# PH and It's Effect on Plants



## aquabillpers (Apr 13, 2006)

What effect does pH have on plant growth?

Assuming the same KH and GH, will the same plants grow equally well in water with a pH of 6.8 and of 8.4? Why?

Are there some plants that prefer high or low pH?

Bill


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## isu712 (Feb 8, 2008)

I don't think you can have the same GH and KH at both a pH of 6.8 and 8.4. My reasons for thinking this aren't necessarily based upon strict fact, but rather my experience with chemistry (I studied biophysics in college and have been working as an analytical chemist for the last 1 1/2 years). When you change something like pH it is going to significantly change the equilibrium which will cause other chemical reactions to take place and make it so that things that contribute to KH and GH are either going to be more prevelant as free ions or less prevelant.

For example, I had a really low pH in a newly set-up NPT (close to 5, yikes). Also the KH was not registering it was so low. When I added some crushed coral to raise the KH it also brought the pH up. This just shows how so many things are intertwined when it comes to chemistry.

Anyway, like I said before, I don't have any real evidence to back my claims, it's just my instinct. In fact I could be completely wrong and if I am wrong I am sure someone here will be more than happy to set the record straight.


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## mommyeireanne (Oct 24, 2007)

D Walstad shows in the book how _most_ plants will do better in hard water, even many that grow in lower pH, softer conditions. She shows studies as well as her own experiment results. So when LFS's tell you it's a acidic or softer water plant, it's probably fine anyway. Lots of variables influence plant growth, if overall the tank is doing well, you may just need to remove an individual plant that isn't growing as well. This book is worth a read or three. Good reference for any fishkeeper with plants.


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## helenf (Mar 24, 2008)

It probably depends on where the plant is from and the specifics of that ecosystem. 

The following is based on my understanding of a discussion in Tim Flannery's "The Future eaters", a book I highly recommend to anyone interested in ecosystems and in how humans interact with their ecosystems:

I'm guessing that, like animals, in ecosystems where there is little nutrient available, it pays plants to specialise, to be able to survive in odd niches where something is a little different - maybe a high PH, maybe a low PH, maybe different water chemistry or whatever.

I'm also guessing that in ecosystems where there is a lot of nutrient available, it doesn't pay to specialise but to generalise and to be able to take up as much nutrient as possible as fast as possible, so as to be able to reproduce faster. 

This effect means that you get more diversity of species in ecosystems that are not very fertile. Like tropical rainforests and like most of Australia. You get less diversity of species are more "generalist" type animals (and plants) in ecosystems that are more fertile, like most of Europe and North America. 

So, plants from a low PH ecosystem that has a lot of nutrient should be, in general, able to adapt and do well at high PH. 

Plants from a low PH ecosystem that is also barren (not much nutrient available) might be surviving there because they have adapted to a specific set of circumstances. They are specialists, not generalists. They won't be so able to adapt to and take any advantage of a situation that is different to the one they are designed for. So they won't do well in high PH water.

The same argument would apply to moving plants from high PH to low PH, and from moving plants from water of different hardness, etc. I think 

Now, I'm a physicist, not a biologist. So there are probably things I don't know about that contribute to all this. Would be very glad to learn about them if so, as it's interesting!


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## Diana K (Dec 20, 2007)

pH and KH are closely linked. High pH is generally linked with high KH, but not always.

Some plants can use carbonates as a source of carbon. If you put some of these plants in a CO2 deprived tank, but it had high KH (and likely high pH) then these plants, able to utilize carbonates, would grow better than the plants that do not have a biological mechanism to handle carbonates. It might get to the point that the carbonate metabolizing plants would out-compete the CO2 using plants to the point that the CO2 using plants would eventually die. The pH is not _directly_ responsible for either the plants that lived or the ones that died.

Many minerals that plants use are indeed found in different forms at different pH. Some minerals are bound in such a way that plants can easily break that bond and take the mineral in and use it. At a different pH that mineral may bind to a different molecule, or in a different way that plants cannot break the bond, so they show a deficiency, even though you may be adding that mineral to the tank. 
In general most land plants can take in the most minerals and other nutrients from soil that is pretty close to neutral (6.5 to 7.5 for most plants) I am not sure if it is the same optimum pH in aquatic gardening, though I do know the concept is the same.

There are a few highly specialized plants that really are particular about the GH, KH and pH of the water, but in general most of the commonly available plants are fine at a wide range of conditions.

I have found that Hornwort will not do well in soft water, but thrives in a moderately hardwater tank. The pH, GH and KH are all higher in the tanks where Hornwort thrives. None of the parameters are a stand-alone figure.


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## aquabillpers (Apr 13, 2006)

Let me rephrase my question.

We all know that water with a higher GH is a much more important source of plant nutrition than water with a high KH. Most plants don't use KH as a nutrient.

We also know that the amount of CO2 in the water is not affected by the KH.

And we know that pH is largely a function of the KH, all other things being equal.

So the restated question is, "In order to promote the best plant growth, should the KH in the hypothetical tank be maintained at a level sufficient to keep the pH at 8 (about 20 DH) or at a lower level to maintain a pH of about 7?" Please ignore the problems involved in adjusting KH.

Thanks again.

Bill


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## Diana K (Dec 20, 2007)

Phrased that way I would aim for whatever pH the fish wanted, actually, then choose plants accordingly. There is no hornwort in my Discus tank, for example. 

