Re: pH<7

[Thomas Barr <tcbiii at earthlink_net>]

> From: "Hopkins, Samuel" <Samuel.Hopkins at marconi_com>
> Subject: PH <7.0 for plants?
> 
> Is it generally true that with plants you want the PH to be <7.0? And if so,
> why?
> 
> Thanks,
> 
> Sam

Hummm....well there's a number of different ways to approach your question
but......
Take a look at the pH/KH/CO2 table(www.sfbaaps.com).
See what good ranges of CO2 levels are(20-30ppm in what I think is optimum).
You can have great CO2 levels at high KH values and pH's above 7. Many folks
have pH less than 7 but also use CO2 gas to do this. You cannot for example
add acid "buffer" to make plants grow etc, it's CO2, not acid "buffer" that
is making them grow well.

It's not so much an issue of pH, it's an issue of pH ****AND**** KH.
Plant care about CO2, they are not concerned about the pH.
CO2 levels are determined by both pH and KH (refer to the table).

Water with high KH values(HCO3) at equilibrium (like a glass of it sitting
on the table for 24 hrs say) has less CO2 and more HCO3 than water with low
KH values.

That *****equilibrium**** part is very important. Seldom is water EVER in
equilibrium. Plants remove the CO2, this raises the pH, we add CO2 to
counteract this.
Some folks don't use CO2, some plants and all algae use HCO3 instead of CO2
as a carbon source splitting it HCO3=> CO2+ OH. This raises the pH and also
softens the water(Removes the KH). Plant growth is slower in these tanks and
some plants cannot grow well in these tanks also.

Soft water(in terms of KH) has more CO2 than hard water does at
****equilibrium****. But the water is quickly CO2 depleted when you add
decent light to a fully planted tank. Doesn't matter if it's soft or hard
then. There's not enough for the plants in both cases. The soft water starts
off a little better(it has more CO2 in it) than the hard water if you don't
use CO2. For non CO2 tanks, low KH values are common as the plants have
removed it. This allows more CO2 in the water right before the lights come
on. After the lights have been on for an hour or so, the CO2 levels get
depleted and the pH rises up high(and low CO2 results).

Simple question but complicated answer, I know but it's not just a matter of
pH. It is a slow process getting CO2 from the air into the water also. This
creates a deficit when the plants are removing as they remove it faster than
the air can add it back in. Also, plants are in a much thicker, viscous
medium to "breath" in. It needs to be richer in CO2 to get the same amount
of CO2 to the leaves for the same growth. Diffusion is slower in thicker
media(gas vs liquid).
Imagine trying to breath in molasses.

For a simple response of what to do, do you use CO2? What's your KH?
From there you use the pH/KH/CO2 table and add enough CO2 to get the pH in
the desired range. For a non CO2 tank, add some HCO3 users like Vals,
hornwort, Egeria etc and a few floaters. After some time has passed the
plants will reduce the amount of HCO3 and soften the water.

Regards, 
Tom Barr