Substrates for Aquarium Plants

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Introduction
Types of Substrates
Substrate Materials
Submerged Soil Properties
Organic Material in Substrates

The Transitional Period following Submergence
Organic Decomposition Products
Roots and Root Hairs

Fertile Substrates
Low Fertility Substrates
Gravel Washing & Cleaning
Soil Collection and Treatment

Precautions

Suggested Research
Acknowledgements

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.

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

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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 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 clayuse 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 bals, 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 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.

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 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 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 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.

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Submerged Soil Properties

There is an excellent link page for soil properties maintained on a NASA web site.

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 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.
Cation Exchange Capacity of Common Materials
Soil Component CEC
(cmol/Kg)

humus 200

peat 100-150

vermiculite 150

smectites 100

illite 30

chlorite 30

kaolinite 8

Fe, Al oxides(laterite) 4

Cation Exchange Capacity of Common Materials
Soil Component	CEC (cmol/Kg)
humus	200
peat	100-150
vermiculite	150
smectites	100
illite	30
chlorite	30
kaolinite	8
Fe, Al oxides(laterite)	4

Different cations are adsorbed more or less easily than others.
Cations Ordered by Adsorption Strength
Al > H > Ca > Mg > K > Na

Cations Ordered by Adsorption Strength
Al > H > Ca > Mg > K > Na

- source: Jim Kelly from information he gathered primarily from N. C. Brady's book, "The Nature and Properties of Soil"

pH - the measure of hydrogen ion concentration. The lower the pH, the higher the concentration of H+ ions in solution. pH greatly affects chemical reactions and biological processes. A low pH in the substrate will inhibit micro biological activity and interfere with root nutrient uptake. A high pH will favor the wrong types of bacteria and chemical reactions and may create toxic conditions for plants, fish and other aquatic organisms. Macro nutrient (N, Ca, Mg, P, K, S) are most available at a pH of 6-7. Micronutrients (Fe, Mn, Zn, Cu, Co) are more available at a low pH. pH below 6 may cause Al toxicity from clay (aluminum silicates).

Alkalinity - a measure of the amount of alkaline earth metals (Ca, Mg) and alkaline metals (K, P, etc.) in compounds. Generally associated with higher pH.

Mineral completeness - a measure of how many of the trace minerals are present and available in the soil.

N-P-K nutrients - a measure of the major nutrients which play a dominant role in fertility. These nutrients can be added by chemical fertilizers although this is probably unnecessary for the first several months of an organic substrate.

Porosity or permeability - a measure of how fast water or dissolved nutrients can diffuse through the substrate. A high permeability (permits rapid diffusion) is not really desirable in a fertile substrate as it will allow nutrients to escape more quickly into the water. Terrestrial soils need to be porous in order to permit oxygen to circulate in the soil. Insufficient oxygen will cause toxic conditions which can kill many types of terrestrial plants. This is not a serious problem for aquatic plants because they are entirely adapted to the anaerobic environment.

Labileness or lability - a measure of how unstable an organic material is in terms of its tendency to decay. Animal tissue, foods, feces or manure are extremely labile and will decay rapidly consuming large amounts of oxygen, releasing many decomposition products and supporting possibly toxic bacteria. Humic material is organic material with the lowest labileness. Note that fertility is highly correlated to labileness.

Oxygen demand - closely related to labileness. Too much oxygen demand may create an oxygen shortage in the aquarium water. Moderate oxygen demand will increase CO2 content of the aquarium water and the interstitial water of the substrate. Moderate to low oxygen demand produce favorable conditions for reduction of Fe and Mn to soluble states.

Redox potential - oxidation reduction potential (ORP). A gross measure of the amounts and proportions of various chemicals acting as electron donators or acceptors. In practical terms, this determines the concentrations of various chemical forms which result from chemical and biochemical reactions in the substrate. The presence of oxygen or high oxidation state chemicals such as nitrate increase the redox potential. The absence of oxygen etc. and the presence of low oxidation state chemicals such as sulfide ions or ammonium lower the redox potential. A low redox potential greatly increases the solubility of minerals like iron and manganese which can change oxidation state. It also favors the concentration of ammonia from nitrate reactions and this enhances the availability of nitrogen and phosphorus to plants. A very low redox potential produces toxic sulfides from sulfates.

