pH, nutrients and clays
As a plant newbie, I have never had anything useful to say (instead just
sucking up information from the rest of you). But regarding the few
posts about water chemistry and nutrient uptake, I may have something to
1) pH: George Booth was correct, there is little effect that
temperature will have on pH within the range of tropical tanks. We know
that neutral pH is defined where the concentration (or better, activity)
of OH- and H+ (or H+ and H3O+) are equal. At 25 degrees C, this falls
right at pH 7. However, the equilibrium between OH- and H+ varies with
temperature ( and pressure, and salinity - for salt tanks!) just a little
bit, so that at 20 degrees C, in freshwater, at normal atmospheric pressure,
"neutral" pH is really at pH 7.08, and at 30 C, neutral is 6.92. This
really is almost no change off of pH 7, but I thought that you might want
to understand a little more about it.
2) nutrient uptake: in any chemical system, there are three means by
which chemical species can move about - Flow, diffusion and reaction.
And each is important within specific scales of time and space. Flow
will act on large scales, carrying material large distance (Peru to the
Amazon delta or from your filter media into the tank) on time scales we
can observe (days for the river, minutes for your tank). Diffusion is just
the random motion of ions and particles. If there is a gradient in
concentration, then diffusion will tend to smooth out that gradient,
and within a middle-small space (a few centimeters), more or less complete
mixing will occur within a few minutes. Of course, flow will help
diffusion along by generating turbulent mixing (i.e. undergravel heating
heating coils or UGF). What most likely controls nutrient uptake by
plants is most likely a combination of reaction (how fast the plant can
actually 'pick up' a nutrient ion) and diffusion. Not all reactions are
especially fast - otherwise, all ammonia/um and nitrite would immediately
react to form nitrate, all fish waste would immediately decompose, etc.
On your hardness test kit, if you use EDTA, you may have noticed that the
change in color from pink to blue is not immediate, but takes some time.
This is sped up by shaking the vial, but still it takes maybe 2-3
seconds. EDTA is a chelating agent much like that used by plants (or
rather, the symbiotic mycorrhizae fungi that live among plant roots) to
pick up nutrients. In the absence of some great hand shaking the roots
up like your test vial, and with rather low concentrations of chelating
agents (Much less the 4 drops or so you put into the test vial) this
uptake reaction may take some time. And on the scale of millimeters or
less, flow is not going to speed up diffusion much.
So what does this all mean for vermiculite and clays in general as a
substrate? Clays are wonderful for storing cations (positively charged
ions like Fe2+, Ca2+) because their surfaces contain lots of OH- sticking
out the sides. Laterite ( which technically to a geologist is not really
clay, but just a soil with lots of aluminum and iron oxi/hydroxides) also
has these OH- sites. Fine grained clays probably would serve to restrict
fluid flow and diffusion, simply because there is some much less pore
space for the water and ions to flow through, that diffusion of oxygen
into the substrate could not keep up with consumption of oxygen during
bacterial decomposition of waste, dissolved organics, and even other,
dead bacteria. Even clays the size of vermiculite would most likely not
be 'open' enough to keep oxygen concentrations up in the substrate.
However, the mixing and mashing ala the Jim Kelly article on 'the
Krib' breaks up the alignment of the clay grains, creating more
porosity and space for fluid flow - so roots can access an oxygenated
environment richer in nutrients.
I am sorry - this is long. I get excited when I feel like I can have the
chance to explain something a little bit.
Steve Petsch - stpetsch at morpheus_cis.yale.edu