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Substrate Ideas


  1. [PLANT] Supplement for Cultivation Methods
    by (Jim Kelly) (10 Mar 1995)

[PLANT] Supplement for Cultivation Methods

by (Jim Kelly)
Date: 10 Mar 1995
Newsgroup: sci.aquaria

This information is meant to accompany the previous posting "Aquatic
Plant Cultivation Methods" as a technical reference for soil/plant
relationships.  It was previously posted in Sept 1994 on sci.aquaria.

Jim Kelly
     The following is a summary of ideas about substrates for
planted aquaria which I picked up from recent readings in soil
science.  The material is found in most introductory texts.  My
favorite (and the main source for me) is N. C. Brady's
book, "The Nature and Properties of Soil."  I looked through many books 
(originally motivated by the quest for information about what makes 
laterite so wonderful) and this one had just the information I was 
looking for and at a level which I could understand.  The info in the 
book applies to many soils, not just submerged aquatic environments, so 
I'll leave it up to you to theorize about whether any of it is 
applicable to to the hobbyist or not.  I believe that it is and have 
commented (as best I could) on the relevance of the ideas to the hobby.
My opinions are enclosed in square brackets [...] to help
distinguish them from the textbook info.  I would appreciate any
feedback, as I haven't seen most of this discussed on the net
previously and would like to hear the opinions of the net experts
on the practical relavence of the material.  In all cases more 
depth is available in Brady's book, but I have tried to cull anything
which isn't of relavence (IMHO) to aquarists.

1)  Cation Exchange Capacity and Plant Nutrition

     It has been suggested by those in the know that many of the
aquatic plants kept by hobbyists which have extensive root systems
prefer to take some (if not most) of their nutrients in through
their roots.  Plant nutrients (in their available forms) exist in
solution as positively charged cations and negatively charged 
anions.  Since + and - charges attract, each cation in solution
is accompanied by an anion so that the charges match.  In natural
soils, however, there exist microscopic clay particles which have
excess negative charges at the edges and surfaces of their crystal.
These charged sites attract cations, which would otherwise be 
moving in solution, and hold them to the crystal's surface with a
tenacity that depends upon the cation and the type of clay 
involved.  Decayed organic matter (humus) has the same property
as these clays.  The attracted nutrient cation is (usually) not
permanently fixed to the particle, but is periodically freed due
to thermal agitation, and other cations from the solution can take
its place.  This replacement is called "cation exchange" and in 
this way an equilibrium is formed between water and soil particle.
(I.e. a higher particular cation concentration in the solution
means a higher percentage of that ion adsorbed on the clay
compared to other ion species, since the reactions are reversible.)

     This abundance of nutrient cations being held in the substrate
is used by plants to their advantage; it forms a storehouse of
nutrients, preventing them from being leached into the water
column.  By cation exchange, H+ ions are released from root hairs,
which in turn exchange with nutrient ions adsorbed on the surfaces
of clay particles, forcing the nutrients into solution where they
can be assimilated by plants.  As a general rule, the ability of
a soil to hold cations in readily exchangeable positions is 
considered good for plant nutrition.  This ability is measured
quantitatively in centimoles of exchangeable charge positions per
kilogram of substrate and is called the Cation Exchange Capacity 
(CEC).  Here is a table of approximate CEC's of organic matter
and various common clays taken near pH=7.0:

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

     Before briefly discussing what the various components are,
here are two comments.  First, humus, vermiculite and the
smectites are far superior in CEC to other common soil components.
Second, laterite (a tropical clay subjected to extensive
weathering over geologic times scales which is often sold as a
soil additive for plant tanks) is composed mostly of Fe and Al
oxides and kaolinite-like clays, which will contribute very 
poorly to the CEC of the substrate.  [This is in contrast to what
has been said on the net.  However, laterite may have other redeeming
qualities.]  Now here are some notes on the CEC table soil components:

* 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.

* Layer silicate clays form small colloids in the soil with a
layered structure.  They have a large external (and frequently
internal, between-the-layers) surface area.  VERMICULITE and
the SMECTITES are called expanding clays since water and cations
are allowed to move between the layers, forcing them apart.  This
creates an internal surface area which exceeds the external and
gives them their large pH-independent CEC's.  ILLITE, CHLORITE,
and KAOLINITE do not expand in this way, resulting in lower
CEC's.  Approximately half of their CEC is pH-dependent (goes to
zero at low pH).  The mechanisms causing the CEC's of the layer
silicates is described by Brady.