Sulfur - one of the essential macro nutrients. Excessive amounts of sulfur compounds in the substrate can lead to production of toxic H2S. This could arise from indiscriminate use of sulfate fertilizers and excessive sulfur rich organic materials such as manure.

Iron - The availability of iron from the soil depends upon how much iron is in the soil and how much oxygen the roots of the plants are putting into the substrate which will greatly reduce Fe availability as the substrate becomes root bound. It is likely that you will need to resort to additions of chelated Fe water in a mature system. Some plants notably Cryptocorynes, may have methods of extracting Fe from the substrate even under high oxygen conditions according to observations by Paul Krombholz. Iron is abundant in most soils and available from those with a fine texture such as those containing clay. Lateritic soils may have as much as 300mg/g (30% by weight) of iron. Median or typical values for all soils and sediments are reported to be 40 mg/g. (from Diana Walstad's article "Iron The Limiting Nutrient for Algae?" in TAG 6:4) Coarse textured materials like sand and gravel will not be able to provide significant Fe. Granular laterites may provide Fe initially but their texture is not fine enough to provide Fe in the longer term.

The iron has to be dissolved in order to be available to most plants. It won't dissolve unless its chemically reduced to Fe++ (ferrous ion). That requires the absence of oxygen caused by anaerobic bacteria acting upon organic material (i.e. peat). Specialized anaerobic bacteria are also responsible for the reduction of the iron. The humic acids of peat also prevent the Fe++ from being oxidized and precipitating (going out of solution) by attaching to the dissolved iron ion (a process called chelation).

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Organic Material in Substrates

Humic acids will affect the pH of the substrate and water. CO2 and hardness measurements will be affected by humic acids since they are usually acid or base titrations. Humic acids are fine in small concentrations and we need to take care to control their concentration by regular water exchanges and possibly by the use of activated carbon filtration.

The Transitional Period following Submergence

Another point to remember about organic and soil substrates in general is that they will go through a transition period that lasts several months upon initial submergence. During this time, depending upon the amounts and type of organic material, there will be a lot of bacterial activity and the release of certain compounds which not all plants are prepared to cope with equally. Aquatic plants are adapted to doing this by conducting oxygen to their root system via air channels called aerenchyma. This oxygen diffuses from the roots and detoxifies many harmful compounds such as ammonia, H2S or excess Fe.

I've found that Crypts, Alternanthera spp, and Lobelia (with thick roots and leaves) were the fastest early colonizers in a transitional high organic soil, then followed closely by Hygrophila spp and Bacopa. This may have been more related to the setback by bleaching and the thick stemmed and thick leafed plants had a big advantage there. During the transitional period I think the plants were also sensitive to having too much top leaf material harvested since this affects their ability to generate oxygen needed to protect the roots. It is probably also best to use healthy plants from another algae free aquarium for the initial planting. You really don't want to introduce any types of filamentous algae and established clean, fast growing plants are good for consuming the early, high amounts of nutrients which may be released.

Another problem of the transition period is the possibility of the release of ammonia as a result of bacterial reduction processes on nitrates in the substrate. This ammonia can be a tremendous nutrient boost to the plants so that you may observe other nutrient deficiency symptoms caused by the rapid growth. The ammonia can also be a problem for fish if you have them in the tank. It would be a good plan to use daphnia and snails to break in a tank especially if you can guarantee that it will be free of filament algae (refer to the Krombholz bleach method). Normally I use little or no biological filtration (nitrifying bacteria) so as to conserve ammonia for the benefit of the plants. During the transition period, you can use biological filtration to prevent toxic ammonia levels.

