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Re: Muddy thoughts
> >>Again, if you [meaning Steve] don't know what to do with the answer,
> >>there is no point in looking for one.
That's not true. I can't critique an hypothesis if you won't advance
one! ;-) This is still a diversion from the question about whether
substrate heating coils are useful at improving nutrient availability to
rooted aquatic plants. But let's skip the finer points of the scientific
method and get something concrete to talk about.
> Steve fired back:
> >If you [meaning George] don't know what to do with the numbers ...
> I believe I said if "you", meaning "Steve, the guy who claims he can
> tell us if the numbers are good". If the flow rate was known and the
> CEC value was known (as you recall, laterite has a CEC of 4) and the
> nutrient concentration in the water was known and perhaps other
> factors where known, those values can be plugged into the
> Theoretical Substrate Nutrient Equation (and I trust there is one).
> Once we know that the substrate could theoretically hold X amount of
> nutrients, against what standard would we compare X to determine if
> that substrate was "good"?
I'm going to raise this theory because I want to point out the problems
with it. Please feel free to revise a new theory and we can poke at it
It seems to me that if the "nutrients moving into the substrate" theory
is to have any validity, you have to assume that water containing
nutrients moves into the substrate that it somehow increases the
availability to plants and this implies an increase in concentration of
nutrients in the substrate. To do that there has to be energy used up,
in this case, by the weak electric bonding of the nutrients at the CEC
sites. That gives an increase in concentration all right but doesn't
necessarily mean an increase in availability because now those ions are
weakly bonded to the CEC site so the plant has to give up energy to get
the nutrient. This may be fine if the plant has more energy than it has
nutrients and so by other reactions, the plant is able to secrete acids
which release hydrogen ions which liberate the cations from their
exchange sites. No problem, the plant has lots of energy from
photosynthesis to accomplish a lot of bio-chemical tasks.
So in this proposed theory, CEC plays an important role and it implies
there is a linear relationship between CEC and the potential for
nutrient concentration in the substrate. The real problem with this
theory is that there are more calcium and magnesium ions in the water
than ammonium and these ions generally have a higher affinity to the
cation exchange sites. Also I don't think phosphate usually occurs as
cations. There are several other macro and micro nutrients so I suppose
something could be proposed about that...
I don't think moving water through the substrate improves the
availability of iron in the substrate at all, in fact the opposite. I'm
not really advocating any particular theory here about substrate heating
coils at all. I think this question about X amount of nutrients is the
same question as "How much CEC is appropriate?" asked in another way. My
answer to that was that the precise CEC value is not critical because I
don't think the key to keeping nutrients in the substrate is CEC
although that is certainly an important consideration. For one thing,
one of the most important nutrients, iron, stays there because its very
insoluble in the presence of oxygen. Phosphates also have insoluble
forms and the biological processes in the substrate also improve
phosphorus availability. Its tricky because phosphate also has some very
soluble forms, and is liberated by plants as the leaves die. Controlling
phosphates in the water is probably best achieved by having a whole lot
of growing plants in a given volume of water and by not introducing too
many sources (i.e. fish food, fish, decaying plant leaves, stirring up
If you start with a higher concentration of nutrients in the substrate,
CEC can play a minor role in retaining the nutrients there. In trying to
understand the nature of CEC, I asked before if the centimoles in the
centimoles per kilogram referred to moles of ions (mono-valent or
divalent) or moles of ionic charge. If I understood the answer I got
here, it means centimoles (1/100th of a mole) of ions not charge and
therefore in one kilogram of material where ALL the exchange sites were
somehow occupied by a single ionic species, then the theoretical limit
would be obtained by multiplying by the gram wait of the ion in
question. So for calcium (a likely candidate) with an atomic weight of
40, if it were sorbed into one kilogram of pure very fine clay with a
CEC of 100 (to choose a number out of a hat), then we could retain 40
centi-grams or 0.4 grams of elemental Ca. Now calcium does not travel
well through the plants transpiration systems for reasons best explained
by an expert. I chose it for an example to show George how such a theory
could be constructed (assuming that one as inclined to defend such a
theory) If someone were interested in modifying the theory for other
nutrients we have to factor in all their concentrations and the
respective cation binding affinities. The binding affinity is a function
of pH (hydrogen ion concentration), temperature and the peculiarities of
that particular material.
But the system is not static, its dynamic so a complete theory has to
include a measure of the transport rates into the substrate and the
consumption rates of the plants. This is where the substrate
permeability (its resistance to water flow) comes into the picture.
Permeability of an homogeneous material can be experimentally measured
and so we come to the next question.
