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Chemistry help? pH issues driving me crazy

Posted by brainmedicine USDA 10b / Sunset 17 (My Page) on
Wed, May 28, 14 at 13:06

so i'm in SF, with water avg pH 8.5, TDS avg 108, alkalinity/hardness avg 61 and 62, respectively. i've got a ton of container plants, some in gritty mix, others in a 3-1-1 of bark/turface/lava rocks, even a few in potting soil. very sunny patio that traps heat, starting to need to water/fertigate up to 3-4 times a week.

i use dyna gro nutrients and acidify with white vinegar shooting for about a 6 pH. i bought some hoses, clamps and valves from local hydro shop and make large batches of fertigation solution for gravity drip in 5 gal buckets. i fertilize with foliage pro and a little bit of protekt, and adjust pH with vinegar. that mix is fairly reliable in its ratio.

but when i mix up my solutions for irrigation i can get seriously different pH readings from the same exact amount of nutrients in each 5 gal bucket. for instance- this morning i went to irrigate with water, protekt and vinegar. 5 gal water, 1.25 tsp protekt, and 1.75 tsp vinegar put me right around 6-6.5pH.
but- when i went to fill the second bucket up i had to use 2.25 tsp of vinegar and that got me maybe to 6.5-7pH. thats nearly 30% more vinegar and still wound up with a slightly higher pH than the first batch.

i'm trying to get my ratios correct to cut down on the amount of time i have to spend watering, (mix, check pH, adjust, rinse, repeat, etc) and this is driving me crazy. i know our water pH fluctuates throughout the year based on our water supply mix, but it shouldnt change in the same day, or even week. and i'd think with so little TDS/hardness/alkinity/etc that the water isnt really the issue. the chemicals maybe?

any body have a similar experience? and ideas what i might be unwittingly doing to cause this- or things i need to do to prevent it?


Follow-Up Postings:

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RE: Chemistry help? pH issues driving me crazy

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.


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RE: Chemistry help? pH issues driving me crazy

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?


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RE: Chemistry help? pH issues driving me crazy

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.


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RE: Chemistry help? pH issues driving me crazy

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 H₃O⁺ 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.


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RE: Chemistry help? pH issues driving me crazy

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.


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RE: Chemistry help? pH issues driving me crazy

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.


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RE: Chemistry help? pH issues driving me crazy

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 ?


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RE: Chemistry help? pH issues driving me crazy

How do you measure the pH of your liquid?


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RE: Chemistry help? pH issues driving me crazy

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


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