The transition period may also be associated with very high levels of nitrate and phosphate dissolved in the aquarium water. Chelated Fe should be added cautiously lest algae proliferate. Blue green cyanobacteria will often develop. Snails and shrimps can be useful consumers of this pest. Green unicellular algae, associated with green water, may bloom. Daphnia are an excellent remedy for this condition; however, all types of fish, even algae eaters, eagerly consume daphnia. Persistence, water changes and a good quality micron or diatom filter will help solve green water.

It should also be noted that primarily mineral substrates such as gravel, sand and laterite, will not have the same type of transitional period. These types of substrates have a different transitional period where fish feces, plant detritus and uneaten food get worked into the gravel to form the important organic components of a productive substrate.

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Organic Decomposition Products

"Organic carbon may be metabolized to produce ethanol, lactic acid, methane or complex, refractive, polycyclic organic compounds. Ferric iron is reduced to Ferrous iron with the consequent release of phosphate from ferric hydroxide precipitates. Manganese oxide is reduced to manganous oxide. Sulfates are reduced to sulfides. Ferrous iron and sulfide combine and produce an insoluble ferrous sulfide precipitate. Nitrates are reduced to ammonia. One note ... there is a definite order to these events based on the redox potential of the substrate ...therefore, not all of these processes will occur unless the redox potential is low enough." - DH

Ammonia - The production of ammonia (the preferred nitrogen source) by reduction reactions in a fertile substrate occurs readily. The ammonium ion can be captured by cation exchange sites in clays and humic material and will greatly enhance growth of the aquatic plants. Much higher levels of ammonia can be tolerated in planted aquariums without fish. Ammonia should be monitored during the transition period following submergence if the substrate is fertile or nitrogen rich. Note that ammonia will be produced in large quantities from nitrate fertilizers in the substrate.

CO2 - There is compelling evidence to suggest that aquatic plants can make use of carbon dioxide generated within the substrate. Where there are no fish in the aquarium, a labile substrate can provide plenty of CO2 to satisfy plant requirements at the expense of free oxygen in the water. Plants may absorb the CO2 through leaves or roots. We don't know if all aquatic plants can absorb through roots.

Ethylene - a mildly toxic gas by-product of decomposition. Generally not produced in sufficient concentration to be of concern.

Hydrogen sulfide (H2S) - can be produced if there is sufficient organic material without oxidants (uncommon) to prevent its formation. It is generally not a problem in a well planted aquarium especially where iron containing soils are used. See also Sulfates / Sulfides.

Humic acids - This is a catch-all term to refer to a diverse group of water soluble, acidic, toxic complex organic compounds. There are probably phenolic groups in most of these compounds. They are so complex and diverse that they are just to difficult to classify separately. These are natural products from the decay of humus and organic material in general. In concentrated form they would be highly toxic but at natural concentrations, they are primarily beneficial. They tend to inhibit bacterial and algae growth and act as low grade natural chelating agents for iron. These are the substances found in black water extracts useful for Amazon rain-forest species. They are also probably highly important in pH buffering systems and help to provide pH stability.

Methane - This is probably one of the primary gaseous products generated in substrates. It is not toxic at normal concentrations.

N2 - A certain amount of nitrogen gas will be generated by the action of denitrifying bacteria in the substrate. This represents loss of useful fixed nitrogen; however, this may not be significant in the overall nitrogen budget. This decomposition product is not toxic. Further experimental investigation would give us greater insight into the conditions for denitrification or the degree to which it occurs in aquariums.

Nitrate - Elevated levels of nitrate will probably occur with fertile substrates but this is generally not a problem. The use of nitrate additions (i.e. PMDD) should be avoided until it has been determined that the nitrate levels have subsided.

Phosphate - is released readily during the decomposition of organic materials. Fish foods and feces provide a surplus of P. Phosphate is extremely soluble and natural soils which have been subjected to rains will usually have very low P values. During the dying off phase of plant leaves, the phosphorus will be withdrawn by the plant or liberated to the aquarium water very rapidly. The total volume of phosphate in an aquarium's water may be reabsorbed and released during a two hour time span.

