Table Of Content1
Substrates for Aquarium Plants
Introduction
There is a wide range of materials and approaches used for aquarium substrates. The
processes and interactions that occur within the substrate are extremely complex. While
some aspects are well understood, we are able to discuss these biological processes only in
the most general of terms. I will attempt to present a relatively complete survey of substrate
techniques. I will quote the intended benefits of each technique but I will also comment upon
these techniques so that the reader can have the benefit of an informed judgment. I believe
that too much information is being passed on without critical validation. In making such
judgments, it is inevitable that they will be influenced by my personal values and biases. Let
me stress that no single approach should be considered "the best" since many approaches
will give highly successful results and a particular application needs to take into consideration
the requirements of that aquarist. Factors to be considered are: types of plants, growth rates
desired, maintenance, automation, budget, technical expertise and the aquarist's tastes and
beliefs. Let me stress that so-called fertile substrates are not optimal for all people. Please
see the section entitled "Low Fertility Substrates".
The substrate serves two purposes in a planted aquarium. It provides a suitable medium to
hold rooted plants in place and (to a greater or lesser degree) it provides nutrients to the
plants necessary for growth. It also may have a dominant effect upon three parameters of the
aquarium water: the pH buffer system, mineral hardness and dissolved organic compounds
(DOC). Note that substrates may be designed to minimize these effects. pH buffering effects
complicate or introduce errors into measurements of CO2 content and carbonate hardness.
Moderate or low levels of DOC, i.e. humic acids, inhibit bacteria and algae. Excessive DOC
due to inappropriate choice of substrate materials or too many fish negatively affect fish
health and inhibit plant growth.
Essential mineral nutrients are conveniently separated into two categories. Nutrients used by
plants in relatively large amounts are termed macro-nutrients. Besides carbon (C), hydrogen
(H) and oxygen (O), they are nitrogen (N), phosphorus (P), sulfur (S), calcium (Ca),
magnesium (Mg) and potassium (K). Nutrients used by plants in small amounts are termed
micro-nutrients. They are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum
(Mo), cobalt (Co), and boron (B). Other mineral elements, such as sodium (Na), chlorine (Cl)
and nickel (Ni), are also present in plants, and may or may not be essential.
It is possible to grow plants in completely inert substrates such as plain gravel; however, it is
common to use a nutrient strategy where some or most of the nutrients are obtained from the
substrate. This has several benefits. Many nutrients are absorbed preferentially by plant
roots. This means that plants can grow faster when nutrients are provided in the substrate.
We can limit the nutrients that are dissolved in the aquarium water to restrict algae growth.
We can employ higher concentrations of some nutrients in certain kinds of substrates and
thus get improved growth rates. It is not necessary to perform nutrient additions as frequently
nor to measure nutrient levels in the water as frequently.
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On the other hand, it may be necessary to take precautions when using various additives
intended to produce more fertile substrates. The substrate should be prepared so that
excessive amounts of phosphates, nitrates or ammonia are not released into the aquarium
water. The use of excessive amounts of organic materials or the wrong types may result in
high rates of bacteriological decomposition that consumes oxygen in the substrate and may
release harmful substances. Different aquatic plants are adapted to different types of
substrates. Organic plant materials such as leaf mulch, peat, composted particles of wood
release a variety of humic acids. Some soil amendments may contain excessive amounts of
some minerals that can interfere with nutrient uptake by plants or be toxic to invertebrates
(snails) or fish. Fine texture materials such as clay, silt and organic particles commonly used
in fertile substrates tend to cause problems with turbidity (cloudiness) when the substrate is
disturbed during planting or uprooting. These materials may settle after a period of time;
however, they may settle upon plant leaves giving an unsightly appearance and encouraging
the growth of certain algae types on the surface of those leaves. Clays are extremely fine
particles which can create very cloudy water if disturbed.
Some nutrients are utilized by plants much more efficiently from the aquarium water and
these include K, Ca and Mg. Although some plants have been shown to get C from carbon
dioxide in the substrate, it is probably best to supply this in the water.
