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More about pH

Posted by lathyrus_odoratus 5A-IL (My Page) on
Sun, Jun 24, 12 at 17:12

Al have pointed out that we shouldn't worry much about pH in our container plants - it's hard to determine as many things affect it and it's always changing.

But, some of us have water with high pH (but not alkalinity) and we see our plants suffer - changes in leaf color - from yellowing to lighter greens in some places on the leaf to losing the red backing on some plants.

In response, we often use vinegar or citric acid to lower the water pH so that the resulting soil solution is acceptable to the plant.

On another forum, someone has questioned that reducing pH would have any effect. The quote was, " What causes the pH to stabilize at neutral when you add the water to soil, is the presence of thousands of buffering sites on the humic acids in the soil that soak up the hydrogen ions from the acidic water and exchange them for other cantions, bringing the pH to whatever the buffering power of the soil is, somewhere near neutral (7.0)."

The poster is assuming a peat-based soil, which many people use.

Clearly, those of us who do this see a benefit. So, seems that the above cannot be completely correct.

I want to understand the process that takes place and none of the few plant books I have discuss it. I have tried some searches, but am using the wrong terms as I come up empty handed.

Al, I am hoping you can help me understand or point me to a resource that explains it.


Follow-Up Postings:

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RE: More about pH

I'm not sure thatim correct, but I think part of the benefit of using vinegar, etc... To lower you tap waters pH is that it makes th calcium and magnesium in your tap readily available to your plant... Not 100% sure, but I believe that is one of the benefits.


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RE: More about pH

I believe the quote, " What causes the pH to stabilize at neutral when you add the water to soil, is the presence of thousands of buffering sites on the humic acids in the soil that soak up the hydrogen ions from the acidic water and exchange them for other cantions, bringing the pH to whatever the buffering power of the soil is, somewhere near neutral (7.0)." is referring to soil not soilless potting media.

That would be like saying, "I put 20cc vinegar in my tap water with a pH of 7.8, alkalinity at 1.42 meg/l. Five minutes later my pH was back to 7.8 pH." This is not true.

I'm sure others will be by to help, but I can assure you, vinegar will work to lower the Ph in soilless media for the proper nutrient uptake by the roots.

Ron


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RE: More about pH

Ron, my experience also shows it to be true. So, now we just need to know why!


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RE: More about pH

I don't know much as I'm still a garden newbie but I searched and found some things Al has posted re: vinegar. Maybe we can save him some typing. :)

"Vinegar and citric acids are indeed poor as remedies for mineral soils (because of their high buffering capacity) with high pH, Which is undoubtedly what Dr Gillman was referring to, but they are both very effective and inexpensive ways to temporarily lower the pH of the soil solution in container culture, 'temporary' being all the container grower needs. Acid injection is used on a regular basis by commercial growers to lower soil solution pH and curb the tendency toward the upward creep in pH of soils irrigated with alkaline water, so there is no reason to believe it wouldn't be equally effective when regularly mixed into the irrigation water or fertigation solution by hand.
I can attest to its effectiveness by offering the fact that during the winter, I often see Fe deficiencies in plants under lights because I'm not able to flush accumulating dissolved solids from the soil as effectively as in the summer when I'm watering with a hose, which causes a rise in media pH and the precipitation of Fe into insoluble compounds. Acidifying the irrigation water with vinegar or citric acid lowers soil solution pH and makes Fe (and other minor elements) more readily available, evidenced by an easily observed change in the color of the foliage of 'tell' plants after acidifying the irrigation water."

"The primary goal of acidifying irrigation water and fertigation solutions isn't to reach an end point pH of the water; rather, it is to achieve a sort of stasis where the water has no effect on raising or lowering the pH of the soil solution.

