Phosphoric Acid Discussion/Clarification

Ive been growing a few years now, but PH down (particularly phosphoric acid) has always made me suspicious that maybe the PH down isn't simply adjusting the PH of the solution going in. I've since learned more about carbonate/bicarbonates, alkalinity, etc. but I'm still unsure if PH down works as intended. I will say I use Promix HP which as you know is amended with lime and shouldn't require phing. The manufacturer website also says phing input doesn't matter in Promix as the lime and alkalinity and it's impact on medium PH drift are all that matter. Here is a post regarding the issue that sounds a lot like what I've dealt with the last couple of years.

"When you "pH" your water in soil, the acids become more concentrated as the water evaporates. pH down doesn't evap with the water, so as the concentrations rise with every watering it will eventually lock out a lot more than just Ca.. Please trust me, it took me years of following bad advice to realize that I was doing it all wrong.

Not everyone has the same issues, because most people us r/o water or DI water. For instance, my tap water is pretty hard, like 200ppm, so when I mix a nutrient solution at full strength it sits around 7.0. When I was doing it wrong it was taking me ~1/4tsp of pH down to bring it to 6.0, and more if I used Protekt (the more dissolved solids in the water, the more acid it takes to drop the pH). With DI water just a few granules of pH down will cause the pH to plummet. The plants always looked great at first, because the pH down hadn't accumulated yet, but towards the beginning/mid flower they would start showing all kinds of signs of nutrient deficiencies. So, following bad advice I bought some cal mag, and it never resolved the issue."

Now I'm not saying this observation is correct or not (God knows I need to take everything online with a grain of salt), but it is thought provoking and I'm curious what growers here would say in support or against this theory and why. I've since gone to balancing the PH to around 6.5 with just foliage pro and skipping the protekt since it drives up PH, but my tap is around 180 PPM and the carbonate/bicarbonate content makes it very resistant to PH down. I feel like I have to use a lot of phosphoric acid to overcome the alkalinity buffer. My theory is the amount of phosphoric acid it takes to bring the solution "within range" winds up resulting in wild PH issues as the water/nutrients are evaporating before the acid, resulting in the acid building up in the medium. As a follow up question: does chlorine impact alkalinity? Would allowing off gassing of chlorine reduce the need for phosphoric acid or at least impact the alkalinity buffer where it would take considerably less acid?

Soo you are saying your plants will not eat phosphor?
But yea in dwc first time you use a lot then less and each time water evaporates or gets used you refill.problem solved

amneziaHaze said:

Soo you are saying your plants will not eat phosphor?
But yea in dwc first time you use a lot then less and each time water evaporates or gets used you refill.problem solved

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I'm not saying they can't consume phosphor, but just like overfeeding and creating salt buildup from lack of absorption, I'm wondering if an excess of phosphoric acid can build up in Promix due to water evaporation outpacing the absorption of the acid. To clarify, I'm using Promix HP, not DWC.

Of course thata why you test runoff ph

I suppose it depends on HOW you water your mix. I like to water to the point of excessive runoff and allow the pots to fully drain each and every time. There wouldn't be the chance for the pH Down to ever build up. My tap water is at about 7.0-7.5, so, by itself, I would add about four drops per gallon to bring it into the ~6.5 range. BUT....when I mix up the fertilizer with the water, the pH is too low, so I have to add about six drops of pH Up to bring it back.
Initially, the mix I was using, by itself, was too acid and I got a runoff that was significantly low (4.0). By testing and adjusting the incoming water until the runoff was in the proper range, I was able to restore the pH.
Assuming you aren't getting enough runoff, then I suppose the phosphoric acid could build up over time. Again, it's all about your watering technique.

amneziaHaze said:

Of course thata why you test runoff ph

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Even runoff PH only tells so much though. Without a soil tester, at best I'm seeing what PH goes in and what passes through the bottom for that feed, not necessarily what the overall PH of the peat itself is. I'm doing a test with a cup of the sink water before and after 3 days to see if the chlorine off gassing helps the alkalinity. Tap is 140 PPM and 7.3 PH this time of year. With 1 1/2 tsp of FP per gallon, I can get the PH to around 6.5.

tstick said:

I suppose it depends on HOW you water your mix. I like to water to the point of excessive runoff and allow the pots to fully drain each and every time. There wouldn't be the chance for the pH Down to ever build up. My tap water is at about 7.0-7.5, so, by itself, I would add about four drops per gallon to bring it into the ~6.5 range. BUT....when I mix up the fertilizer with the water, the pH is too low, so I have to add about six drops of pH Up to bring it back.
Initially, the mix I was using, by itself, was too acid and I got a runoff that was significantly low (4.0). By testing and adjusting the incoming water until the runoff was in the proper range, I was able to restore the pH.
Assuming you aren't getting enough runoff, then I suppose the phosphoric acid could build up over time. Again, it's all about your watering technique.