Potassium - is highly soluble and may not be present in large amounts in decomposed organic materials. It is probably wise to supplement potassium in the aquarium water on a regular basis. Potassium is released from organic material early in the decomposition process.

Sulfate / Sulfide - Sulfide ions may be produced in a low redox environment; however, these are rapidly oxidized to non-toxic forms. Iron will also react with sulfide compounds to form non-toxic FeS. Sulfides are oxidized to sulfates by free oxygen which exists in the top half inch or so of the substrate and are never present in oxygenated water. The concern over hydrogen sulfide formation in organic substrates seems to be greatly exaggerated. Aquatic plants have adapted to protect their roots via the oxygen conducting channels, the aerenchyma tissues. An established aquarium substrate may be so filled with roots that no regions have sufficiently low redox potential to cause sulfide formation. An aquarium with insufficient light or conditions necessary for photosynthesis may suffer oxygen deficit and hydrogen sulfide formation may result.

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Roots and Root Hairs

"Most rooted aquatic plants have been found to possess root hairs, and several different species have been found to develop mycorrhizal associations much the same as terrestrial plants do. Rooted aquatic plants are well adapted to growing in an anaerobic substrate. They are able to 'pump' enough oxygen to the roots so that in many cases the oxygen actually diffuses into the surrounding sediment. They can also respire anaerobically if necessary and produce lactic acid or ethanol instead of CO2 as a byproduct. The root meristems (growing tips) of some species are even inhibited in the presence of oxygen." - DH

Fertile Substrates

"Rooted aquatic plants grow best on a mineral soil such as a silt loam with low organic matter. Rooted aquatic plants require no N, P, S, or micronutrients in the water column when grown on a fertile substrate. Rooted aquatic plants grow best when the substrate is anaerobic ... in fact some roots will not produce root hairs UNLESS the substrate is anaerobic. In most cases, it appears that inorganic carbon limits growth of submerged aquatic plants... not because it is too low in concentration or because the uptake mechanisms are inefficient in aquatic plants (in fact both factors are comparable to terrestrial plants), but because the diffusivity of CO2 in water is about 10,000 times slower in water than in air.

"Aquatic plants undergo cyclical growth, even under constant conditions. This means that even under optimal conditions your plants will slow down and maybe even die back every once in a while.

"Rooted aquatic plants, unlike their terrestrial counterparts, can absorb mineral nutrients both from the water through their leaves and from the sediment through their roots. Unfortunately, it is often assumed that rooted aquatic plants can satisfy all of their mineral nutrient requirements from the water through leaf absorption. This is, however, incorrect. As early as 1905 a researcher by the name of Raymond H. Pond stated that, " ... a soil substratum is requisite for normal growth." and that, " [rooted aquatic plants] make a better growth on a good loam soil, just as many land plants do." Since then, the dramatic and consistently superior growth of plants rooted in soil compared to plants rooted in sand has been shown repeatedly for many different aquatic plant species from many different types of habitat.

"While the reasons for this superior growth are not completely understood, certain facts are clear. First, submerged soils are generally lacking in oxygen. This is of benefit to rooted aquatic plants since under anoxic conditions Fe, P and N are more readily available than under aerobic conditions. Second, nutrient concentrations are higher in a fertile soil than in the overlying water. Third, there is no competition with phytoplankton for available nutrients." - DH

We call these "fertile" substrates; however, all of our efforts amount to one thing and one thing only: conspiring to provide a safe, low level of nutrients for plants at the root zone. A fertile substrate simply means that we attempt to provide a greater proportion of nutrients to the roots.