Vinegar and citric acids are particularly good choices for hobby growers to add to their irrigation water and fertigation solutions because they acidify w/o adding nutrients that need to be accounted for as a fraction of the TDS in the soil solution. IOW, if the hobby grower was conscientious enough to wish to acidify the water supplied to his plants, it would follow that he would also be conscientious enough to make allowances for the nutrients in the acids. E.g., if the hobby grower used nitric or phosphoric acid to acidify his water, he would have to adjust the amount of N or P supplied in his nutritional supplementation program. As you can imagine, the calculations can be bothersome and difficult for those w/o a background in chemistry, and the problem of deciding just how to fertilize using what's readily available can also be a challenge. Using vinegar or citric acid as the acidifying agent eliminates the calculations and allows us to use a fertilizer in the ratio of our choice with confidence that the ratio is not being skewed by the acid used.

It should be noted that describing blueberries and other plants as acid-lovers is probably a bit of a misnomer. Blueberries will grow perfectly well at rather high pH levels if you can limit the availability of Ca. The plants we call acid-lovers have adapted to acidic soils. Their need of calcium is still the same as any other plant, and their nutrient requirements do not differ from plants that thrive in alkaline soils. The problem for acid-loving plants is that they are unable to regulate their calcium uptake, and readily absorb too much Ca when available, resulting in cellular pH values too high. Some 'acid-loving' plants also have difficulties absorbing iron, which is tightly bound in alkaline soils, another reason why they thrive in low pH soils. Note that this all pertains to the plant's ability to handle nutrients, and not to the actual nutrient needs of the plant."

"The goal of acidifying irrigation water and fertigation solutions is to make (primarily) micronutrients more available."

"Finally, organic acids (humic, fulvic, phenolic .....) are very active biologic components of soils. In soil, these acids retard the leaching of essential nutrients by organically bonding them to colloidal surfaces (chelating) in the root zone, and they also have buffering effects on soil pH shifts. Not only do these organic acids increase absorption of nutrients essential to plant health, They additionally are known to improve plant respiration, photosynthesis, and root growth. And while adding acidifying agents to water & fertigation solutions for containerized plants is primarily focused on managing pH-related issues, it's not a leap to think it possible that they also bring some of the benefits that many other organic acids provide in addition to their help in managing pH-related nutritional issues."


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RE: More about pH

If the question is why does it work the answer is simple enough:

Acetic acid (vinegar) is a weak acid. This means that it does not completely ionize in water - which just means that it doesn't always give up its proton (H). But it *is*, afterall, an acid (in water) and it will give up some protons. For our purposes all this means is that it takes much more acetic acid to lower the pH of any solution than it would something like sulfuric acid. Recall that pH is just a measure of H ions in water.

With respect to humic acid and buffering capacity: it's true that humic acid acts as a buffer in soil. The pH value of natural humic acid is 5.99. It has a higher buffer capacity for bases than it does for acids and it resists pH changes in the pH range of 5.5 to 8. Above or below it will tend to lose it's ability to buffer.

But remember that it's just a buffer. It's not a brick wall. If you look around online you can find titration curves showing the addition of various acids to humic acid solutions and the effect on the pH. Add enough acid and the pH will drop. In fact the pH in any buffer nearly always drops some - just far less than if the solution is unbuffered.

Vinegar will work just fine if you use enough of it. I've seen it said by someone in this forum that the effect is "temporary." This is partially true in that plants are producing exudates which can effect pH and a whole host of other things are going on in soil (bacteria, CRF, nitrate/ammonia uptake, etc) - I wouldn't try to lower the pH of my yard with vinegar. But fertigating with vinegar daily or every few days works - or at least it did for me (pH 10.2, alkalinity 110 CaCO3 equiv).

I recently switched to sulfuric acid because I got tired of buying 4 gallons of vinegar on a regular basis... now I just goto a big box store and pickup some Qual battery acid (35% sulfuric acid). If you want to know how to use it then pick a target pH / alkalinity and check out this website: http://extension.unh.edu/agric/AGGHFL/Alkcalc.cfm

Foliage pro is very deficient in S so this has worked very well for me...


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RE: More about pH

@ redshirtcat - what kind of protection do you use when working with sulfuric acid and the like? I'm a chemist, but when I work with things like sulfuric acid (rare in my field) I always do it in a fume hood with serious protective wear. Obviously that's not available at home.