"Potassium is needed in the water column because it is used to maintain the ionic balance
within the plant. There is a dynamic equilibrium between the K inside and outside the plant.
Calcium and Magnesium are required in the water because they do not transport readily
within the xylem of aquatic plants. Calcium in particular is necessary in the water because
there is an extra-cellular requirement for Ca ... if the Ca level drops too low then the cell wall
does not form properly.
"Experimental evidence clearly indicates that K, Mg and Ca are required in the water column
for the 'normal' growth of at least a few submerged aquatic plants. The reasons for this are
less clear but can be discussed on the basis of their chemical properties and
physiological/biochemical roles. As mentioned, potassium, while a mobile element, is required
for ionic balance and so must be present both internally and external to the cell membrane ...
remember that aquatic plants, unlike terrestrial plants, have their apoplasm (that part of the
plant external to the membrane) in direct contact with a solution into which ions can diffuse.
Calcium is also necessary external to the cell membrane for normal cell wall development ...
Both Ca and Mg in land plants do not 'move' as readily in the xylem as other ions. The
hypothesis is that because the movement of water through aquatic plants is less than in land
plants, Mg requirements can't be satisfied by upward movement. At no time have I said that
Ca and Mg can't move within the xylem of aquatic plants ... in fact experimental evidence
clearly shows that at least Ca can move up or down in aquatic plants tissues (just not at a
rapid enough rate to satisfy external requirements)." – DH
A complete discussion of K, Mg and Ca requirements is not within the scope of this article.
The reader may refer to the popular PMDD recipe for guidelines for K and Mg dosing. Ca can
often be a critical nutrient in soft water particularly if water changes are not frequent or under
high lighting intensities. A safe guideline for tap water with general hardness below 2 GH or
with low Ca content would be 1 teaspoon of CaCO3 for each 10 gallons of water changed.
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Types of Substrates
The simplest substrate is plain fine gravel (approximately 2-3 mm grains). This is the most
common type of substrate used by people keeping fish. Over time, the substrate may become
enriched by the wastes of the fish and food to the point where it may be able to provide
nutrients for aquatic plants although poorly. On the other hand, with a careful balance of
nutrient additions, it is quite simple to grow aquatic plants even in a completely inert sand or
gravel substrate. Generally one or more types of substrate additives are used in addition to or
in place of the standard aquarium gravel. These include various natural soils, various clays,
peat, humus, leaf mulch or commercial products such as Duplarit (laterite). These additives
are intended to perform one of the following functions:
increase substrate nutrient holding capacity (CEC) thereby increasing the effective
fertility of the substrate.
provide iron and possibly other trace mineral nutrients
directly provide macro-nutrients (as chemicals or by decomposition)
decrease porosity to inhibit nutrient diffusion (out of the substrate)
increase porosity to permit nutrient diffusion (into the substrate)
increase the density of the substrate to prevent a fluffy components from being dragged
up by uprooting
Substrate Materials
Fine gravel - typically the stuff sold in aquarium stores for excessive prices. Try to find a store
that will sell a bulk bag for a reasonable price or something like TexBlast sand for
sandblasting. Avoid gravel with limestone in it as it will continue to dissolve causing high pH
and hardness. See caution under topsoil for collecting. Gravel is largely unreactive due to it's
low surface to volume ratio so it is not useful as a source of mineral nutrients. It works well to
anchor stem cuttings. A one inch layer on the surface helps to contain fine substrate materials
such as clay, vermiculite or peat during planting. It is a good choice for substrates where the
primary source of nutrients is from the water or fish wastes because it will allow nutrients to
diffuse into the substrate where they can be adsorbed.
