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Re: Laterite, kitty litter, et al

Thanks, Steve for this post.  The quotes from Dr. Dave and from Karen
Randall are great.  I'd like to expand (gasp!) on one point.

> We should avoid confusing the properties of laterite with the various
> clays and other soils. Laterite is quite different, it is chiefly
> composed of iron and aluminum oxides/hydroxides. 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)...

This is correct.  CEC is very important to the distribution of Ca++, Mg++,
Na+, K+ and NH4+ between soils and water, but there is also a second
factor called surface adsorption, which is independent of the ionic
charge.  This is a tendency for solutes to become attached to the surfaces
of particles in the soils or sediments.  Surface adsorption is measured
with a value called the "distribution coefficient" (Kd).  Freshly
precipitated metal hydroxides provide very high Kd values, and phosphates
have a particularly high affinity for being adsorbed.

> As Dave Huebert points out above, rooted aquatic plants do best when
> they can get an adequate supply of phosphorus from the substrate.
> Laterite and other iron compounds in a substrate help to sequester
> phosphates. Clays, kitty litter and vermiculite are not able to do this.
> Natural, amorphous soils commonly contain significant amounts of iron
> (typically 5% according to Diana Walstad).

Well, actually pretty much all materials (including glass beads) have some
capacity from surface adsorbtion, but there is a big variation in the
distribution coefficient for different materials and solutes.  The biggest
variable seems to be the amount of surface area provided by the sediment.
Fine-grained sediments of all types - especially clays - can have
significant capacities for adsorbing solutes because they provide a large
surface area.

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.

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

Organic particles in sediments are very effective adsorbers for organic
solutes - particularly those that aren't very soluble in water.  I haven't
read of natural soil organics being very significant adsorbers of
inorganic solutes.  They do provide CEC and may chelate some solutes, but
those are different reactions.

> Roger, have you tried adding any form of phosphate fertilizer into the
> substrate? Probably the best way would be to use something like enriched
> clay balls or Jobe's plant sticks which are made from a material
> designed to slow the release of the nutrients. I don't think the plastic
> coating on Osmocote pellets does an adequate job of this and prefer to
> use these inside iron rich clay balls. I'll probably put some of these
> into my 75 gal shortly.

I haven't tried this yet, but it gives me a good angle to persue.  Thanks.

Roger Miller