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Low Fertility Substrates

The problems of turbidity, transition and substrate preparation may simply be too complicated for many people. Under conditions of low or moderate lighting intensity, the aquarist may have success using the natural nutrients which accumulate from keeping fish. He will derive great benefits from the addition of chelated Fe and CO2 fertilization. As the intensity of lighting is increased and with the addition of CO2, nutrient deficiency symptoms will occur without the use of fertilizers. The Dupla method as described in The Optimum Aquarium book or as refined by George Booth is highly successful and easily repeatable. The PMDD fertilization method as described on the internet is also highly successful. This method also considers the problem of nitrogen fertilization which most other methods do not. There are countless other commercial or do-it-yourself methods for providing nutrients to the aquarium plants via the water. It should be repeated that all of these methods can be used with low fertility substrates but this article is not about fertilization of the water.

It is worth expanding upon the Dupla substrate strategy to understand its benefits over a fine gravel substrate or a laterite enriched substrate without heating. The laterite provides iron which is an important nutrient. The CEC of the laterite and the other organic material which accumulates in a substrate helps to adsorb nutrient cations where they may be absorbed by plant roots. During the period of transition when there is little organic material from plants, rooting tablets which are added to the substrate during its construction probably helps to supply several nutrients to the substrate. The application of heat to the substrate enhances biological activity which has a two fold effect. This encourages decomposition which liberates nutrients from organic detritus. It creates a low redox condition which greatly enhances Fe availability in the substrate. Heat or heating coils also encourages circulation of water through the substrate by diffusion and convection. This promotes the recharging of cation exchange sites with dissolved nutrients.

We think of this type of substrate as sterile or devoid of nutrients but the opposite is in fact the case. Every single substrate device is about one thing and one thing only: conspiring to provide nutrients for the plants at the root zone. The low fertility substrate simply means that we do not count upon providing a large proportion of nutrients in the substrate; we have a second strategy which ensures that these nutrients are available in the water where they may be absorbed directly by the leaves and indirectly by the roots. There is room for a spectrum of opinions about which additives or substrate devices are practical.

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Gravel Washing & Cleaning

The practice of gravel washing is necessary in aquariums where there are no plants and where the accumulation of organic materials would eventually give rise to enough harmful byproducts to poison the aquarium fish. In a healthy, planted aquarium, this practice is often unnecessary since these materials can be allowed to decay naturally producing nutrients for the plants. If you are experiencing excessive algae or if it looks ugly, you can lightly gravel wash the accumulation of mulm on the surface of the substrate. Decaying waste material can contribute to excess nutrient levels which could be encouraging the growth of algae. It is often difficult to move the gravel washing tube amoung heavily planted areas. Disturbing the gravel can also uproot your plants or disturb their root systems.

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Substrate Equipment

1. Under gravel filters (UGF) - Perhaps the best that can be said about under gravel filters is that they do not kill aquatic plants. While you can still fairly successfully grow plants with one in place (active or not), it should be clear that they serve no useful purpose in a planted aquarium. Truthfully, an UGF will perform mechanical filtration requiring little maintenance; however, it should not be used with soil, clay or laterite additives. Under gravel filters interfere with the biochemical reduction of iron and manganese in the substrate which makes these micro nutrients available.

2. Reverse under gravel filters - Same comments as for under gravel filters.

3. Slow reverse under gravel filters - The idea is to provide a very slow circulation of oxygenated water through the substrate to prevent anaerobic decomposition. This is unnecessary and largely counter-productive since it will interfere with the beneficial reduction processes which provide important nutrients and will cause diffusion of nutrients out of the substrate when we desire a higher concentration there.

4. Substrate heating - This encourages biochemical decomposition of organic material and thus liberating nutrients. For low fertility substrates this may produce a beneficial increase in growth. It is probably unnecessary or not beneficial for fertile substrates. Higher temperatures will increase nutrient mixing by diffusion (although this may not necessarily be desirable).