Sand - very similar in properties to gravel. Used alone in a substrate it does not have any
fertilty or iron. As a top layer it makes planting stem plants difficult because the particles are
not heavy enough to hold a buoyant cutting in place. Sand is very good for mixing with other
materials such as peat or vermiculite to increase their density and reduce their tendency to
float up when uprooting plants. It can be with a small amount of micronized iron or other
source of trace minerals. It could also be mixed with organic materials such as compost, peat
or earthworm castings to reduce their fertility and oxygen demand. Layers of sand or gravel
are not very effective for sealing nutrients into the substrate. Nutrients are still able to diffuse
through sand at a high enough rate to cause problems with algae if you have too much
decomposable material or fertilizers in the substrate.
Clay - is an extremely fine mixture possibly of several mineral compounds and which has
particles smaller than two microns in size. Pure clay contains no organic materials and clay
soils often have little organic material. It is typically composed of iron and aluminum silicates
and oxides but it can also contain many other mineral substances as well. Some clays or
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mineral subsoils may be unsuitable for substrates because they may tend to produce
unfavorable concentrations of certain minerals. These minerals such as manganese, boron,
iron, zinc and copper are needed in low, trace concentrations but can be toxic in greater
concentrations. The simple addition of humus, peat or similar organic compost will greatly
detoxify these minerals by forming organic complexes. Clay soils rich in iron are typically red
in color. If the proportion of iron in the clay is unknown, micronized iron or other fine iron oxide
powders may be used to enrich the iron content. Clays have very high CEC.
The exact amount of clay used in a substrate is not critical. It will probably be quite beneficial
used 1 part per ten of sand or other material but could also be used in higher ratios. Clay
should be used in the bottom layer of the substrate since it can be drawn up by plant roots
during transplanting especially if the substrate is fluffy by having peat or vermiculite. A top
layer of sand or fine gravel will greatly help reduce problems with turbidity (cloudiness).
Ideally clay should be well mixed with sand so that its fine surface area occupies a larger
volume. Wet clay is very difficult to mix but it can be cut into small chunks and soaked in
water for several days to form a loose mud which is easier to mix with sand. If dry powdered
clay is available this can be mixed dry with sand quite easily. If you only have wet clay which
is too thick to mix, cut it into small pieces and press these onto the bottom of the aquarium to
form a thin layer on the bottom.
Caution: if handling dry powdered clay, use a mask to avoid inhaling the fine dust as silicates
can be harmful to your lungs.
Caution: if used improperly clay can produce extremely cloudy water when you uproot plants.
This cloudiness can prevent plants from getting enough light for photosynthesis and if not
correctly promptly, the plants may be unable to produce sufficient oxygen to protect their roots
in an organic substrate. Cloudiness could kill the plants. Clay turbidity can be cured by a
complete water change or by the use of chemicals sold in aquarium stores to fix cloudiness. I
have also had good results using an ordinary filter with old, bacteria covered media. If you
only use the clay as the bottom layer or for preparing clay fertilizer balls, you should not have
problems with cloudiness.
Clay is very useful for preparing fertilizer balls for enriching the substrate after it has been
established. Clay balls can be used periodically such as about every six months or so. I take
about 10 granules of Osmocote or a similar fertilizer and mix them together into the center of
a one half inch ball of clay. These are dried until hard and then inserted about 2 inches under
the surface of the substrate about an inch from the stem of a large plant which needs feeding.
The clay prevents the nutrients from diffusing out too rapidly into the aquarium water. The fine
texture of the clay material also binds the dissolved nutrients from the fertilizer and helps to
store it for the plants. These clay fertilizer balls can be used in all types of substrates and will
greatly improve the growth of the aquatic plants especially in a gravel only substrate.
Laterite - A red soil found in tropical and subtropical regions that has been leached of soluble
minerals, aluminum hydroxides and silica. It is primarily composed of iron oxides and iron
hydroxides. Often the term laterite is used to refer to an expensive commercial aquarium soil
product that is mined in the tropics. Lateritic soils or latersoils are commonly fine clays (< 2
microns); however, latersoils may have any texture. Dupla laterite appears to be a mixture of
silt and sand textures with some fine clay components. This may avoid some problems with
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turbidity when the aquarium is first set up or facilitate the circulation of interstitial water. This
coarser texture will reduce the CEC of the Dupla laterite in comparison to fine clays and
reduce Fe availability due to reduced surface area.
