# soil and co2



## Mitchel (17 May 2021)

Hello

a quote from a supplier website:
_The soil lowers and stabilizes the pH at 6. At this pH value there is naturally sufficient CO2 available for good plant growth. The CO2 content of the 
water is very important for optimal plant growth. The CO2 absorption at a pH of 6.0 is approximately 80%. At a higher pH value the absorption decreases quickly_.

2 questions:
1) So, if I use this soil and the pH drops and stabilizes at pH 6, is there enough CO2 for the plants in the water? 
2) is the acidity way too low for healthy plant growth?

thank you,
Mitchel


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## dcurzon (18 May 2021)

hmmmm, sounds suspicious to me, but im really no expert at all so maybe wait for a more knowledgable person....
CO2 causes Ph to drop. As co2 is absorbed into the water, the ph drops.  Usual target is around 1ph drop, so if your normal ph is 7ph (as an example) then the target to reach with co2 injection is for that to drop to 6ph, which signifies roughly 30ppm of co2.  Likewise, a drop from 8ph to 7ph would also be around 30ppm.

I really do not think that just because the soil lowers the ph value, that it magically introduces co2 into the aquarium.


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## dw1305 (18 May 2021)

Hi all, 


Mitchel said:


> _At this pH value there is naturally sufficient CO2 available for good plant growth. The CO2 content of the  water is very important for optimal plant growth. The CO2 absorption at a pH of 6.0 is approximately 80%. At a higher pH value the absorption decreases quickly_.





dcurzon said:


> I really do not think that just because the soil lowers the ph value, that it magically introduces co2 into the aquarium.


It doesn't. 

It is an answer that needs a bit of unpacking.  If you don't add CO2 the amount in the water is dependent on the amount of CO2 in the atmosphere, and at 415 ppm CO2 that value is <"theoretically ~0.6 ppm"> and usually assumed to <"actually be about 3 ppm">. 

The 80% value comes from the CO2 ~ carbonate ~ pH equilibrium, where the Total/Dissolved Inorganic Carbon (TIC / DIC) doesn't change in amount, only in form, from <"CO2 to HCO3- to CO3-- with increasing pH">. 






cheers Darrel


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## Mitchel (18 May 2021)

That makes sense.
Thanks for the answers


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## dw1305 (18 May 2021)

Hi all,


Mitchel said:


> is there enough CO2 for the plants in the water?


I use a floating plant (usually Amazon Frogbit (_Limnobium laevigatum_), partially because it has access to 420 ppm of atmospheric CO2. Diana Walstad talks about this in <"The Ecology of the Planted Aquarium"> as the <"_aerial advantage_">.


Mitchel said:


> is the acidity way too low for healthy plant growth?


A lot of plants do better at lower pH values, partially because the TIC is in the form of CO2 and partially because many nutrients are more available at lower pH. Have a look at @Roland 's <"Soft water tank">.

cheers Darrel


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