Laterite balls/shards: low surface area

Earle Hamilton <ehami at sunny_ncmc.cc.mi.us> wrote:
> [snip, really good laterite overview]  
> Dupla sells the powder and 
> "pellets".  At one time the fellow who is the potter and I talked about 
> marketing this stuff and would have mixed up a batch of clay and run it 
> through the extruder he has.  We would then cut off hunks similar to 
> Dupla that would be about the size of a marble.  FIRING THESE CHUNKS 
> WOULD DEFEAT THE PURPOSE OF THE LATERITE.  It has been suggested we could 
> get laterite in our tanks by breaking up clay pots.  The stuff used for 
> clay pots has very little iron and I suspect firing bonds the stuff 
> together in a way that would be of little use to plants.
> I have seen no evidence that the clay chunks are of any real value.  They 
> do hold up well and are easy to form from the powdered laterite from 
> Duply or A.R.T.  If you work with this stuff you will find it takes very 
> little water and that the result is very sticky goo but it is easy to 
> make up the marbles.  Would be a great project for a fish club.

I just got back from 2 weeks away, and just reviewed this discussion.  I
will put in my two cents and let the thread die (I guess it seems to be

I don't understand ANY benefit to fired clay (other than it can provide
the same benefits that a gravel substrate may provide).  Having been 
fired at 1100+F, the iron is lost.  Even if it were there, virtually all 
of it is inaccessible because we now have a "stone" with a very LOW
surface area/volume ratio.

Back in my undergraduate studies in Soils, a very tremendous attribute
of all clays were their VERY HIGH surface area/volume ratio.  We are
talking hundreds or thousands of times higher than that we can observe
in sands or silts.  Those little flat platelets have tremendous surface
area with (often) a high CEC, and can tightly bond to each other.  This
is why clay holds its shape, and is why clay pots (fired or not) hold 
water.  Those really big H20 molecules just can't squeeze between two
platelets trying to grab cations off each other.  However, this tight 
bonding between clay particles also implies a clay material can quickly
go anaerobic because circulation is very difficult to achieve (although
heat coils seem to achieve circulation through convection through a short 
depth: a few inches of gravel and clay mix).

Thus, while non-fired clay balls may have iron available, we must get
a root hair in there to exchange those cations.  Because the ball is not
fired, the roots can (eventually) penetrate the mass (possibly dying if
it goes too deep, if it cannot get sufficient oxygen for respiration).

This is one reason why I think I like vermiculite:  A clay with a high
CEC, but "puffed up" into static structures (like puffed rice) that
inherently allow for circulation and CEC exchange.  I just have to
figure out how to "charge it" with iron and other macro- and micro-
nutrients, because it comes essentially sterile (it was fired in
its "puff-up" preparation, and to give gardeners a medium free from
possible parasites/bacteria/virii).

It seems much simpler to distribute the laterite around the substrate
in its smallest (manageable) form:  More surface area is *readily*
available to rootlets, it is more distributed so more rootlets can be
accommodated over the same cubic inch of substrate, and circulation
is (potentially) improved at the actual rootlet bonding site.

Of course, laterite in the form of "talcum powder" will compact more
over time and possibly find itself much more easily into the general
aquarium water body.  My guess is that if we are going to use laterite,
the most efficient leveraging of its CEC capacity and iron (and other
nutrients) would the smallest particle that would not easily be 
suspended in the aquarium, possibly going larger if we want a lot of 
lee-way when up-rooting our vegetation.  Something BB sized 
(say, 2-4 mm) seems appropriate.

I'm not sure how well it weathers over time, though.  If you start
with 2" balls of laterite and a year later you have a 6"x1/4" patty
of clay (to be extreme), then maybe larger is OK:  we will get the
surface area simply because time will make it so.

cbay at gr_hp.com		or	charley at agrostis_nrel.colostate.edu