It has been suggested that excessive concentrations of some minerals have been leached out
of laterite in comparison to other mineral clays. This may render it safer as the sole addition to
a non-organic substrate; however, the use of humic materials will also act to absorb or buffer
concentrations of soluble minerals.
It has been said of laterite that it possesses a high CEC or the ability to chelate nutrients. The
CEC of laterite is much less than for clay or humus. This can be misleading since the CEC of
lateritic soils depends strongly upon its texture and the presence of various silicates which
have higher CEC. While the CEC of commercial laterite is comparatively low, it is a
tremendous improvement over sand or fine gravel alone. Only organic compounds can act as
chelators. Laterite does not help preserve the solubility of minerals like Fe or Mn.
In my opinion, laterite has five important beneficial properties:
Laterite is rich in iron which is an important nutrient for plants. Upon initial submergence,
it will provide a considerable release of iron into the water; however, this does not
continue for long. In the anoxic areas at the bottom of the substrate, the ferric iron
compounds will be acted upon by bacteria and reduced to the more soluble ferrous
form. In this role it is similar in function to many common soils. Without benefit of some
organic material to promote bacterial activity, laterite is an inferior source of iron
compared to natural soil mixtures. The use of laterite in the optimum aquarium regime
as described by Horst Kipper and Kaspar Horst of Dupla does not include significant
organic material although it is noteworthy that Dupla laterite does contain 0.2% humic
matter in contrast to Aquarium Products laterite which had 0.0% humic matter
(according to an Internet posting by Shaji Bhaskar).
Laterite may help the control of phosphates when used in conjunction with proper
wattage substrate heating cables. Iron oxide has the ability to capture and fix
phosphorus in small quantities from the aquarium water. Laterite is not a panacea for
the control of algae but it may be useful in conjunction with a phosphate limiting strategy
such as frequent water changes.
Laterite will improve the CEC of pure sand or gravel substrates.
The iron will help stabilize sulfides.
The last property is the lack of carbonate salts and organic material which would
otherwise affect the pH chemistry of the water and complicate CO2 measurements.
Laterite makes automatic control of CO2 injection by pH much simpler while still
enjoying the benefits of iron within the substrate.
Often red, iron rich clays or soils are erroneously referred to as laterite; however, only those
soils which have undergone considerable weathering over thousands of years are true
laterites. There are sources of latersoils in many locations throughout the world which were
deposited thousands or millions of years ago during times when the global climate was
significantly warmer. Weathering and leaching of laterite is only important for benefit 5.
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Is the leaching of silicates from laterite to differentiate it from iron rich silicate clays important?
Possibly not. We lack further evidence and theories.
Caution: some fine laterite clays can produce cloudy water when you uproot plants.
A good functional substitute for laterite is micronized iron which has the similar chemical
composition to laterite.
Vermiculite - "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 vermiculite CEC is pH-dependent. 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." - JK
It should be noted that CEC is determined by ratio with mass. Expanded vermiculite has a
very low density and so it's contribution to the cation exchange capacity of a volume of
substrate will be much less. The fine particles of vermiculite tend to float around in the tank for
a while and settle upon the leaves of plants when the substrate is disturbed. It has attracted a
great deal of interest and speculation; however, its usefulness remains to be proven
conclusively.
Vermiculite is readily and cheaply available at almost all gardening supply outlets in North
America, Europe and Australia. It is also available in many other places but may be more
difficult to locate or identify. Before addition to an aquarium substrate, it should be soaked in
water for several days or weeks to allow the water to saturate between its layers so that it will
sink. It should be stirred and kneaded to help break apart the particles. When it has been
processed into a fine texture and sinks, it is ready to be mixed with other substrate materials.
