pH and alkalinity in plant database


New Member
An example: Ammannia pedicellata. Recommended pH value: 5 to 7. Recommended alkalinity: 2 to 12 °dKH. Put together: at pH 5.0, with alkalinity 2 °dKH, the Tillmanns' chart returns CO2 content 703 mg/L.
What do you think about such a recommendation?
There are plenty examples of this fault in Flowgrow plant database. As well as in other, similar databases. I'd like to know who and how has arrived at such numbers? Can anyone tell me, please?


Well-Known Member

Don't feed the troll. Es ist jedem klar, daß man nicht die Extremwerte miteinander verknüpft.



New Member
Not truly sure whether I can understand, but I've chose the more appropriate combination, i.e. pH 5.0, with alkalinity 2 °dKH. If you suggest combining pH 5.0 with alkalinity 12 °dKH, then Tillmanns' chart returns CO2 content 4217 mg/L. That would be the extreme combination.
So, plant database suggests pH 5 and CO2 content between 703 and 4217 mg/L. You got it?


Well-Known Member

the plant database does not claim to be an academic treatise. Use your wits instead of splitting hairs.

Regards, Markus


That would be the extreme combination.
Use your wits ...
Come on! There's a lot of space between academic treatise and talking nonsense.
You're right, anyway, could you describe your concerns in more detail?

Why do you think it is necessary to go into such detail? This reminds me of the instructions in the microwave oven manual not to dry dogs in it.

I know the hamster limps when you compare it, but incidentally, the pH can be influenced in various ways. How much detail would you go into?
Or maybe it makes sense to tacitly assume a certain degree of personal responsibility? Is it necessary to add this notice everywhere?

Best regards


New Member
I like Flowgrow plant database. In fact, I consider it the best on-line database in Europe. Above all, I appreciate it's botanical accuracy.

I'd like to believe that the database has been created by people who have got many opportunities to keep described plants and know a lot on their demands.

In theory, pH may be influenced by many factors. In practice, though, there's only one - alkalinity. Nitrification pushes pH down by consuming alkalinity (bicarbonates). Plants' photosynthesis pushes pH up thanks to carbon dioxide uptake. Humic substances? They're not bicarbonates, of course, yet they too affect alkalinity, and pH accordingly.

The only commonly used way to decrease pH while not affecting alkalinity is carbon dioxide injection. I don't do it but I understand that many hobbyists do. What is the maximum level of CO2 which can be considered rational? 40 mg/L, perhaps?

Ok. So, CO2 content may vary between 0.4 mg/L - based on partial pressure of CO2 within air - and 40 mg/L. I believe that the people who contribute to the database keep CO2 content within this range. And that they've got personal experience with those plants for which they provide guidelines.

And then - I can see that their recommended values are completely distracted from reality. Let's look again at the example (only one from many similar ones) mentioned above: If I choose MEDIAN values, i.e. pH=6 and alkalinity 7 °dKH, it returns CO2 content 246 mg/L. Is this a joke? In fact, you can combine the suggested numbers in any way you choose, and you get rather fair numbers solely for pH=7.

So, can I TRUST these guidelines? Do the authors truly speak out of their experience? Or are they just suggesting random numbers without much thinking?

You say, use your own wits. Fair enough. But what are these guidelines good for, then? Wouldn't it be better to cancel them completely? Or - that's my suggestion - to cancel data on alkalinity? In my experience, pH is the parameter which truly matters.

I still want to trust this database. I still want to believe that there are people who know and are willing to share their knowledge. It must have been a lot of effort to create and maintain the database. It deserves to be corrected not to speak nonsense.


Hey Maq,

what if the suggested ranges are from different keepers?
One of them could have had success with pH of 5 and the other wasn´t interested in pH (that could be me!) but had an alkalinity of 2°dH and both kept the plant with success.

Back then when I was researching I read the database kind of different compared to you.
$Ammannia pedicellata
Here, I see a range of values where the plant can be kept. But I would not expect the values to match within their range. E.g. CO2 is upper limited by 40 mg/l. And this value is exceeded quite quickly by changing pH with moderate alkalinity.

But I´m pretty sure this plant can also be kept at 0 °dH of alkalinity ^^ What is, in detail, not possible in the presence of CO2 when the pH is above 4.3. Just another issue almost no one notices and nobody is caring about it.



New Member
As I see it, °dKH is rather big unit. Yes, like you say @Kejoro , zero bicarbonates are impossible upon normal conditions, but 0.05 °dKH is quite a normal value.
What intrigues me is your attention to alkalinity and neglect for pH. I see it the other way around. But I'd like you to develop your stance: What makes you take alkalinity for THE main factor?


Hi Maq,

have fun with measuring 0.05 °dKH :D Imho these low values have no practical relevance. I can add salt to have a calculated value in that range, but I must ensure the salt is equally mixed and hasn´t adsorbed any water. But most salts are hygroscopic and so on ^^ It´s quite a mess to ensure these values.

Why I neglect pH: I use RO water and can adjust the values which I think to be best. Measuring alkalinity (KH) is quite easily done with the Salifert Test at a very good accuracy ( ~0,2°dH normal setup) and CO2 is checked by a drop checker. Which leads automatically to a pH in the targeted range.
Of course, using tap water introduced some limitations and practice changes. But you always end up in modifying KH.

There is also an empirical reason why I just don´t care about pH when I can ensure that KH exists. I had a 30l Cube with some (very) difficult plants in it according the database. $Syngonanthus macrocaulon for example.
The KH sweeped from 3°dH down to 0,1°dH (verified with Salifert and streched acid). Not doing any water changes for months, just adding ferts. CO2 inlet was kept constant but its concentration varies due to changes in plant mass. KH decreased naturally by metabolism.
Although pH must have changed there was no change in plant growth over time.
The only thing I pay attention to: 4,3 < pH < 7.



New Member
Excellent rationale, @Kejoro . I too am using RODI water and mineralize it. But I prefer targeting and measuring pH, and adding bicarbonates to reach given target. So I think the difference is only in the means. I guess your way is better for those who inject CO2, which is not my case.
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