# Walstad: Photosynthesis and Denitrification increase pH?



## ocellatus (Dec 18, 2011)

I don't recall that her reasoning was ever explicitly stated. 

Photosynthesis uses light energy to convert water and CO2 to glucose (eventually starch) and O2. Would the rational be that the decrease of CO2 in the water during heavy photosynthesis results in an increase in pH? 

Since photosynthesis outcompetes the ever present process of plant respiration, (converts glucose and O2 to CO2) perhaps there is a net increase in pH through overall drop in CO2?

Likely someone with a bit more biochem experience can clarify / correct this.

O


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## plantbrain (Dec 15, 2003)

CO2(an acid) is removed shifting the pH up during the day if the CO2 is removed from the water column. If the plants are all emergent but have roots is saturated soil, then during the day, often the pH can drop due to the O2 being pumped to the root zone, and bacteria and plant respiration releasing CO2.

For our purposes, we generally assume the 1st scenario.

Denitrification and PS typically reduces the KH, so the pH tends to drop if you took a sample and compared it to the starting water sample. If you had limestone etc in the tank, then it might be higher pH/KH, but without non inert rock etc, the pH/KH tends to drop.

It's nothing to worry about though. Seems to help the tank over time and the plants.


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## Mxx (Dec 29, 2010)

The point about photosynthesis raising pH I'd somehow missed in my first reading of the book. And no it's certainly not explained there, implicitly or otherwise. And a great book, but that point in it seems a little misleading. It seems to suggest that a sufficient density of plant mass to animal mass will result in the pH balancing. 

I thought it might be the case of plants removing just CO2 to increase the pH, but if that is true then the pH will in fact continue to slide and/or crash over time. I note she suggests the addition of dolomite in the substrate, which could be to counteract the production of hydrogen sulfide, (somehow?). But I suppose the dolomite is what is necessary to provide a continual balance to neutralize the organic acids produced continuously, just as in nature where rainwater and streams slowly erode mountains and flush those minerals through the watercourses to the sea. 

Thanks!


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