Topsoil - By this I mean natural soils found in the garden, fields or forests. Topsoil or garden
soil sold in nurseries or retail gardening centers commonly has high proportions of compost or
manure added which make it unsuitable. Manure is too labile so soils containing manure
should usually be avoided. Natural soils tend to contain appropriate amounts of humus and
can often be a valuable source of many nutrients, especially trace nutrients. The porosity of
topsoil is determined by its primary constituent: sand, clay or organic material. Like subsoil, it
is primarily a mixture of varying proportions of sand, silt and clay but with a greater proportion
of organic materials. Garden soil, depending upon how much it has been enriched with
compost, probably contains from 1% to 10% organic materials and is an excellent source of
humus. It is moderately fertile and has a moderate oxygen demand upon initial submergence.
Garden soils which grow vegetables well probably have a good balance of essential mineral
nutrients and are a good choice for use with aquarium plants. Topsoil from forests may
contain much larger proportions of organic materials including leaf mulch. This may make a
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good substrate for certain plants like Cryptocorynes. Soils from conifer forests are generally
too acidic and contain allelopathic compounds to inhibit the growth of other plants. Sandy soil
has low CEC. Soil with good organic content and clay can have CEC of 25 cmol/Kg or
higher. Oxygen demand low to high depending upon organic content. Good pH buffering
characteristics (~6.5).
Subsoil - typically a mixture of varying proportions of sand, silt and clay. It is very low in
organic materials and macro-nutrients. It may be a potential source of iron and may be useful
as a diluting mixture in conjunction with other substrate materials. It may contain some
additional micro-nutrients; however, this depends upon the type of subsoil. See the University
of Minnesota Soil Science information on soil horizons. Subsoil has high CEC if it contains
clay. Sandy subsoils will have very low CEC. Subsoil can have greatly varying nutrient
mineral content. Darker subsoils will contain more iron, very light subsoil has been leached of
iron and other minerals.
Commercial additives - would include the various laterites or similar high cost materials. You
can get very good results using many of these; however, a review of commercial products is
not within the scope of this article. Manufacturers almost never list the ingredients of their
substrate additives, publish any experimental studies or offer any scientific theories to explain
how or why their products work. My opinion is that you can duplicate these results without the
expense of such products. Most commercial products avoid organic components or use it only
in trace amounts.
Dolomite lime - a natural crystalline mineral comprised primarily of calcium and magnesium
carbonate. It is sold in granular form in garden stores and is a cheap source of calcium and
magnesium but is very difficult to dissolve. It is probably not a good addition to a substrate
and should be used sparingly if at all. Since it dissolves very slowly, it would be very difficult
to control calcium content, hardness, pH and alkalinity. Calcium and magnesium are
absorbed best from the water through the leaves of the plants. Calcium carbonate powder, or
calcium chloride or magnesium sulfate/carbonate/chloride are all good alternatives which can
be added at water change time or in regular daily doses and are readily dissolvable in water.
Dolomite lime can be used to increase the pH of an overly acidic soil. Calcium and
magnesium ions will displace other cations from cation exchange sites thus temporarily
increasing available nutrients such as N (ammonium) and K.
Peat - a type of soil comprised of partially decomposed plant material. It is typically found in
swamps or bogs where the acidic conditions prevent complete decomposition. It is composed
primarily of cellulose and lignin and is rich in organic or humic acids. Peat or other humic
materials in soils are very important for establishment of the complex biological ecosystem of
a stable submerged soil. Humic materials and carbonates form an important part of the pH
buffering system. The amount of peat material desirable for any particular type of aquarium
plants varies. Peat is less labile than other organic materials due to its acidic nature and the
resistance of cellulose and lignin to decay. There are several types of peat whose properties
may vary considerably. Sphagnum moss is often sold as peat but it has not actually
undergone the acidic decomposition process and leaching of nutrients that true bog peats
have. My experiments with sphagnum peat moss indicate that it does not release large
amounts of nutrients however it will lower the redox potential of the substrate quite
significantly causing higher concentrations of dissolved iron in the substrate. Peat is acidic.