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You've brought up some interesting points. I also run a top to bottom pour through of about 3 gallons every 2-3 days with a solid amount of runoff. So you're saying the constant flushing of the old build up in the runoff means the odds of the acid being retained in any real amount in the medium itself is basically non-existent? I still wonder if the PH of the water at time of feeding reflects what the meter actually says or it starts drifting once it's inside the medium due to uneven breakdown of the acid, nutrients, and water.

Yes, you are adding H+ but that is the point. The type of fertilizer you give the plant will cause the plant to kick off either H+ or OH- which will change the pH over time along with the alkalinity in your water. Adding H+ to adjust your nutrient solution is adjusting both the alkalinty of you water and what OH- that the plant is kicking out everytime it uptakes a NO3 ion. The alternative is using some NH4 as nitrogen source. Or adjusting the amount of lime added in the beginning, depends on if your pH is going up or down over time. Its a balancing act really but peat has enough of a buffer to give you leeway. You sure its not a EC problem instead of pH.

Keep in mind, these "soil" mixes aren't really dirt-soil -like you have in the vegetable garden. Most of the time, they are inert mixes with a peat moss base. And peat moss is acidic. So, just to make sure we aren't equating soil (less) mixes and dirt-soil. Dirt-soil has all the little bugs and microbes and worms and "critters" roaming about through the soil, eating and buffering and adjusting things the natural way....BUT, soil-less mixes aren't that. Soil-less mixes may have some microbes added and maybe some worm castings, etc., but they are mostly "dead" and inert -especially if you water the way I have described. Whatever might be in the mix at the beginning, will be leached out after a month or so. After that, it's basically a drain-to-waste hydro op.

I run small pots (typically, 3-gallon) and I base all my watering on the weight of the pots. My mixes are very open and they do not retain a lot of water, so I am constantly checking the weight to make sure I don't let them get too dry. It would be difficult to over-water the type of mix I use. I never water based on time. I only water based on weight.

RuggedWombat said:

You've brought up some interesting points. I also run a top to bottom pour through of about 3 gallons every 2-3 days with a solid amount of runoff. So you're saying the constant flushing of the old build up in the runoff means the odds of the acid being retained in any real amount in the medium itself is basically non-existent? I still wonder if the PH of the water at time of feeding reflects what the meter actually says or it starts drifting once it's inside the medium due to uneven breakdown of the acid, nutrients, and water.

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In my experience, yes.

Roguedawg said:

Yes, you are adding H+ but that is the point. The type of fertilizer you give the plant will cause the plant to kick off either H+ or OH- which will change the pH over time along with the alkalinity in your water. Adding H+ to adjust your nutrient solution is adjusting both the alkalinty of you water and what OH- that the plant is kicking out everytime it uptakes a NO3 ion. The alternative is using some NH4 as nitrogen source. Or adjusting the amount of lime added in the beginning, depends on if your pH is going up or down over time. Its a balancing act really but peat has enough of a buffer to give you leeway. You sure its not a EC problem instead of pH.

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I did have an EC issue with underfeeding during veg (trying to gauge minimal EC to prevent def), but I think I'm just looking for justification that with the lime already in Promix maybe I'm overdoing it adding PH down and the natural buffer of the medium should bring everything in range. I did have EC issues earlier in veg, but I ramped up to around 1.7-1.8 EC in early flower and the plants look great now with maybe a bit of burn in some newer growth. However I see no signs of N, P, or K loss, just interveinal damage most likely from underfeeding resulting in the plant consuming its own fan leaves. Also I had been using protekt which took even MORE ph down so making sure I'm not overcomplicating things by adding more to the mix than necessary. It seemed like I was putting in way more PH down to overcome the protekt and alkalinity to get the PH in range than your average grower on here was using. For example someone here said 3-4 drops of PH down, but Ive has to use like 15-20ml of PH down to get 3 gallons within range. 1 1/2 tsp FP per gal brings it to 6.5, but that's with no protekt.

I dunno, but I will say, I use White Distilled Vinegar to pH my water, for a couple reasons. 1) its much much cheaper than pH down, like 4 bucks a gallon at wal mart. 2) Its a natural acid MADE by bacteria, so its usable BY the soil bacteria(learned that on a garden talk with Mr Grow It episode) and its also less stressful on the soil biology.