I'm not trying to criticize you, just genuinely wondering what precautions you're taking. I'm toying with the idea of sulfuric acid or phosphoric acid. Were you able to add foliage pro, or your diy equivalent, to acidified water without any problems?

I also got really tired of buying vinegar, so I moved to powdered citric acid. You can buy it bulk online, but it's also available in smaller quantities at retail in most asian and indian grocery stores, as well as brewing supply stores. Besides being powdered and relatively safe, the other thing I like about it is that it's a pretty strong chelator, much stronger than acetic acid. It's also one of the major organic acids secreted by plants naturally to capture poorly available mineral ions, tying into Al's suggestion that this is part of the benefit hobby growers see from using it. I'm particularly interested in the ability to solubilize metal bound phosphorus.


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RE: More about pH

The sulfuric acid I use is ~35%. I understand it corrosive but not nearly as dangerous as nitric acid. I have not noticed any fuming at this concentration.

When working with the acid I take a few precautions. I wear eye protection (splash proof safety goggles) and nitrile gloves. I keep non-acidified running water nearby and I keep a box of baking soda handy to neutralize any spills (*only* for spills - do not put baking soda on your body if you spill acid on yourself - you'll add a heat burn to your chemical burn).

I should note that it is my understanding that nitrile will not stand up to concentrated sulfuric acid but so far I've had no issues with 35% and very minor splashing. I remove the gloves immediately if they come in contact with the acid.

Having never spilled 35% sulfuric on me I don't really know how dangerous it is. I basically follow the directions on the box.


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RE: More about pH

Thanks for the responses. I was away for a few days and didn't get to check back until today.


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RE: More about pH

Just to follow up on this here is an article on CEC in container media and its effect on pH that you should check out: Understanding Plant Nutrition: Nutrient Sources: Media Cation Exchange Capacity

My take home message from the article (which matches with my experience with the 5-1-1 and the gritty mix) is in bold below:

"Consequently, the effective CEC of organic materials like peat measured on a volume basis (i.e. per pot) is about 40 percent to 50 percent less than that of a field soil. On balance, then, CEC of soilless container media is low and provides little buffering.

Research has shown that the CEC of soilless media has little effect on resisting change in pH, or in supplying nutrients to the crop. Several experiments were completed at Michigan State University that tested the effect of CEC on long-term pH and nutrition management using impatiens as the test crop. The media tested ranged in buffering capacity from one considered very low (5 meq/liter, a 70 percent rockwool and 30 percent perlite mix) to one that would be considered highly buffered (76 meq/liter, a mix of 70 percent highly degraded consumer grade sphagnum peat and 70 percent perlite).

Hydrated lime was used as the lime source to increase the initial pH to about 6 in all media because it reacts quickly and completely and did not influence long-term pH management. The amount of hydrated lime needed ranged from 0 lbs/yd3 with the rockwool/perlite media, 0.8 lbs/yd3 with the coir perlite media, 2.5 lbs/yd3 with the grower grade sphagnum peat/perlite and 4.5 lbs/yd3 with the consumer grade sphagnum peat/perlite media.

When an acidic fertilizer solution was applied to the impatiens grown in the different media, pH of the rockwool medium tended to be higher than for the other media. In all media, however, the pH dropped very quickly to a low of about 4 (Figure 2) regardless of the CEC of the media.

When shoot-tissue calcium was tested after four, eight, 12 or 17 weeks of growth, there was little difference between plants grown in the media with low CEC (rockwool perlite) or relatively high CEC (consumer grade peat/perlite). The media-CEC therefore did not act as a buffer to nutrient levels available for plant uptake.

The conclusion of these and other experiments was that CEC from peat has little or no effect on either pH management or calcium and magnesium management in container grown crops."


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RE: More about pH

And here's the followup article on Mg, Ca and residual lime and its effect on pH: http://www.greenhousegrower.com/article/3901/understanding-plant-nutrition-limestone-calcium-and-magnesium


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RE: More about pH

Great find, Redshirtcat! Thank you!


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