* Fe and Al OXIDES often occur in temperate regions mixed with
layer silicates, and sometimes dominate in the weathered soils 
of the tropics (e.g. laterites).  At high pH they carry a small
negative charge (less than kaolinite), and at low pH they become
positively charged and can counteract the CEC of layer silicates.
The most common are gibbsite (Al2O2.3H2O) and goethite 
(Fe2O3.H2O).  [Thus these are poor at holding nutrient cations
for plants but at low pH they may hold anions]

* Not listed in the table but also of importance are some 
amorphous minerals of volcanic origin (allophane) which can have 
both a high CEC as well as considerable anion adsorption capacity.

     Different cations are adsorbed more or less easily than
others.  Here is a partial list of cations ordered by adsoption

           Al > H > Ca > Mg > K > Na

Adsorbed H and Al cations tend to acidify the soil (Brady gives
Al acidification mechanisms) and tend to dominate the adsorbed
cations in very acid soils.  Most other cations, called
"exchangeable bases", neutralize soil acidity and dominate in 
neutral or alkaline soils.  The "% base saturation" is defined

                         (# adsorbed exchangeable bases)
           % base sat. = ------------------------------- x 100
                           (total # adsorbed cations)

% base saturation generally increases (decreases) with 
increasing (decreasing) pH, since at high pH Al forms insoluble
compounds and H+ --> H2O.  Note that laterites from tropical
regions generally have a low % base sat., and are called "acid

     Four final comments on cations exchange deserve emphasis:

* It is not just the total number of a particular nutrient ion
adsorbed in the substrate which determines its availability to
plants, but also the fraction of the CEC occupied by that ion.
For example, 6 cmol/kg of exchangeable Ca in a substrate with
a CEC of 8 cmol/kg probably means Ca is readily available, but
6 cmol/kg out of a total CEC of 30 cmol/kg might result in

* Different soil components hold the various cations with 
differing tenacities (i.e. the ordering given above is a

* The presence of certain nutrient ions may limit the 
availability of others (in ways more complicated than those
suggested above).

* Anion exchange:  Though usually overshadowed by cation exchange,
certain materials (especially hydrous oxides of Fe and Al) can 
have positive sites on their surfaces which hold anions in 
exchangeable positions.  Laterite may participate in this because
of its high oxide content.  

2)  Effects of Soil pH

* Soil pH buffering - exchangeable acids and bases can to a 
certain extent buffer the pH of a terrestrial soil's liquid phase.
[Presumably this is not of lasting importance for aquaria, where
the substrate is continually leached by the water column, which
is replenished often.  It might be of some importance
for substrates with little or no water circulation, but it seems
that decaying organic matter or bases such as Ca in the primary
minerals would still dominate the pH in the long run.]

* Most macronutrients (N, Ca, Mg, P, K, S) are maximally available
at a pH of 6 to 7.  Most micronutrients (Fe, Mn, Zn, Cu, Co) are
more readily available at a low pH.  [Note that most aquarists,
in contrast to farmers, would not consider very soluble phosphorus
to be a good thing!  Also, at mid to high pH the available N is
NO3-, which is rumored NOT to be preferred by aquatic plants.  The
preferred ammonium N is most available near pH=6 or below.]

* Most bacteria operate their best above pH=5.5.

* Different plants like different pH, but generally a low pH
prohibits root growth and nutrient uptake, even though many
micronutrients are more soluble at low pH.  [See C.A. Black,
"Soil-Plant Relationships."]

3)  Nitrogen

* Vermiculite and the smectites can hold NH4+ (ammonium) in both
exchangeable and fixed (unavailable for plants) positions.  (See
Brady for fixation details.)  There is a reversible equilibrium
between the amounts of ammonium held in these two states (although
equilibrium shifts happen much more slowly than ion exchange), so
that an addition of ammonium to the solution will quickly change
the percentage of the CEC occupied by ammonium, some of which will
slowly take up fixed positions.  Similarly, if plants take in NH4+
from exchange sites, more will be released from fixed positions
(slowly) and become available to plants.  Thus, NH4+ fixation can
be viewed as a storehouse of N which helps to buffer changes.
[This would seem to be a positive attribute for an aquarium
substrate because it makes it more stable against ammonium
availability fluctuations.  With such high CEC's in the first place
these clays seem to be the best substrate for plant nutrition (at
least N nutrition).  Note that the "fixing" is not part of the 

* Nitrification will occur only with sufficient oxygen and ammonia.

