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Re: CO2 uptake and pH

Steve Amor wrote:


> So, I suggest that you get your pH down first (Maybe it would be a good
> idea to determine what it actually is first).  Then see what DIY CO2
> injection does for your plants.  As someone has already said though, I'm
> not sure if your plant load is great enough to warrant CO2 injection yet.

There might be a little misconception here.

The starting pH of an aquarium doesn't limit the CO2 concentration that
you can get in a tank.  You can get 15 ppm CO2 into a tank starting from
pH 10 almost as readily as you can get it into a tank starting from pH 7. 
The solubility of CO2 is not determined by the pH.  We use pH and
alkalinity together to measure the CO2 concentration so it sometimes seems
like CO2 is controlled by pH, but that is not true.  CO2 concentrations
are controlled mostly by the amounts of CO2 moving into and out of the
aquarium and the amounts being used up or generated in the aquarium. 
pH-dependent reactions usually cause only fleeting changes in the CO2

> My question to the rest is... I'm interested in the relationship between
> pH and the plants' ability to take up CO2.  What is the limiting level
> above which plants cannot utilise the dissolved CO2?  If this has been
> written about previously, can you provide me with a URL.

To the best of my knowledge, the uptake of CO2 by plants isn't a pH
dependent process.  If you have 15 ppm CO2 then plants will use CO2 no
matter whether there is such a huge alkalinity that the pH is 8.4, or
whether the alkalinity is lower and the pH is 6.8.  No diff.  Normally,
though the alkalinity isn't that high and a pH of 8.4 implies a pretty 
low CO2 concentration. 

At low CO2 concentrations and with other things being conducive, some
plants will start using bicarbonate as their carbon source, and some will
not.  I don't think there is any pH where that change occurs, and
certainly not one that's applicable to all plants. 

Roger Miller