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Matthew Mason asked some astute questions about CEC. Mat, your questions
reveal that you know a little about the subject yourself so I hope you
will share with us too.

> 1.  Has anyone seen/performed a _real_ experiment set (one with +/-
> controls and same condition experiments) with laterite, kitty litter,
> vermiculite, and sand?

I'm not aware that anyone has ever done a carefully controlled,
comparative experiment with laterite and other substances (except sand).
The Optimum Aquarium mentions some comparative studies they did between
laterite and sand. It's pretty easy to observe the differences in growth
between plain gravel/sand plant tanks and those with laterite, clay,
soil or even with a Jobe's sticks. The differences are dramatic enough
to observe quite easily.

About a year ago James Purchase wanted to do a comparative study with
soil, gravel and laterite. He did do a study with the assistance of
Diana Walstad but I think the focus of the experiment was changed and
the results have not been published. I haven't heard from James for a
while so if you're reading James, please email me.

Diana Walstad did some controlled experiments with soil which she
describes in TAG 7:5 which compare garden soil, potting soil and
subsoil. The study also compared the effects of macro & micro nutrient
additions to substrates. I'm not going to summarize that study; it's too
lengthy of a discussion. I recommend that people with a keen interest in
aquatic plants and related subjects join the Aquatic Gardeners
Association and order many of the back issues. I ordered the complete
set of back issues and they are excellent for research and the
references mentioned are great for anyone with ready access to a
university library. If anyone is willing to invest the time, I will be
happy to exchange references and ideas. I have little spare time for
library research but avid interest.

Jim Kelly did a lot of library research and wrote some excellent
articles on substrate properties, CEC and so on which he posted to the
internet around September 30, 1994 and which I have copies of. A lot of
Jim's material has also been published in TAG. See TAG 9:4 July-August
1996 for his article "Notes on Key Soil Characteristics for Aquarists"
which is the best reference material on the subject of CEC that I've had
access to so far. This material is derived from the earlier internet
postings. Roger, Mason: if you have access to TAG 9:4 I'd be eager to
hear your comments. If not, contact me offline; I'll email the earlier
net postings.

If anyone is interested in performing a set of controlled comparative
experiments or research in general relating to aquatic plants, please
contact me, Paul Krombholz or any of the other AGA technical committee

> 2.  Has anyone _personally_ read data that shows that a high CEC actually
> releases the cations after surface bonding - remember that in certain
> cercumstances high CEC may be strong bond energies and therefore difficult
> to remove an ion once bound (hypothesis here).  And if it does release the
> ions then at what concentration in relation to binding; i.e., is the
> release proportional to the binding or is is very un balanced?

I like this question. My understanding of what I've read indicates that
many high CEC materials do not release cations so easily however there
is an equilibrium with interstitial cation species. Proportional to the
temperature, cations of various types will trade places at the binding

Even more to the point, CEC may be pH-dependent or pH-independent. That
is, at low pH, the material will release (exchange for H+) adsorbed
nutrient cations. The CEC of humus is almost entirely pH-dependent and
this permits root hairs to free nutrients by secreting organic acids.
The layer silicate clays (Vermiculite and Smectites) have a large pH
independent CEC and fine texture silicate clays have about half of the
CEC ph-dependent. Gibbsite and goethite (dominant in lateric soils) are
pH dependent and may be able to bind anions loosely at low pH according
to Jim's paper.

> 3.  What is the longest time frame that a laterite, kitty litter,
> vermiculite tank has been stable and growing _without_ water column
> fertilization?  It seems to me, and other authors, that substrate
> fertilization is a more efficient manner to deliver nutrients.  Now, before
> people get their undees in a bunch, re-read the question.  Only answer the
> question if you can satisfy _all_ parts of the question concerning a
> specific substrate, please.  I do not doubt that any/all of these methods
> can work.

Hmmm... trick question. I think the correct answer is 0.

