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Re: clay, laterite, et al

> Date: Sun, 28 Dec 1997 23:04:40 -0800
> From: Stephen Pushak <teban at powersonic_bc.ca>

I'm afraid I must take issue with this posting. Much of this comes across as 
fact when, in actuality, it is conjecture.  I have no problem with anyone 
posting conjecture as such, but I do not want to see this passed of as what 
sounds like scientific evidence. 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). I would much prefer see a few "in my humble opinion" and 
"I think" scattered appropriately throughout claims such as these.  
> In comparison to
> ordinary soil (with a clay & humus component), or any of the various
> fine textured clays, laterite has a relatively low CEC (cation exchange
> capacity = the ability to adsorb positively charged nutrient ions).
> Improving the CEC of your substrate is quite easy; vermiculite or other
> expanding layer clays (like kitty litter) are just one option for this.

What is the proper level of CEC in the substrate?  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?  Why would one want to 
gratuitously "improve" the CEC if there already is enough?  What are the 
consequences of having too much CEC?  Does a larger amount of a safe but low CEC
material such as laterite produce the same results as a smaller amount of more 
dangerous but higher CEC material?  If laterite has such an abysmal CEC, why 
does it work so well?

> 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?  Is this by 
weight or volume?  I have 200 pounds of gravel.  Does this mean I need 2 to 10 
pounds of humic material?  Given the nature of humic material, that sounds like 
a HUGE amount of crud to add to the tank.  

> 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. Just what is the 
scientific definition of "optimal"?  I hope it's more complicated than growth 
rates. Perhaps the researchers performing the plethora of studies just didn't 
know how to grow plants!  For example, most of the aquatic plant "bibles" 
published prior to 1980 (Rataj and Horeman is a shining example) clearly show 
that the authors were rather clumsy at their approaches to growing plants.  
References to maximum height and propagation characteristics indicate that their 
husbandry was marginal to say the least. 

> Laterite certainly provides a certain amount of useable iron in the
> substrate. Substrates with very low organic content do not have a low
> enough redox potential to provide a continuous supply of substrate iron

Are you saying that laterite does or does not continuously supply iron (since 
you include it in the category of low organic content)? Do you have some 
reference for this?  How does the redox potential of a laterite based substrate 
differ from an organic substrate?  What redox levels are needed to produce a 
continuous supply of iron? How do you measure these levels (and I mean "you" 
personally, since you claim to have so much experience in this area)? How do you 
know if plants aren't receiving enough iron from the substrate?   

> 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?  I have seen nothing you have posted that gives 
credence to this assertion.  Have you measured the redox potentials before and 
after?  How much peat do you have to add to how much substrate?  What else is in 
ths substrate?  How does iron get there in the first place?  Is this state 
sustainable for more than a few months? 

I think you are making this up. I challenge you to prove this statement or 
retract it. 

> 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. 

Perhaps it is not wise for YOU to do this. It works just fine for me and others 
who have tried it and does NOT promote filamentous algae growth. 

> I hope that we can describe some procedures to safely use natural soils
> which are also suitable for novices but it should be noted that
> following the procedures is more critical.

And just who is this "we" that you refer to? If you are presuming to speak for 
the collective APD, you are very misguided. If you are presuming to speak for 
the AGA, you are way off base. If you are speaking for yourself and a few 
others, state exactly WHO is giving this advice. 

> In my 75 gallon tank, I had very good growth of C balansae for several
> months without loss of older leaves but more recently the older leaves
> have begun to melt and are replaced by new leaves. The substrate in this
> tank contains a lower organic material content and much of this is from
> peat which is a very poor source of phosphates. I suspect that I am also
> beginning to have reduced levels of phosphates. Another plant which has
> shown a reduction in growth rates in this tank is Aponogeton crispus.
> The fish load in this tank is very low.

Let us know when you find a substrate recipe that works for more than a few 
months.  Then you can be the self-proclaimed King of Substrates and I will not 
have to get all excited when you post your substrate idea du jour. Until then, 
and as long as I don't have a stroke reading this stuff, I will continue to take 
issue with pompous postings like this. 

George "should have taken vacation over the holiday break"