If you wanted to target a certain pH then I would suggest aiming for something close to neutral (6.5 -7.5) and avoid the extremes. 

Better to target the KH and GH that you want and let the pH fall where it may.


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## aquabillpers (Apr 13, 2006)

Thanks for your responses.

I asked the question because for years I had thought that plants did better in slightly acid environments but I didn't know why. The answer I got here contradicted that idea, and I didn't like that. 

So I did what I should have done and went to Google.

Google led me to a site that told me that at a pH of about 7.8 dissolved iron precipitates and and is unavailable to plants. The availability of manganese and phosphorous also is much less at high pH's. http://www.floridadriftwood.com/plantinfo/pH_level.htm

This confuses me a little, because I am getting reasonable plant growth in most of my tanks, and they have pH levels of around 8. Most have soil substrates. I wonder if the precipitated minerals enter the substrate and somehow become available to the plants via that route?

Also, at higher pH levels the relatively innocuous ammonium (NH4) becomes fish-killing ammonia (NH3). Each point increase in pH increases the ammonia level by a factor of 10. The ammonia level in aquariums with high pH needs to be monitored fairly closely. http://fins.actwin.com/aquatic-plant.../msg00139.html

Bill


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## Dr. Demento (Jul 26, 2007)

One of the main factors that pH affects is the ability of enzymes to "do their job." Since it is enzymes that allow biological chemical reactions to occur, getting the pH at a "optimal level" is imperative. The question then becomes - What's optimal? The unsatisfactory but correct answer is "Depends." Plants have adapted to a wide range of environment and therefore utilize enzymes capable of sustaining life (often the same enzyme, but with differences that allow it to function at differing pH). Best bet - go with the middle of the road (7'ish, give or take).

However, plants (like all life) are adaptable. If you grow them at a pH that's not native nor ideal, they may adapt (or fail to thrive). It's possible to get good growth outside of their normal range.

As far as the iron precipitating - it may be a moot point. Reason being is that while plants are unable to use reduced iron, some bacteria often use it as an electron acceptor (similar to how others use hydrogen sulfide and protect the roots) and this will free up the "precipitated" iron for plant use. The other explanation has to do with how their testing method/kit measures iron (which I'm ignorant of). Even though it registers "zero" iron, there may be sufficient to sustain plant life.


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## aquabillpers (Apr 13, 2006)

That's a good point about enzyme activity being affected by pH levels.

I've also read that biological activity tends to be greater at lower pH levels, so the microbiological activity that NPT's rely on would seem to be enhanced by lower pH levels.

So from now on I will try to keep pH levels around 7.0. I will also dose iron to my one tank that isn't doing as well as the others.

Thanks again.

Bill


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## Diana K (Dec 20, 2007)

pH in the water may not be the same as pH deeper in the substrate. A lot of bacteria (And other microorganisms) activity in the substrate, with less water flow may alter the pH, and therefore the availability of fertilizers and other nutrients to the roots.


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## vvdo (Mar 7, 2006)

aquabillpers said:


> Google led me to a site that told me that at a pH of about 7.8 dissolved iron precipitates and and is unavailable to plants. The availability of manganese and phosphorous also is much less at high pH's. http://www.floridadriftwood.com/plantinfo/pH_level.htm
> 
> This confuses me a little, because I am getting reasonable plant growth in most of my tanks, and they have pH levels of around 8. Most have soil substrates. I wonder if the precipitated minerals enter the substrate and somehow become available to the plants via that route?
> 
> Bill


I believe what I have read is that it depends on the chelate iron is bound to. Availability or solubility of iron changes with pH. That's not the same as saying that at pH 8 all iron precipitates, more like a percentage say 50-60%...










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## aquabillpers (Apr 13, 2006)

I'm respomding to my posts of 12 years ago. I disagree with them. 

I had said that I was going to maintain my NPT's at an acid pH, because I believed that 
most plants and fish did better in a that environment. My well water is hard and alkaline, so I tried a number of ways to make it acid; most worked. The plants and fish seemed to do as well as they did before I treated the water. But keeping the water on the acid side was a major pain.

So I stopped the treatment. The water return to its normal alkaline state. My back felt better; I no longer had to lug gallons of acid swamp water home. And the fish and plants continued to do well.

Now I think that in most cases, one is better off working with the water that he or she has, rather than trying to make it "better." 

Bill


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## dwalstad (Apr 14, 2006)

Here is where soil comes to the rescue.

Yes, a high pH will make iron and manganese unavailable in the water. But whatever the water conditions, a soil substrate can provide these nutrients. First, ordinary soil contains plentiful iron and manganese. Second, soil makes them available to plant roots, because the soil is anaerobic and contains lots of DOC, which naturally chelates these nutrients, thereby making available to plants. 

The beauty of all this is that iron and manganese are NOT available to algae. That is a major reason why I advocate using a combination of soil and strong rooted plants. 

High pH, no problem!


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## vvdo (Mar 7, 2006)

aquabillpers said:


> I'm respomding to my posts of 12 years ago. I disagree with them.
> 
> I had said that I was going to maintain my NPT's at an acid pH, because I believed that
> 
> ...


Tapatalk is doing some screwy things... wonder why an 11 year old post turns up at the top of my feed... meh!

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