5. Substrate heating coils - The intended purpose is to draw nutrients from the aquarium water into the substrate via convection currents. This would not be desirable in fertile substrates since the concentration of nutrients is higher in the substrate. In fine substrates (sand, soil or clay mixtures) the viscosity effects of the particles will make convection effects negligible. We currently do not have experimental evidence to support the convection theory; however, I am skeptical since the extremely small pressure differences caused by the small changes in density over the small path length will be negligible in comparison to viscosity effects even with clean gravel substrates. We may be able to explain the experimental differences of substrates with heating coils to higher diffusions rates and decomposition rates. It would be enlightening to perform controlled experiments designed to compare heating coils versus uniform heating of the substrate or to measure convection transfer rates.

Another purpose for substrate heating may simply be to increase the activity of bacteria in the substrate to break down organic debris into mineral forms which can be used by plants (mineralization). This will improve the availability of iron from sandy textured laterites as well as increasing phosphorus and nitrogen availability as well as many other micro-nutrients. In my opinion, this is the principal advantage of substrate heating. Note that heating substrates which contain greater amounts of organic materials may be unnecessary since it will cause decomposition to occur more rapidly than desirable.

George Booth supplied the following information on substrate heating coils:

"There is no known information supported by scientific experiment that shows whether or not substrate heating coils are effective. Various manufacturers such as Dupla, Dennerle and Sandpoint have offered commercial systems but none seem to agree on the specific details of the system. The avid Do-It-Yourselfer has little background to draw upon when deciding how to construct their own system.

"1. Substrate heating coils will NOT improve the rate at which plants grow. If you are looking for the Silver Bullet to improve plant growth, don't look to substrate heating coils. You might want to investigate CO2 injection instead.

"2. Substrate heating coils WILL provide long term stability in your planted tank. If you tend tear down your tanks every year or so for whatever reason, don't bother with substrate heating coils. If you set up a tank for the long haul (longer than one year), substrate heating coils can greatly improve your chances of success.

"3. Substrate heating coils are only effective if a rather high heat density is achieved. This is the area where commercial systems differ the most. In my experience, the heat density achieved by the Dupla system is the most effective. The Dupla system provides around 25 watts per square foot of substrate area with the coils spaced 1 3/4" apart.

"The reason that substrate heating coils provide long term stability is generally thought to be due to water circulation in the substrate produced by the coils. If the coils produce enough localized heating, convection currents will be set up and nutrient laden water will be moved into the substrate. Once the nutrients are in the substrate, they are attached to binding sites provided by clays (like laterite) or other substrate additives. Thus, the nutrients in the substrate are constantly being refreshed and the substrate does not become exhausted by plant uptake.

"My experience has convinced me that heat density is the key to success. Thermodynamics theory shows that with "low" heat density, heat is carried through the heated medium by conduction and no flow is present. At a certain threshold dependent on the medium, heat will produce convection currents through the medium. Due to the complexity of a typical substrate, the correct coil temperatures are impossible to calculate and extremely difficult to determine experimentally." - GB

6. Oxygen plenums - The intended purpose is to prevent formation of toxic H2S in the substrate. A plenum is simply a chamber located under the substrate such as the cavity below and under gravel filter plate. An oxygen plenum would permit oxygenated water to flow under the substrate thus increasing the redox potential of the substrate. This is not necessary since a proper substrate will not have a problem with H2S. It will also interfere with reduction processes which provide iron and manganese to plant roots.

7. Nutrient plenums - The idea is to provide a chamber where a higher concentration of nutrients can be maintained chemically. The benefit of this method versus simply introducing fertilizer capsules or enriched clay balls is not clear. Any slight difference in hydraulic pressure caused by the addition of water to the nutrient chamber would result in nutrient transfer through the substrate into the aquarium water.

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Soil Collection and Treatment

Natural soils differ greatly from those soils usually found in gardening retail outlets. Natural top soils usually have a relatively low ratio of organic to mineral components. The humus of a natural soil tends to be much more completely decomposed. Depending upon the geographic location, the local soils may contain either large percentages of dense clays or sands. Optimally, a soil should be a mixture of sand, silt and clay. Depending upon the desired fertility of the substrate, the soil could then be enriched by the addition of peat, compost, potting soil or earthworm castings. Soils which are low in iron content may be enriched by the addition of small amounts of micronized iron. Heavy clay soils should be mixed with vermiculite or sand to improve their porosity.

