mixing. a digital pH meter probly wouldnt be too bad either, compared to basic pH reagent /color coding kit. although they seem finicky and require lots of care- which sort of foils my plans for quick fertigation routine.

as for the process of mixing, i typically will make a 5 gal batch by filling with 4 gals of water or so, then adding FP, then ProTekt, then vinegar. i "stir" with a blast from the hose to mix things around and top off to 5 gals. for irrigation its just protekt then vinegar. i add nutrients according to my "typical" mix ratio, then measure, and adjust as necessary.

now that i'm thinking about it- our water is pretty low in TDS. and the mix i seem to get fairly accurate is the one with the most nutrients- foliage pro 1/2tsp per gal, protekt 1/4 tsp, then vinegar.

my irrigation mix is only getting protekt, and acidified with vinegar. much less nutrient in solution..... so could my problem be a lack of buffering? i'm comparing this mentally to trying to make a batter for 1 cupcake as opposed to batter for a dozen- extremely hard to do in precisely the right ratio.

is there something else i could use as a buffer that would be fairly inert to plants? or should i just go with what seems to be easiest and irrigate with protekt at around pH7 ?

How do you measure the pH of your liquid?

just a typical reagent pH kit, reds are acid, green is 7, and dark green/blue is base