# [Wet Thumb Forum]-Plants consuming KH



## Steve Hampton (Feb 13, 2004)

Phrasing this question correctly will be a bit of a challenge but here goes.

Is it possible for plants to convert/consume/"eat" KH without causing biogenic decalcification? Meaning if a tank doesn't have CO2 injection, and that tank experiences a steady decline in KH would the plants be responsible? I thought that the pH would climb rather dramatically when plants had to resort to biogenic decalcification. Is there a "lower" level of this KH "carbon extraction" that can occur that wouldn't cause an increase in pH?

On a couple of other forums I've seen it come up that plants are consuming or eating KH. I have contended that the plants weren't consuming KH, but rather a stray acid was causing a drop in KH. I'm confused and very possibly wrong. Could someone please set me straight...at least on this issue.


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## Steve Hampton (Feb 13, 2004)

Phrasing this question correctly will be a bit of a challenge but here goes.

Is it possible for plants to convert/consume/"eat" KH without causing biogenic decalcification? Meaning if a tank doesn't have CO2 injection, and that tank experiences a steady decline in KH would the plants be responsible? I thought that the pH would climb rather dramatically when plants had to resort to biogenic decalcification. Is there a "lower" level of this KH "carbon extraction" that can occur that wouldn't cause an increase in pH?

On a couple of other forums I've seen it come up that plants are consuming or eating KH. I have contended that the plants weren't consuming KH, but rather a stray acid was causing a drop in KH. I'm confused and very possibly wrong. Could someone please set me straight...at least on this issue.


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## 2la (Feb 3, 2003)

Steve, if you're pulling bicarbonates or carbonates out of the system and incorporating them into plant tissues, shouldn't that tend to _decrease_ pH?

This assumes that in the equilibrium

*CO2 + H2O <---> H2CO3 <---> H+ + HCO3-*

the rate of HCO3- consumption exceeds the rate (or is one step ahead) of the reaction between CO2 and water, thus producing an excess of (unopposed) H+ in the water. I don't know if this is what actually occurs, but fundamentally my understanding is that increasing the KH of a system (by adding bi/carbonates) _increases_ the pH. Shouldn't the reverse be true, then, and decreasing the KH should likewise _decrease_ the pH if the rate of KH (bicarbonate) decrease exceeds the rate of the bicarbonate buffer system to replenish it?


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## Roger Miller (Jun 19, 2004)

You're right!

When plants use bicarbonate for their carbon source they don't lower the KH. In the overall reaction they split two bicarbonates into one CO2, which they use, and one carbonate. The one carbonate produces the same KH as two bicarbs, so the KH stays the same. Carbonate forces the pH to rise.

KH can be lowered in the process, but it isn't done directly by the plants. In water that has enough calcium the carbonate produced by the plants combines with calcium to precipitate marl. When marl forms it takes carbonate out of the water and the KH drops. Very soft water may not provide enough calcium to precipitate marl. In that case the KH doesn't drop.

When KH drops without high pH it is most likely to be because of acids produced in the tank. The process is most noticable in soft water tanks that don't get frequent water changes. Nitrification produces quite a bit of acid and is pretty universally present in aquariums. There are also organic acids that may be produced in some aquariums.

I don't know if any plants will use bicarb if there is a significant amount of CO2 present. Some algae may. In the case where an algae or plant is using bicarb in the presence of CO2 then the carbonate produced in the process will just react with CO2; there won't be a big increase in pH and the KH won't change.


Roger Miller


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## wetmanNY (Feb 1, 2003)

The drop in alkalinity is due to more than some stray acids. All aerobic metabolisms pump carbon dioxide into the system. And nitrification processes pump nitrous, then nitric acid into the system. These acids have the effect of eroding the carbonate buffer. Some sulfuric acid is also bacterially generated in the substrate and matured biofilms.

Marl precipitates at pH values in the eights, I thought. Isn't this more a Rift Lake phenomenon than a planted tank phenomenon, generally speaking?

At any rate, plants are more involved in "bio-acidification" in death than in life, as more alkalinity is consumed in the bacterial/fungal degradation of their senescent tissue.


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## 2la (Feb 3, 2003)

Okay, I guess I misunderstood the original question, but even with Roger's and wetman's explanation I'm still having trouble wrapping my wee brain around how theoretically pulling bicarbonates out of the system (even if this isn't what actually occurs) would be expected to lead to an increase in pH? Is it more complex than the equilibrium equation above? Put it this way: If you add sodium bicarbonate to an aquarium, you increase the KH and you also get an increase in pH--so why would you expect the opposite (_removing_ bicarb) to _increase_ the pH as well? Thanks...

Dazed,

2la


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## Roger Miller (Jun 19, 2004)

2la,

I think your answer is fine for the case where CO2 is present in the tank, the pH is fixed by the CO2-bicarbonate buffer and plants or algae can use HCO3- without producing CO3--. I don't think that's the usual case.

The only cases where I know plants use bicarbonate are cases where CO2 is depleted, so the carbon dioxide - bicarbonate buffer doesn't work. CO2 is pretty much gone by the time pH gets to 8.4, which is when carbonate starts making a significant appearance. At that point the buffer is not set by carbon dioxide and bicarbonate; the pH is fixed by the buffer formed between bicarbonate and carbonate.

In the bicarbonate-carbonate buffer, bicarbonate is the acid side of the system. When plants or algae remove bicarbonate and produce carbonate they swing the buffer to the alkaline side. It swings hard. The pH can easily reach more than 10.


Roger Miller


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## 2la (Feb 3, 2003)

Got it! Thanks, Roger. Theoretically, then, should an adequately CO2-injected tank never experience biogenic decalcification? I ask because at one time during my DIY-CO2 days I decided to increase the hardness of my water by adding some R/O Right, and within a week the bolbitis was covered in mineral deposits. I don't think my CO2 had fallen off at any point during that time.


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## Roger Miller (Jun 19, 2004)

2la,

I don't think the mineral deposits could have been the result of biogenic decalcification. Can you thing of anything else it might be?

Biogenic decalcification requires the water at the leaf surface to be saturated with respect to CaCO3, and I think that happens only at high pH and fairly high calcium concentrations. By my experience it also only happens under very bright light -- sunlight, for instance. By my reading at least some and possibly most plants use bicarbonate only if there are specific environmental triggers; e.g. pH over 8.4 at the beginning of the lighted period.

Roger Miller


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## 2la (Feb 3, 2003)

It must have been due to some other process then--perhaps it was the plant's way of dealing with a sudden increase in osmolarity. In truth, I can't say with confidence that it was a calcium deposit since it wasn't white at all but something closer to a dull lime green. I of course discontinued use of the R/O Right for awhile but tried it again recently with good effect on my Eusteralis. This time the bolbitis (albeit the small form; the first was the large form) showed no ill effects at all, so I don't know how to explain it. I really wish these plants would read the books sometime.


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## Roger Miller (Jun 19, 2004)

They just need glasses. I understand they prefer to read Kasselmann in the original German.

Roger Miller


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