Natural soil should be collected from a well drained location so that the soil will not be alkaline or contain naturally occurring metals such as lead, tin, or mercury. Soils from arid regions should thus be avoided. The user may choose to screen for rocks and twigs and incompletely decomposed organic fibers. The soil could also be submerged in water for a period of time to remove excess soluble nutrients although this may not help if the organic components are not well composted.

It is a wise precaution to mix any organic soil with sand if it is too rich in nutrients. To test for soil fertility, mix a cup of the soil in a bucket of water and allow it to settle. Measure the nitrates and phosphates in the water after a day or two in order to determine how rich the soil is. I hope to have some experiments performed soon to establish comparisons.

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Precautions

Avoid soil from around lakes and streams as it will probably be contaminated by various undesirable filament algae. Sand which is collected from the seashore may contain marine algae and calcium carbonate shell fragments. Soils in urban locations could be contaminated by automobile oil, hydrocarbon wastes or other toxic chemicals. Soil which is used for growing vegetables for human consumption should be free of contaminants. Soil adjacent to buildings may contain lead leached from house paint or calcium carbonates leached from cement work. Soil from coniferous forests may be excessively acidic; if small plants and grass are not able to grow beneath the trees, they may be releasing allelopathic chemicals into the soil to inhibit competition from other plants. Soil from low lying areas adjacent to agricultural areas may contain high levels of chemical fertilizers from leaching. Agricultural soils may contain recent additions of chemical fertilizers, herbicides or pesticides. Water logged soil from bogs or lakes may contain nematodes or insects which could potentially be unwelcome to the aquarium. Bog soil may contain H2S, mercury, lead, various mineral salts or alkaline compounds.

The advantages of clay (CEC and Fe) may not justify its disadvantages in some applications. As a barrier for diffusion of nutrients, it is far superior to sand; however, it can cause serious problems with cloudiness especially after uprooting plants. Like vermiculite, it may settle upon plant leaves creating an unaesthetic appearance and providing a place for the growth of algae. As alternatives, you could use fine sand and micronized iron. If you find yourself with cloudy water, filtration with a well used media such as floss, can often be effective. The bacteria and sludge seem to adsorb the suspended clay particles.

Relatively fertile organic substrates can also be troublesome. These invariably increase the amounts of nutrients dissolved in the aquarium water which may lead to transient algae problems. Generally the algae can be dealt with; however, the user needs to be very familiar with the approaches to algae control. Under situations of strong lighting, it may be necessary to be very cautious with chelated Fe additions. On the other hand, the dramatic improvement in growth of many plant types especially Cryptocorynes may be well worth the effort. You should try to strike a balance between the amount of organic material or fertilizer you have and your intended application. Rapid growth may be gratifying initially but the extra trimming can become tiresome. Uncontrolled growth of stem plants seldom produces aesthetic results.

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Suggested Research

The most promising research is probably in the investigation of the effectiveness of various mixtures of soil additives such as sand, clays, vermiculite, organic materials, types of compost and mineral supplements. It is important that some experiments should investigate long term effects. We should investigate:

problems with transition after initial submergence
suitability of substrate types for various plant types
factors contributing to H2S toxicity
factors contributing to micro-nutrient toxicity
loss of substrate fertility
other factors causing long term degradation (toxicity)
strategies for enriching substrate fertility

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Acknowledgements

I would like to acknowledge the assistance of the following individuals who have assisted in the preparation of this article or whose contributions I have drawn upon for facts: Shaji Bhaskar, George Booth, Neil Frank, Dave Huebert, Jim Kelly, Paul Krombholz, James Purchase, Karen Randall, and Diana Walstad. Where I have used another person's words I have shown them in quotations with the original author's initials appended.

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