OK, AH, you did your homework. Thanks for the corrections to my incomplete
and impulsive response. I spoke too quickly in generalities when I was
meaning to apply it to the situation of acid soil and paphs and not systems
where the pH is neutral or above or within the effective buffering range of
bicarbonate. Acid solutions are not covered in your arguements. My
references are not here with me, but I believe a warm acid will dissolve
carbonates better than a cold one. So my terms of "wrong" and "negate" were
inappropriate. "Not fully applicable to the situation" would have been
better. Let's both stick to chemistry applicable to this board and its
purpose - to grow better orchids.
Gary
"Aaron Hicks" wrote in message
...

The skies did open, and through the rent the mighty voice of
"Gareth Wills" spaketh thusly:

Ok Aaron, the buffering ability of the bicarbonate ion is well known. The
solubility of CaCO3 decreases with increased temps above normal. This
does not mean that the solubility increases with decreasing temps below
ambient (look up buffers).

Check your CRC. The solubility of calcium carbonate INCREASES
dramatically below ambient. In the event you have misplaced it, I refer
you to a chart from Faure's "Principles and Applications of Inorganic
Geochemistry," page 223, posted for your viewing pleasure at:

http://members.cox.net/ahicks51/calciumcarbonate.jpg

Two partial pressures of carbon dioxide are given; the upper curve
is closest to actual. As can be seen, the solubility of calcium carbonate
(as measured by the [Ca+2], coming from dissolved calcite) increases
dramatically as temperature drops, particularly *below* 20 C. Solubility
does, in fact, increase with decreasing temperature.

"The solubility of calcite [calcium carbonate, limestone] is also
affected by the temperature because of changes in the numerical values of
all the equilibrium constants. Garrels and Christ (1965) compiled the set
of values of the relevant equilibrium constants between 0 and 50 degrees
C that is listed in Table 12.6 [not shown here]. We see by inspection that
the dissociation constants of carbonic acid _increase_ with increasing
temperature, whereas the solubility product constants of calcite and CO2
_decrease_. As a result, the solubility of calcite in pure water in
equilibrium with CO2 of the atmosphere actually _decreases_ with
increasing temperature, as shown in Figure 12.4 [as above]. For example, a
saturated solution of calcite in equilibrium with CO2 at 3x10^-3 atm
contains about 75 mg/L of Ca+2, at 5 degrees C but only 40 mg/L at 30
degrees C."

_Emphasis_ is that of the author. [This stuff] is mine, as are
typos.

In cooler temps, CaCO3 precipitates since the solubility coefficient is
so low.

No. This is why hot water heaters fill with calcium carbonate
precipitate in regions with hard water. Cold water in, hard water out,
leave the calcium carbonate behind. Wait five years until it's full of
"sand."

The major factor here is the acidity which negates LeChatelier and the
above discussion since CO2 is no longer one of the major players.
Acetates, humic acids, chlorides, and nitrates of calcium are soluble and
plentiful in peat moss mixes which overwhelms the buffering capacity of
the bicarbonate ion, which was the point.

Ah! A correct statement. I do not dispute this. However, stating
that Fowlie is wrong because the solubility of calcium carbonate is
proportional to temperature is wrong, and I will take issue with this. I
will happily admit that the other components of the system, when in
sufficiently high concentrations, will quickly overwhelm the system (but
not "negate" LeChatelier- one does not negate laws of chemistry and
physics without congressional approval, or at least help from the
president of the American Chemical Association).

One last factor of potential interest is the ability of roots to
remove carbon dioxide from their environment, fostering the precipitation
of calcium carbonate and other minerals within proximity of their roots.
When fossilized, they form "rhizocretions." Do a Google search. They're
cool. These trace fossils may also occur after plant death, so they're not
an absolute indication of this sort of activity on the part of the plant.


-AJHicks
Chandler, AZ