* Denitrification occurs under low aeration conditions, which is 
unhealthy for plant roots (they can't grow or take in nutrients
without respiring).  [This seems to indicate that some water flow
through a substrate is good.  It has been found that certain
aquatic plants can use O2 from photosynthesis for root respiration
by supplying it through hollow (gas filled) passages in the roots,
but most net people seem to agree, despite this fact, that 
anaerobic conditions are harmful to aquatic plants.  Thus,
encouraging denitrification IN THE SUBSTRATE would not be 
recommended for someone trying to help the plants as much as

4)  Phosphorus

     Aquarists are primarily concerned with lowering the soluble
phosphates in the water (to avoid algae), since even undetectable
levels support most aquatic plants quite well.  The following seems
VERY relavent to keeping low soluble phosphate levels in the 

* Commonly formed inorganic phosphorus compounds which are 
relatively insoluble are of two types:  Ca compounds and Fe/Al
compounds.  Phosphorus solubility is determined by

  a.  soil pH
  b.  presence fof soluble Fe, Al, Mg and minerals containing these
  c.  available Ca and Ca minerals
  d.  amount and decomposition rate of organic matter
  e.  activities of microorganisms

* In strongly acid mineral soils, soluble Fe, Al, and Mg can exist
and react with existing phosphates rendering them insoluble.  Often 
the amount of soluble metal ions greatly exceeds the amount of 
soluble phosphates, and only minute amounts of soluble phosphate
will remain [for plants and algae] at equilibrium.  Even greater
amounts of phosphates are removed from solution in acid soils by 
oxides of Fe and Al.  In acid conditions these oxides have a net 
positive charge and they attract phosphate ions from solution into
exchangeable positions on their surface.  With time the phosphate 
ions either migrate into the center of the oxide particles and 
become unavailable, or react with the hydrous oxide to form an
insoluble compound, or are use by plants and microorganisms.  Since
several insoluble compounds can be made by the reaction of 
phosphates with hydrous oxides of Fe/Al, it is thought that
phosphate may be fixed over a wide pH range extending from low
through the neutral zone, even though little charge exists in the
neutral/alkaline region on the oxide surface.  Kaolinite can also
fix phosphorus under moderately acid conditions, but the mechanism
is unknown.  Other silicate clays hold a very small pH-dependent
positive charge which is generally insignificant.  [Hydrous oxides
and kaolinite are the primary components of tropical 'laterite' or
'latisols' (the more current name).  This suggest that laterite in 
the substrate with water circulation through it may play a role in
reducing phosphate levels in a tank, especially in acid water.  It
also suggests that any form of iron oxide in the tank (rusting steel
wool in the filter or unchelated garden Fe supplement?) may help
reduce phosphate levels in the tank.  Since a low pH is better,
substrate additions may be the best bet.]
* In high pH soils the availability (solubility) of phosphorus is
determined largely by the solubility of various Ca-phosphate
compounds which are formed (and which Brady lists).  [I couldn't
find much practical or usable info here, but this seems like an
important starting place for experimentation and further research
for those of us with high pH and phosphate problems.]

* Fe and Al phosphates have a minimum solubility at a pH of 3-4.
However, even at a pH of 6.5 much of the phosphorus is chemically
combined with Fe, Al.  Near pH=6, Ca precipitation of phosphates
begins; at pH=6.5 the insoluble Ca salts are the dominant factor;
above 7.0 even less soluble Ca compounds are formed.  The maximum
solubility is in the range 6.0-7.0.  [This is unfortunate for
aquarists since many of us target this range for keeping plants.
Fortunately both types of precipitates have a tail which extends
usably into this range.]

* Freshly added soluble phosphate goes through several stages on
its way to insolubility.  First either (a) fresh precipitates
with soluble Ca or Fe/Al are formed or (b) similar compounds are
formed on the surface of Ca carbonate or Fe/Al oxide particles.
These are less soluble than the phosphate ions, but still soluble
to a degree and have a high total surface area.  As time passes, 
the crystal size of type (a) precipitates increases, decreasing
the relative amount of surface area available for extraction by
plants/algae, and type (b) phosphates tend to migrate into the
centers of the particles holding them, decreasing their 
availability.  As mentioned above, less soluble compounds might
also be formed along the way.  [Again laterite sounds ideal for
long term removal of phosphates from the water, since it's
mostly particulate Fe/Al oxide.]

* Most compounds which react with phosphorus are in the finer
soil fractions, so that the higher clay content soils have
greater phosphate fixing power.

