# pH in the substrate vs. pH in the water column



## Poe835 (Sep 16, 2004)

I was wondering if the pH in the substrate layer will be the same as the pH of the water column? 

thanks!

Anyone know whether the pH in the substrate layer will be the same as the pH of the water column? Will it be different if I have a minieralized soil substrate vs. granular ADA aquasoil?

thanks!


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## davemonkey (Mar 29, 2008)

I'm not great at chemistry, but I think the pH of the soil will affect the water pH, although they won't necessarily be the same. Hard or high pH water can be softened or acidified by an acidic substrate, but if you do frequent water changes, I don't think they'd really equal out. (I may be wrong. )

Aquasoil is acidic as far as pH. Mineralized topsoil can be acidid or nuetral or basic depending on the parent material, how much organic matter is in it, etc... Either way, Aquasoil or mineralized topsoil with high organic content should provide similar pH values (unless you have a topsoil high in calcium, such as a soil with a limestone parent material). (Again, I may be wrong. )

-Dave


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## armedbiggiet (May 6, 2006)

For aqua soil, Yes the same PH cause water do circulate in the sub. like aqua soil cause it is less packed, if they don't your roots would rotten.

the aqua soil release a different kind of acid which promo both breeding and growing.


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## dwalstad (Apr 14, 2006)

Poe835 said:


> Anyone know whether the pH in the substrate layer will be the same as the pH of the water column? Will it be different if I have a minieralized soil substrate vs. granular ADA aquasoil?
> 
> thanks!


Actual measurements of submerged soils suggest that soil pH is independent of water pH. However, over time (2-3 months), all soil pHs gravitate towards neutrality, irrespective of their starting pH. Soil pH is determined by various oxidation/reduction reactions within the soil itself.

Ponnamperuma's study, which shows the pH measurements for 6 different soils over time, is in my book on page 130.


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## Poe835 (Sep 16, 2004)

Thank you everyone. This is very helpful.

Ms.Walstad, I've read your book and I must say it is incredibly helpful in understanding the physicochemical nature of the soil in planted tanks. Thanks so much for writing this book.
I also wanted to ask if there is any way to speed up the pH equilibration process so that the time it takes to reach neutral pH is closer to 1-2 weeks? For acidic soils, would it help speed up the process if we introduce more iron oxide (Fe2O3) to the substrate?

thanks again!


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## dwalstad (Apr 14, 2006)

Poe835 said:


> For acidic soils, would it help speed up the process if we introduce more iron oxide (Fe2O3) to the substrate?


I don't know. However, oxidizing reactions (e.g., iron oxidation) always generate acid. Reducing reactions (e.g., iron reduction) always consume acid. Adding oxidized iron might, therefore, increase the pH of an acidic soil-- provided that the iron oxides are subsequently reduced.

That said, I don't think that soil pH is a major factor in starting up an NPT. In general, its not something that I would worry about.


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## dwalstad (Apr 14, 2006)

Just had a second thought of how to quickly correct an acid soil.

I have a 1991 soil analysis of Dupla's Laterite. It showed unnaturally high levels of phosphate and potassium and has an unnatural (for a tropical and highly weathered soil) pH of 7.0.

My guess is that Dupla added a solution of dibasic potassium phosphate (K2HPO4) to the laterite. Or they might have added a potassium phosphate solution prepared to pH 7.0 [This requires titrating a dibasic solution of K2HPO4 with a solution of monobasic potassium phosphate (KH2PO4) until achieving the desired 7.0 pH.] That would not only immediately increase soil pH but add two important nutrients to the soil.

I don't think that I would use iron oxides to correct an acid soil. Too slow or might not work at all. However, adding a dilute solution of dibasic potassium phosphate to the dry soil might be worth trying.


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## Poe835 (Sep 16, 2004)

dwalstad said:


> Just had a second thought of how to quickly correct an acid soil.
> 
> I have a 1991 soil analysis of Dupla's Laterite. It showed unnaturally high levels of phosphate and potassium and has an unnatural (for a tropical and highly weathered soil) pH of 7.0.
> 
> ...


Thanks for the suggestion Diana. I've actually thought of that earlier, and I tried adding potassium phosphate solution adjusted to pH ~6.6-7 to an acidic clay mixture, mix thoroughly, then let it dried. What I found is that when I submerged the soil, there is phosphate leaching into the water column (based on my phosphate test kit). So I imagine at least a decent amount of phosphate remains in the soluble form. Would this hurt in any way (other than inducing an algae bloom, i guess). 
Again, thanks for the suggestions.

PS. It could be that I added too much phosphate. Maybe I should try again with less and see what happens. Thanks!


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## dwalstad (Apr 14, 2006)

Poe835 said:


> Thanks for the suggestion Diana. I've actually thought of that earlier, and I tried adding potassium phosphate solution adjusted to pH ~6.6-7 to an acidic clay mixture, mix thoroughly, then let it dried. What I found is that when I submerged the soil, there is phosphate leaching into the water column (based on my phosphate test kit). So I imagine at least a decent amount of phosphate remains in the soluble form. Would this hurt in any way (other than inducing an algae bloom, i guess).
> Again, thanks for the suggestions.
> 
> PS. It could be that I added too much phosphate. Maybe I should try again with less and see what happens. Thanks!


Well, you were way ahead of me here. 

I don't think that a little phosphate leaching (1-10 ppm P) would hurt--- nothing that a water change won't fix.

Let us know how the plants do. My guess is that they'll like this soil.


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## dwalstad (Apr 14, 2006)

Poe835 said:


> It could be that I added too much phosphate. Maybe I should try again with less and see what happens. Thanks!


Well, you were way ahead of me here. 

I don't think that a little phosphate leaching (1-10 ppm P) would hurt--- nothing that a water change won't fix.

Let us know how the plants do. I suspect that they'll like the potassium phosphate fertilization.

Good luck!


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## Poe835 (Sep 16, 2004)

dwalstad said:


> Well, you were way ahead of me here.
> 
> I don't think that a little phosphate leaching (1-10 ppm P) would hurt--- nothing that a water change won't fix.
> 
> ...


Well..I didn't actually used that soil since I feared the high phosphate. I just setup a small test bottle with soil and water and checked the water parameters. I've also considering mixing the soil with an appropriate amount of KOH to neutralize the acidity (obviously, I've done titration experiments to know how much KOH to add). I actually noticed while during the titration experiment that soil with high humic content shows some buffering at neutral pH, which I found interesting. This will give me some buffer room in case I add too little/much KOH. 
It might be better than K2HPO4 since there's not phosphate involved, and it doesn't increase water hardness like using lime or dolomite. I know this may sound like too much trouble, but I'm actually having fun with testing these theories out and learning a ton along the way. Thanks again for your suggestion.


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