Different kinds of peat vary considerably in their acidity. The most common type of peat
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available is probably Canadian sphagnum peat moss found in gardening supply outlets.
Canadian sphagnum peat moss has pH ranging from 3.4 to 4.8. High to moderate labileness.
Moderate to high oxygen demand. Low redox potential. Peat has a high cation exchange
capacity (CEC) of 100 to 150 cmol/Kg. See also:
http://peatmoss.com/index.html, the Canadian Sphagnum Peat Moss Association Homepage.
http://karamelik.eastlib.ufl.edu/projects/forum/akm4763/part1.html, the Florida Geological
Survey, Bulletin No. 30 Part I NATURE, ORIGIN, KINDS, AND COMPOSITION OF PEATS IN
GENERAL
Caution: acidic peat mixed with aluminum commonly found in clay may have enough acidity
to create aluminum toxicity if used in large amounts. A small amount of dolomite lime (1 tsp /
square foot of tank area) mixed with the peat will prevent the pH from becoming too low. Note
that peat and other organic matter also prevent metal toxicity by adsorbing dissolved metal
ions.
Leaf mulch - is used by some to grow difficult Cryptocorynes. The leaves of various tree
species have differing amounts of mildly toxic compounds. Refer to: Jacobsen, Niels,
CULTIVATION OF SOME DIFFICULT CRYPTOCORYNE SPECIES IN HUMUS-RICH
BEECH LEAF-MOULD TAG 5:5, 133-137, Sep/Oct. 1992. (Rpt: AQUA PLANTA, 1-92.)
Different leaf types vary in acidity.
Compost (plant or manure) - will be almost entirely organic material unless the compost has
been mixed with mineral soil (which is a good way to complete composting). This can be very
fertile and probably quite labile. Excess amounts could great a large oxygen demand which
can be a problem for some fish types. It can also contribute high amounts of ammonia and
phosphates to the aquarium water which can cause problems with algae. The ammonia
released can produce rapid plant growth but high levels of ammonia are toxic to fish. Snails
and daphnia may be somewhat more resistant to ammonia than fish. Dried cow dung is
mentioned in some older articles on growing aquatic plants mixed sparingly with sand or soil.
Can be slightly acidic. High CEC. Very high levels of macro-nutrients. High to extreme
labileness. High oxygen demand.
Earthworm castings - are created by earthworms as they ingest and excrete organic compost.
Depending upon the medium in which the worms are growing, the castings contain varying
amounts of organic and mineral materials. Often, they can be +25% organic material and very
fertile. They are a good source of humus and can often be found in garden supply centers so
are widely available. Similar in properties to peat but may have much higher nutrient
concentrations than peat. This should always be used as a mixture with other mineral
substrate materials such as topsoil or a sand, clay, silt mixture. High CEC. Very high levels of
macro-nutrients. Highly labile. High oxygen demand. If used, it should be mixed with sand to
reduce the fertilizty of the substrate.
Caution: excess amounts of earthworm castings can liberate enough macro-nutrients to
create severe algae problems. Use in moderation. Suggested only for experienced aquarium
hobbyists.
Potting soil - tends to contain large amounts of sphagnum peat moss or other organic
materials and insufficient mineral soil. This should probably be mixed with a natural topsoil,
subsoil, sand or clay for use with a diversity of aquarium plants. Some plants such as Crypts
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or Echinodorus may thrive in this richer medium. High labileness. High fertility. Slightly acidic.
High CEC. High oxygen demand.
Micronized iron - a commercial iron additive which can be added to an aquarium soil to
increase the amount of available iron. It is commonly available in gardening centers for
around $10 per pound. This should be used sparingly and mixed well with sand or clay in the
bottom layer. If used with together with peat, the amount of peat should be minimized since
the humic acids from the peat and the high surface area of the micronized iron can allow
excessively high levels of iron to be released into the aquarium water. For this reason, I think
the micronized iron should be used in the bottom layer and the peat should be used in the
middle layer.
