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Re: laterite, kitty litter, clay

Dave Gomberg takes issue with the statement:
> >Natural, amorphous soils commonly contain significant amounts of iron
> >(typically 5%

Thanks for the correction Dave. I rechecked my reference and it should
have been 0.4% not 5%. My error. :%]

George Booth writes:
> The author has shown by his own results that he
> does not yet hold the "key" to the perfect substrate (or even one that works
> well in the long run).

Actually, most people find that I grow quite nice aquatic plants. Let
the readers judge for themselves; my website is
http://home.infinet.net/teban/index.html I never said I had the key to
the perfect substrate. In fact I will state (again) that there is no
such thing as a perfect substrate. If you read my substrate article you
will find that I state that there are many factors to consider in making
an appropriate choice of substrate materials.

> What is the proper level of CEC in the substrate?  

There is no such thing as a proper level of CEC. It is well understood
that soils with a higher CEC value are better able to retain nutrients.
There is a table of CEC values for various substances in my
substrate article. Please note that laterite also has a CEC value. One
of the major points of my post was that laterite and kitty litter are
quite different in their characteristics.

> Do different plants do better
> with more CEC than others?  Which plants are sensitive to "not enough" CEC?  How
> do you measure how much CEC you already have?  

I have never heard of specific plants requiring a specific CEC value.
I'm not familiar with the laboratory techniques for measuring CEC and I
don't think we should mislead people into thinking there is any value in
attempting to measure it.

>  If laterite has such an abysmal CEC, why
> does it work so well?

I don't believe that the CEC of laterite is it's key advantage. You are
pretty quick to criticize empirical results as non-scientific but we
should remember that the only evidence we have about laterite is
empirical. We can only surmise that laterite and other substances help
to bind phosphates in the substrate. 

> > 1 to 5 percent humic material in a substrate is optimal. Refer to my homepage
> > for a long and technical discussion of substrate materials
> I read your long discussion (three times!) and did not see how "1 to 5 percent
> humic material" was found to be optimal. Do you have a reference?  

Yes, experiments by Barko and Smart clearly indicate that sediments with
higher than 5% organic content actually inhibit growth. Sediments with
lower than 5% organic content grew plants better as the organic content
increased. Barko and Smart theorized that this is due to the correlation
of fertility and organic material. My substrate article should include a
reference section.

> Is this by weight or volume?  

By weight.

> I have 200 pounds of gravel.  Does this mean I need 2 to 10
> pounds of humic material? 

No. Your layer of gravel is probably too deep for that. I would suggest
that it would be more appropriate to use a layered approach similar to
what is advocated for laterite. I wouldn't suggest adding humic material
to gravel anyhow. It would be better to choose a loamy soil with
approximately these values. I don't think the precise value is critical.

> > Dave Huebert made the following comments in an email discussion last
> > summer:
> > ...
> >      "On the other side of the coin, there have been a plethora of
> > studies which indicate clearly that rooted aquatic plants will not grow
> > optimally on a sand or other infertile substrate no matter how richly
> > you fertilize the water column (perhaps the earliest is by Pond, 1905)"
> This doesn't jive with my experiences with massive plant growth using plain
> gravel substrates, undergravel filters and Dupla fertilizers. 

You are assuming that your Dupla substrates are infertile. If your
substrate helps to grow better aquatic plants, it can only be because it
is fertile. Any other theories?

> > We've also found that the addition
> > of peat to a soil substrate keeps the redox potential low enough to
> > provide sufficient iron to maintain moderate growth rates without
> > resorting to chelated iron dosing.
> How have "we" found this out?

In this case I am referring to Paul Krombholz. I have duplicated this
with my most recent tank however I cannot make any long term
observations at this time. At present I have not added any chelated iron
to my 49 gallon tank for several months. Growth rates are good and on
those occassions when I did make chelated iron additions, there was no
perceivable increase in growth rates except for green spot algae which

> > It's probably not wise to combine
> > really high light levels with significant amounts of chelated iron in
> > solution as this seems to promote many types of algal growth especially
> > filamentous algae.

I will qualify really high lighting to mean 4+ wpg or partial sunlight
and significant chelated iron to be greater than 0.2 ppm. I stand by my
statement. I just want people to be aware that higher light intensities
may mean making adjustments. If your aquarium is in the range of 1-2
wpg, then you probably won't need the high quality iron test kit that
George needs. ;-)

> Let us know when you find a substrate recipe that works for more 
> than a few months.

I have George. Since when did we make a rule that says you're not
allowed to periodically fertilize the substrate?

Roger S. Miller wrote:
>> Metal hydroxides are very good adsorbers for inorganic solutes - in fact I've read the argument that high adsorbtion coefficients measured on clays are actually caused by a thin film of hydroxides on the surface of the clay.  But the Kd value for hydroxides apparently drops off as the sediment "matures" - perhaps because the surface adsorbtion sites are already occupied but more likely because the hydroxide particulates are very "fluffy" when they're new but consolidate over time and lose much of their surface area.  As a result it isn't a given that laterite will have a much higher surface adsorbtion capacity than a clay soil, because the iron and aluminum hydroxides in laterite are probably going to be very well aged. <<

How could these be "renewed"? Is there another way to achieve the same
effect? This sounds like a very interesting area for further discussion.

>> I guess my point is that you should be able to get a good substrate with clay or with laterite.<<

Yup. But I want to stress that the properties of the many various types
of clays and lateritic soils are quite diverse. That was my central

>> Either one provides the ability to hold phosphorus, though laterite might hold more.  Clays (and maybe kitty litter) would have a better capability to hold ammonium.  Also in either case the nutrients in the soil can become depleted over time and you have to arrange some means of replacing the nutrients that the plants have taken out. <<

Yes, I sure don't see any problem with periodically adding some
nutrients to the substrate when we get an indication from the plants.
Sometimes we may want to maintain slow growth and sometimes we may want
to increase it as you probably want to do with your C balansae. In my
case, my plant is already quite large with a large number of leaves so I
haven't felt that its critical to fertilize yet. I suppose it bears
mentioning that I also advocate watching your plants to observe when it
is time to make changes, just as Karen has said repeatedly.

Thanks for the new insights on other adsorption characteristics Roger.
I'd be very interested to learn more about metal hydroxide binding of
phosphates and the other things you mention.