> Steve proclaims:
> >So the flow rates involved must be vanishingly small. I doubt that any
> >of us could come up with a mathematical model to predict it
> I think I already suggested that ("intractable computations") but
> thanks for putting your stamp of approval on it.
> >but measuring it experimentally wouldn't be hard at all.
> Oh? "It would not be hard to measure a vanishingly small flow rate"?
> Please give me a detailed procedure; one that an engineer, not as
> skilled in mechanics as you are, could follow. I have an empty 29
> gallon tank and spare heating cables with which I can perform this
> simple experimental measurement.
I did spend a few hours pondering over this and I came up with a couple
Take a homogeneous material, such as clay, place it in a tube and apply
a pressure (such as the hydrostatic pressure of a column of water) and
measure the rate of flow through the tube. This would give a highly
accurate permeability factor which could be used in a discrete element
What we discussed previously was setting up a tank with a plenum like an
UGF and heating coils in gravel over top the plenum. Turn the heating
cables on and inject a neutral buoyancy ink dye into one of the riser
tubes of the UGF and measure the rate of descent. Multiply by the ratio
of the areas of the tube and the plenum and you should have the average
velocity through the substrate. Compute the depth of the substrate
(assuming an average temperature) and multiply by the temperature
difference and the coefficient of thermal expansion of water and you
should be able to come up with a value for a pressure difference. With
this data you can again compute the permeability factor.
The problem with substrate heating cable analysis is that its not a
simple problem of disjoint columns of water of different temperatures
giving the pressure difference. Also the permeability in clay is vastly
different to that of coarse grains of sand or gravel with no clay. So we
have to use a discrete element computer model again to model the local
temperature gradients and permeability. The solution of this little
problem is something for a post graduate thesis and much as I'd love to
rattle it off, I can't since I lack both the time and the motivation.
This is the last I have to say about substrate heating coils unless
George or anybody else has anything more interesting to actually add to
the theory. I will not answer any further questions on what I have said
so far as its just too bizarre to have to both advance and critique a
bogus theory. :-p
Instead, I'll advance a new idea. (well an old idea dressed up in a
dinner jacket like the kind they give you when you go to an expensive
restaurant in a T-shirt)
I think you can get nutrients to stay in a substrate by putting them
inside little clay balls where the ions in solution just have a tough
time diffusing through the clay. Jobe's sticks use a similar material to
slow diffusion. I think they call it an occluding material but it
amounts to the same thing. Thus you don't need heating cables to get the
nutrients in there, you just poke 'em in with your finger. What could be
I think you get iron in the substrate by using any sufficiently fine
soil material which contains iron compounds especially clay soil. You
don't need a lot. I think many types of soil already contain a lot of
nutrients occupying their cation exchange sites or simply mixed into the
amorphous soil material. There seems to be no question that many types
of natural top soils contain nitrogen and phosphorous and so it is
probably expedient not too use too much of these. Humic matter in soil
plays an important role in moderating the concentration of some minerals
such as copper or other metal nutrients but we don't want a lot of
labile organic material. A small amount of peat might be good so I'm
trying that out in my latest substrate. Peat is relatively stable to
decay and does not contain high amounts of macro nutrients.
Clay when mixed with much coarser materials like sand, forms a thin
layer over the surface of the larger particles. In this way, the clay
still has a high surface availability but the mixture retains its
permeability. I think this is what happens when you mix a small amount
of clay (such as laterite) with fine gravel; it coats the granules.
I have a feeling that heating up the substrate does a lot to increase
the rate of biochemical reactions in there. There will be a certain
temperature threshold when things really start to work well and you'll
get a very large increase in iron and manganese availability. This will
also improve phosphorus and ammonium availability. Confidentially, I
don't think its particularly hard to get all the minerals you could ever
want available in the substrate without ever heating it, but let's keep
that just between us, gentle reader.
A few more mud questions:
Do worms play an important role in the formation of aquatic soils? Or is
it only the terrestrial earthworms that do this and the aquatic plants
mainly get the soils by erosion and sedimentation?
Also somebody else said that aquatic mud was likely to be too high in
macro nutrients. I'm not sure this is true either but would like to hear
Dr Dave's comments. I think most sediment which travels down a river is
getting pretty well leached although the mineral particles could still
contain plenty of micro nutrients. We tend to think that lakes which are
polluted with relatively high levels of nutrients are also going to have
really rich sediments in the bottom. That may also not be true.
Steve Pushak Vancouver, BC, CANADA
Visit "Steve's Aquatic Page" http://home.infinet.net/teban/
for LOTS of pics, tips and links for aquatic gardening!!!