Fritted trace elements (FTE) - a commercial micro nutrient additive. It is commonly available
in gardening centers for under $10 per pound and should be used very sparingly. This can be
used to supply trace micro nutrients in the substrate. It should be used in the bottom layer at
about 1 teaspoon for four square feet of tank area.
Iron-rich pottery clay - does not contain the high iron levels of laterite; however, the iron may
be more accessible due to the fine texture of the clay. One clay which I've tried, Terrastone,
has an iron content of 4%. See precautions for clay. Current experimental evidence shows
that this is a good low cost alternative to laterite.
Caution: Very low pH substrates may cause aluminum which is common in many clays to
dissolve so excessive peat used with aluminum clays should be avoided. Dolomite lime may
be mixed with the peat to increase the pH level in order to prevent aluminum toxicity.
Charcoal or activated carbon - The use of this material has been recommended in some
aquarium plant handbooks especially for Aponogetons. Charcoal is sometimes used in
terrestrial soils to keep the soil "sweet", that is to absorb H2S. We have scarce experimental
information on this material.
Submerged Soil Properties
Texture - refers to the size of particles. Often a mixture. Clay is very fine in texture (< 2
microns), silt is moderate (50 microns to 2 microns) and sand is coarse (2mm to 0.05mm).
Fine gravel can be considered as anything about 2-4mm. A mixture of sand, silt and clay
particles is common. Too fine will not permit nutrient, water or oxygen diffusion and may
compact to the point of discouraging root penetration. Too coarse will not facilitate optimal
root hair contact and will not stimulate root growth or plant growth. Some substrate designs
require a coarse texture to facilitate circulation; however, this would not be good with nutrient
rich or organic substrates.
Density - strongly affected by the total amount of organic material, too compact and
permeability will be affected. Too much organic material will reduce the nutrient volumetric
content making nutrients less available to the roots.
Organic material - material derived from living organisms. Provides many types of nutrients
required by plants for growth which are released during the decomposition process by
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bacteria. Scientific studies indicate that natural sediments with a proportion of 5% by weight of
organic material is optimal for those plant types studied. Different plant types may favor
varying concentrations of organic materials. Cryptocorynes and Echinodorus may flourish in
higher concentrations; however, we can expect more problems with variability and excessive
nutrients contributing to unicellular green algae blooms. Not all types of organic materials are
desirable since they vary in labileness (see labileness). Organic material which is nearly
completely decomposed is generally preferable (low labileness). All organic materials will
release various types of organic or humic acids during the process of decomposition. These
acids have an on affect the accuracy of measurements of CO2 concentration based upon pH.
This effect increases with the amount of humic acid in solution and the aquarist needs to be
mindful of this effect. He may consider controlling the rate of CO2 injection by bubble rate if
he chooses to use organic materials.
Carbonate content - the amount of carbonate salts (CaCO3, MgCO3) which are available in
the substrate. These types of minerals can have a large effect upon water pH chemistry.
Changing levels of carbonate hardness will affect the accuracy of automatic CO2 controllers
which rely upon a constant carbonate hardness value to achieve the desired CO2 content.
Humic material content - the proportion of organic material which has reached nearly
complete decomposition. Humus is composed primarily of the components of plant cell walls
which give them their structural strength, lignin and cellulose. The class of humic substances
is extremely diverse and complex. It has extremely high CEC. The CEC of humus is pH
dependent and low at low pH. This is highly desirable since it permits plant root hairs to
secrete organic acids which liberate nutrient ions adsorbed by the humic material.
CEC - cation exchange capacity. The ability to capture and hold positive ion nutrients such as
Mg, Mn, Fe, Cu, B, Ca, K, Zn, Mo, Co, Na and Ni as well as nitrogen in the cation NH4+.
Units of measure are cmol/Kg (or meq/100 grams which are equivalent).
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.
http://home.infinet.net/teban/substrat.htm#Introduction
Description:The substrate serves two purposes in a planted aquarium. It is possible to grow plants in completely inert substrates such as plain gravel; however,
