Plants "using" KH.

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

	Here we go again.  There is a common misconception that plants
that can get CO2 from bicarbonate will use up KH in the water.  This is
only true if the water is very hard (and has high KH).  What then happens
is in effect:

	Ca++  +  2HCO3-  ->  CaCO3 (solid)  +  CO2  +  H20

You see the CaCO3 on the plants and the (high) KH drops.

If the KH is low, this cannot happen, and the net effect is:

	HCO3-  ->  OH-  +  CO2      but remember the other equilibrium:
	OH-  +  HCO3-  <->  H2O  +  CO3--

In this case, the plant is getting CO2 when the CO2 concentration in the
water is very low.  This causes the pH to rise, maybe to 9 or so.  Even
at that pH, though, the concentration of HCO3- will be higher than that
of CO3-- and much higher than that of OH-.  There will still be some
CO2 there (not much).

The measured KH is the total of HCO3-, CO3-- and OH-, with the CO3--
counted twice (two protons needed to neutralise it).  It does not
change when the plants remove CO2 in this way, and most of it is still
HCO3-.

If the KH is low, the plants cannot remove it.


-- 
Paul Sears        Ottawa, Canada