* Products of organic decay such as organic acids and humus are
thought to form complexes with Fe/Al oxide compounds which can
reduce the phosphate fixation which occurs to a remarkable
5)  Potassium

     A large amount of K exists in most soils, almost entirely
locked up in primary minerals or fixed in secondary clays.
90-98% is in relatively unavailable forms.  1-10% is in slowly
available form, fixed in vermiculite and smectites in 
nonexchangeable positions.  1-2% is readily available for plant
roots (9/10 of which is adsorbed on clay and 1/10 of which is
in solution, but this applies only to terrestrial soils).  The
available and fixed K are in equilibrium with eachother in the
same way as described for ammonium.  Brady notes that alternate
wetting and drying seem to cause the fixation.  [So perhaps we
aquarists needn't consider it?]

6)  Micronutrients

     Too little of thes essential nutrients causes deficiency
symtoms, but too much is toxic to plants;  the window in which
plants will grow well is not large.

* Functions of some micronutrients:

     Zinc:        Promotes growth hormones and starch formation;
                  promotes seed maturation and production.

     Iron:        Important in chlorophyll formation.

     Copper:      Important in photosynthesis, protein and
                  carbohydrate metabolism.

     Manganese:   Important in photosynthesis, N metabolism,
                  and N assimilation.

     Boron:       Essential for cell division and development.

     Molybdenum:  Essential for N assimilation.

* The forms that the micronutrients are present in depends upon
soil pH and aeration.  Simple cations are present under low pH
and aeration conditions, while at high pH and aeration hydroxy
metal cations form.  High pH and aeration render most
micronutrients insoluble and unavailable for plants (Mo and a
few others excepted).  High pH favors oxydation in general.
However, poorly drained acid soils often supply toxic quantities
of Fe and Mn.  [Generally one wants to strike a balance between
too much aeration of the soil (where everything gets oxydized
and becomes unavailable) and too little (which is probably
worse in my experience:  toxic quantities of micronutrients,
inhibites root growth and respiration, anaerobic decay).  People
claim to do this with heating cables or drops/sec RUGF flow
which can be adjusted for the correct flow.]

* There is a marked difference between sensitivities of different
plants to Fe deficiency.  Some respond by secreting compounds
from their roots which reduce iron in the vicinity to its more
soluble forms.

* Some micronutrients can be temporarily "fixed" much like NH4+
and K+.

* Large quantities of soluble phosphates can affect the uptake 
of Fe and Zn.

* It is essential and difficult to maintain the proper balance
of micronutrients.  Some biochemical reactions may be poisoned
by high levels of some other micronutrient not directly 

7)  Chelation

     Organic complex formation may protect micronutrients from
from certain chemical reactions, such as Fe and phosphates
precipitating eachother [which may be good or bad depending on
the context!].  However, this may reduce the availability of
some micronutrients for plants [it depends on which chelate and
which nutrient I suppose].  Note that FeEDTA is available for
uptake by plants.  A chelate is a compound in which
certain metallic cations are complexed or bound to an organic

* Quoting from Brady, "The mechanism by which micronutrients
from chelates are absorbed by plants is still obscure.  
Chelating agents in most cases are absorbed by growing plants,
but their rate of absorption is slower than that of the metals
thay carry.  Thus it would appear that the primary function of
the chelate is to keep metals available in the soil.  At the 
same time there is evidence that some of the benefit from
chelating agents is through increased translocation of the
metals once they are absorbed by the plants."

* Some cations are chelated more easily than others:

            Fe > Cu, Zn > Mg

Thus ZnEDTA + Fe2+ --> FeEDTA + Zn2+, but it happens slow 
enough to allow for some Zn absorption.

8)  Nutrient Uptake by Plants

     The uptake of a nutrient depends not only on solubility
and being in an exchangeable position, but also on it being
in close proximity of the root surface.  Nutrients are supplied
to roots by

     1.  Root interception:  as roots penetrate the soil they
         come in contact with colloids having adsorbed 

     2.  Mass flow:  nutrients move into roots as water is 
         absorbed.  [I have no idea of the degree to which 
         this occurs for submerged aquatics.]

     3.  Diffusion:  nutrient absorption in root vicinity sets
         up a concentration gradient, and ions diffuse toward
         root surfaces.

For cations, diffusion is the most important.  For NO3-, mass
flow is the most important.  [Perhaps this explains why many
aquatics don't like to use NO3- as is said.]  Root uptake of
nutrients requires intimate root-soil association.  [Which means
coarse gravel is not optimal.  The finer the substrate the
better as long as there is aeration.]  Nutrient solubility is
strongly affected by root exudates and microbial activity near
roots [by the latter Brady is presumably referring to N 
fixation by bacteria in terrestrial plants].  Finally, the
entrance of soluble nutrients into root cells is stimulated by
plant root metabolism -- respiration provides the fuel!  

Jim Kelly

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