You don't say which nutrients are not to be fertilized in the water.
Oops, let me unscrunch my undies! ;-) We generally provide K, Mg and Ca
in the water and according to Dave Heubert, aquatic plants need them in
the water. CO2 is probably best provided in the water. It's difficult to
keep P out of the water entirely with feeding and so forth but this
macro-nutrient is probably best provided in the substrate if you want to
stimulate growth. Micro-nutrients can be provided either in the water or
in the substrate. It's easy to have micro-nutrient toxicity and
materials like humus or peat can ameliorate (help prevent) problems like
this. That's a whole 'nother topic. To get back to what I think is the
spirit of your question; I think you can easily avoid adding phosphates
to a "safe" system for several months depending upon how many plants
there are and their growth rates predominantly controlled by lighting.
By safe, I mean an aquarium where the risk of toxicity and algae
problems is minimal. You could probably get away without adding iron or
other micronutrients for years with a peat-soil or other type of
substrate. I don't think you can expect to keep nitrogen in the
substrate for long; my opinion is that high nitrate and rich organic
substrates are not suitable for quite a few types of plants esp. during
the first months. You may avoid problems with filament algae but BG and
green water can be troublesome for the first several months. I don't
have enough experimental evidence to comment. I think you can quite
easily maintain 10 ppm of nitrate in your aquarium water and fertilize
the substrate every 3-12 months with P and N (depending upon growth

By the term "rich" substrate, I mean one which contains something like
2-5% labile materials like compost, manure, earthworm castings... Such
substrates can produce high nitrate and phosphate levels for months upon
submergence. High means 50+ ppm. Yeow.

A peat-soil substrate can have low NP fertility and be classified as a
low fertility substrate (neglecting micro-nutrient fertility)

A soil substrate by itself (or mixed with sand or clay) can be either
rich or low in NP fertility depending upon how much organic material and
NP nutrients are in it. It can also cause micro-nutrient or other metal
toxicity depending upon what's in it (i.e. where you dug it up). Even
clay by itself can cause toxicity especially Al if the pH is high i.e. 8
or more.

If you add too much N fertilizer to your substrate, you can burn the

> I am enjoying the discussions about CEC's.  I do have a tank with kitty
> litter/sand substrate and therefore can only comment on that substrate.  I
> wish that I was not a poor grad student so that I could afford laterite and
> do the experiment that I want to do in order to get some answers.

You have the interest; do you have the space for setting up an
experiment? There are a great many factors which need to be taken into
consideration in such an experiment. It is best to have a mentor with
experience in such things. Also, a critical question which must be
determined is what is the goal of the experiment.

If you wanted to compare for example, a particular clay with laterite,
you would find that all of the other factors taken into consideration
would outweigh the effects of these two materials in isolation. To put
it another way, the differences probably aren't that important given
that you are using other materials and strategies to promote optimal
growth in the same manner in both test environments. Also optimal growth
rates are not optimal at all to the hobbyist since those conditions lead
to all sorts of other management problems. Still there is very often a
simple method to greatly improve the appearance of your plant tank with
just a little substrate fertilization. ;-) I'm not after optimal growth;
the term optimal only came up in the context of a quote pertaining to
phosphate preference in the substrate. My point was that phosphate
fertilization needs to be in the substrate and it is probably the
limiting growth nutrient for a lot of our tanks (even those with
measurable phosphate in the water!!!)

Does anyone have any tabular data on the Kd value for any materials?

Roger, I didn't miss your point about the substrate system changing with
time especially as organic materials from the fishes and food work into
it and about the complex chemical changes which occur there. Natural
soils are the end result of long periods of biological change;
unfortunately terrestrial soils start another set of changes when you
put them under water. 

If we had a stable system, nothing much would grow in it; growth is
about biological state changes. What we want is a dynamic stability
where we have regular, steady inputs and consistent, desirable results. 

I think that for certain plants, especially Cryptocorynes, you can get
much better results with a little clay or soil and organic material like
peat. As for stimulating aquatic plant growth in general with substrate
phosphorus, I think there are a wide range of materials to help perform
this function. Clay and humus are just two excellent choices for this. I
recommend using moderate amounts of both together to help avoid the
possiblity of toxicity. I also recommend avoiding organic material
(<.5%) in layers of the substrate below 2 inches. I also like peat
because it is one of the most stable organic materials (after humus) and
it too provides protection from trace mineral toxicity. I think trace
mineral toxicity is one of the potential problems with certain kinds of

Steve (rambling again)