For pH up I use Potassium Carbonate, you can get it at homebrewing supply companies, its used to adjust pH in winemaking, so its safe for human consumption, plus it provides an extra potassium boost.

tstick said:

Keep in mind, these "soil" mixes aren't really dirt-soil -like you have in the vegetable garden. Most of the time, they are inert mixes with a peat moss base. And peat moss is acidic. So, just to make sure we aren't equating soil (less) mixes and dirt-soil. Dirt-soil has all the little bugs and microbes and worms and "critters" roaming about through the soil, eating and buffering and adjusting things the natural way....BUT, soil-less mixes aren't that. Soil-less mixes may have some microbes added and maybe some worm castings, etc., but they are mostly "dead" and inert -especially if you water the way I have described. Whatever might be in the mix at the beginning, will be leached out after a month or so. After that, it's basically a drain-to-waste hydro op.

I run small pots (typically, 3-gallon) and I base all my watering on the weight of the pots. My mixes are very open and they do not retain a lot of water, so I am constantly checking the weight to make sure I don't let them get too dry. It would be difficult to over-water the type of mix I use. I never water based on time. I only water based on weight.

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I water based on weight as well. On average they are light, but not bone dry medium at the end of 2 days.

IndooorGardnerOhio said:

I dunno, but I will say, I use White Distilled Vinegar to pH my water, for a couple reasons. 1) its much much cheaper than pH down, like 4 bucks a gallon at wal mart. 2) Its a natural acid MADE by bacteria, so its usable BY the soil bacteria(learned that on a garden talk with Mr Grow It episode) and its also less stressful on the soil biology.

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Also an interesting idea as an alternative. Though I am still concerned about the sheer amount of acid that it was taking to bring the PH in range. 15-20ml seems to be far more than what I read on here. I will say though the feeding process is add nutrients, PH down within range and then shake the water up and measure so it's possible shaking it up and not allowing enough time to settle might show an inaccurate PH reading initially before feeding?

RuggedWombat said:

Also an interesting idea as an alternative. Though I am still concerned about the sheer amount of acid that it was taking to bring the PH in range. 15-20ml seems to be far more than what I read on here. I will say though the feeding process is add nutrients, PH down within range and then shake the water up and measure so it's possible shaking it up and not allowing enough time to settle might show an inaccurate PH reading initially before feeding?

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I use about 2-3 ounces of Distilled White Vinegar in a gallon of water to bring my pH into range. Also consider, Distilled white Vinegar is only 4% to 5% Acid, the rest is just distilled water. I also have my vinegar in a jar with dried crushed eggshells, epsom salt and some potassium carbonate for a cheap home made cal-mag-potassium supplement. seems to be working pretty well.

Phosphate (which is the same as phosphoric acid in this context) is quickly taken up by plants. Using phosphoric acid to regulate pH can provide significant amounts of phosphate in a nutrient scheme.
After fertigation phosphate is quickly absorbed so that the soil pH even rises. With calcium it can react to make calciumphospate (same mineral as your teeth) which is completely insoluble and behaves more like sand in a medium than a nutrient. Run off removes the soluble portion reliably.
What I'm explaining is that accumulation is not likely.

Tap water with ~200 ppm is on the soft end. Hard water can exceed 500 ppm.

The lime in the medium is difficult to predict. Promix contains peat (which is acidic and breaks down creating more acid) and lime (which is basic and dissolves slowly, neutralizing the acidity of the peat as it is released). In a perfect world this is perfectly balanced and keeps the pH constant. If you were to add base with your irrigation water you would tip the balance creating less than ideal conditions. In reality the buffering capacity of the lime is too strong initially and you have too basic pH and eventually is exhausted leading to acidic pH. Water with ideal pH, measuring run off pH and adding more lime when required is the best management here.

An alternative acid to use is sulfuric acid. Sulfate is not a concern to cannabis with a high tolerance to overfeeding. Organic acid like vinegar and citric acid should be avoided. They are breaken down by microbes in the media causing pH swings. Those are difficult to control as you can't really measure rhizosphere conditions.

IndooorGardnerOhio said:

I use about 2-3 ounces of Distilled White Vinegar in a gallon of water to bring my pH into range. Also consider, Distilled white Vinegar is only 4% to 5% Acid, the rest is just distilled water. I also have my vinegar in a jar with dried crushed eggshells, epsom salt and some potassium carbonate for a cheap home made cal-mag-potassium supplement. seems to be working pretty well.View attachment 5375523

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Shout out to fucking DDP S&C though shit is delicious.