A discussion of substrate additives

I have been procrastinating on preparing this article for a while
but now seems like a good time. I want to cover some of the technical
aspects of substrate additives with a view to explaining the 
popular theories about why they work. I will be borrowing heavily
from ideas presented by Jim Kelly in an article posted on September
of 1994 on sci.aquaria. Jim did research from a lot of sources but
credits  N. C. Brady's book, "The Nature and Properties of Soil" as 
his favourite. I want to keep this as appropriate to the general
audience as I can. Please feel free to correct me on any points of
error or if you just want to beg to differ! ;-)

Plant nutrients in solution are generally in the form of ions. When
a mineral salt dissolves in water, it separates into two ions: a
positively charged cation and a negatively charged anion. When a
salt dissolves, we call it dissociation which means the salt molecule
is split into two very mobile ions. Not all substances which can
dissolve or mix with water dissociate; for example, alcohols. I think
that EDTA and DPTA chelated minerals are examples of this. A chelated
mineral is a compound composed of an organic acid ion and a mineral
ion such as Fe or Mg. These complex organic acids are actually the
evolved mechanisms by which plants are able to capture and transport
components used in their metabolic processes including the conversion
of water and CO2 to simple sugars (photosynthesis!).

Horst & Kipper (of Dupla fame) did a lot of pioneering research into
aquatic soils and the value of light and CO2. An excellent book which
covers many of their findings applied to create a formula approach to
successful aquarium keeping is called "The Optimum Aquarium". They
measured nutrient concentrations in the natural habitats of aquatic
plants (esp. cryptocorynes) and conducted several comparative experiments
to evaluate techniques. They found that plants grown in a substrate
containing an iron rich tropical clay (laterite) grew much better
than plants grown in conventional sand and gravel substrates. They
also found that several other factors such as light intensity, the
concentration of dissolved CO2, the concentration of ammonium (nitrogen),
and other critical micronutrients such as free iron, magnesium etc.
were even more important to high growth rates of plants.

Horst & Kipper theorized that iron from laterite subsoils was being
leached into the mineral rich springs which feed the Cryptocoryne
habitat. To date, no one has been successful at duplicating this leaching
process in an aquarium to the extent that it provides sufficient iron
to satisfy our plants requirements. We rely upon chelated nutrient mixes
which H&K also pioneered to supplement what is available from the tap
water. Given that one uses these supplements, the value of the iron
in laterite is uncertain. How can we explain the obvious success of
tanks using the Dupla laterite method? Laterite and other substances
possess another property called the Cation Exchange Capacity (CEC).
Remember that several of the important nutrients for plant growth
including ammonium, iron, potassium, phosphorous and magnesium can
exist in solution as cations! (or sometimes in combination with oxygen 
as anions)

Clays, humus and finely powdered mineral oxides have varying amounts
of CEC. The ultra fine particles of clays are tiny crytals of oxides
and silicates usu. of iron and aluminum. These crystals are in effect
giant molecular structures and the edges of these crystals possess
negative charges. Positively charged cations are attracted to these
sites where they form weak molecular (electrostatic) bonds. Since these
ions are not bound tightly, they can be displaced by other ions in the
solution esp. by thermal agitation. CEC quantitatively measures how
many cations can be captured by any given substance. Here is a table
given by Jim Kelly:

          Soil Component           CEC (cmol/kg)
          --------------           -------------
          humus                    200
          vermiculite              150
          smectites                100
          illite                   30
          chlorite                 30
          kaolinite                8
          Fe, Al oxides            4

Substrates with CEC help plants to capture and absorb important nutrients
with their roots. Plant roots secrete organic, humic acids (found in 
humus) which are able to displace the nutrient cations, bind them
into soluble chelates and make them available for absorption into
the root where they are transported to the leaves for use by the plant.

Laterite is comprised of Fe and Al oxides and kaolinite-like clays
which contribute little to the CEC of the substrate. Jim mentions other
amorphous minerals of volcanic origin (presumed in laterite) which
have high CEC and may contribute to laterite's CEC. In addition,
the hydrous oxides of Fe & Al in laterite can have anion exchange
sites which may be important for phosphate ions. Jim didn't talk much
about anion exchange or how important it is relatively. He did talk
about the possible benefit of iron oxides in reducing phosphate
concentration by attracting these anions and making them chemically
unavailable. The iron oxide needn't be in laterite to perform this
function but should be in the substrate if there is some degree of
substrate circulation.

"HUMUS is the end product of the decomposition of organic matter.
It is a complex substance consisting of material either modified
from dead plant tissue or synthesized by soil organisms.  It is
fairly resistant to furthur decay (in contrast to peat moss, which
is relatively undecayed organic matter but which also has a high
CEC) and thus forms the long-lived organic component of the 
substrate (but not as stable a clay).  It has the following
characteristics:  high surface area per volume, exceeding clay
particles; has negatively charged carboxylic and phenolic sites;
has an entirely pH-dependent CEC, which is low at low pH but 
exceeds silicate clays above about pH=6;  when saturated with H+
ions in its exchange sites it can extract nutrient ions (e.g. Ca,
Mg, K) from minerals by dissolving them, and then hold the 
nutrients in exchangeable positions for plant uptake." (Jim's words)
It has excellent nutrient exchange properties.

Humus, laterite and clay all have one undesirable characteristic;
their fine structure makes them subject to compaction and may prevent
the diffusion of water and oxygen into the substrate. Plant roots
themselves can assist in providing oxygen. Vermiculite and sand
are good additives to help prevent this compaction and allow the
roots to penetrate. Many aquarists use heating coils or very slow 
reverse undergravel flow to provide the very low levels of oxygen
and water/nutrient circulation. This does not appear to be necessary
with the use of vermiculite in a low organic substrate.

High organic substrates such as peat moss or commercial potting soil
mixtures are NOT good. There is far too much undecomposed organic
material which will rot in the low oxygen environment producing
toxic sulphide gases. Jim Kelly recommends loam, straight out of your
garden. I'm using commercial humus from earthworm castings. This is
mixed with a larger proportion of vermiculite which has been allowed
to saturate with water and used in the lower layer of the substrate.
The top layer consists of coarse sand or fine gravel (2mm dia).

This is by no means a complete discussion of all aquatic soil properties
but it is long for a posting. I've tried to keep this understandable.
I hope it clears up some of the confusion between chelation of metal
ions and the cation exchange process. I hope it takes some of the
mystique out of laterite and iron oxide clays and encourages people to
try out some new ideas! I also hope Jim has time (and interest) to
share some of his latest findings along with his recipe for vermiculite-
loam substrates.

 - Steve