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