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There are major "trunk" splits in the philosophy of aquarium gardening; "should you use carbon dioxide infusion?" "What, if any chemical supplementation, fertilizers?" "Extra lighting?" For me the biggest yes/no question is "to use of not to utilize soil in, or under the systems substrate".
Certainly, the most magnificent, fastest-growing live aquarium plant set-ups do involve all of these photosynthesis "boosting" mechanisms. And even the simplest, non-filtered, even unlighted system can benefit greatly from adding appropriate soil to the substrate.
For our purposes here we will label an aquariums gravel as the substrate, denoting loam, clay, mud, laterite, peat et al. added separately as soil.
"What, throw mud in my tanks; are you crazy?" Well, I've stated it before, cleanliness is not sterility. Take a look out in the "real world", that over-spun term, Nature. Where do you find 100% pure gravel bottomed waters? Nowhere I've ever heard of. Though many true aquatic plants absorb nutrients by above-root structures, they too derive chemical foods originating in organic and mineral soil sources; all this by way of the universal solvent, water.
Where will your plants get their major and minor (aka trace) mineral and non-mineral nutrition? With a "standard" low-intensity set-up, defined as adequately lit and without CO2 infusion, "simple fish fertilization" might well do with regular water changes re-supplying buffering capacity to provide all essential nutrients. For spectacular results, the other mentioned intensifiers (more light, CO2) with soil and/or direct chemical fertilization really do make a difference. In my opinion, the easiest, lowest cost, most disaster-proof improvement is the addition of soil.
Ahem. So what will this magical improvement get you and your plantings and how? The functions of nutrients? Catalysts, electron transport, structural components, osmosis, cation exchange, augmented nitrification... where to start? We'll leave elaboration to elsewhere. It makes sense that given the necessary conditions of photosynthetic life (think of all the inputs you're aware of, heat, light, minerals, organics...) that some one would be "rate limiting"; that is something would be the controlling, stopping influence of metabolism. In the world of playing an aquatic "Sherlock Holmes" with fellow aquarists, I've found the most common rate limiting effect to be nutrient availability; mainly a lack of useful chemical foods. How to supply these without triggering an algal bloom, fish loss, erratic pH shift...? In a word, soil, added to the substrate.
Materials & Sources:
Soils are made up of three types of particles, sand, clay and silt. Rather than chemical types these fragments of rock are classified by their relative sizes:
Particle Classification Diameter (millimicrons)
Coarse sand 200-2000
Fine sand 20-200
Clay Less than 2
These different soil sizes are generally found mixed together and are described on the basis of the percentage of silt, clay and sand present per the following diagram:
Why's this important? Most of the aquatic (and terrestrial) plants we grow do best in loamy soils. Size does matter; no joking; clay-diameter material acts as a colloid, attracting and slowly releasing water and nutrients. You need only use a small amount of clay to gain the absorptive property of its vast surface area. What about the gravel/sand we call substrate? For our purposes here we will consider the inorganic mineral substrates as not contributing nutriment to aquarium plants.
While we're at defining terms that you're bound to run across, here's one for laterite (from the Latin, later, for brick or tile); this is a reddish iron-containing soil from the tropics; (mainly made up of iron and aluminum oxides and clays) a most suitable amendment type. Which brings us conveniently to:
1) Organic animal manures; I want to make special mention of these to encourage you to avoid them. Some of this matter decays too readily under bacterial action, depriving the sub-stratum of oxygen, and in the processes of reduction creating toxic water conditions. If you must addend with animal manures, do so conservatively, in blind pots, utilizing sufficient fine gravel or sand to contain them.
2) Peat is defined as a highly organic soil, composed of more than half partially decayed vegetable material. Most peats are collected from marshy regions, dried, and used for fuel and terrestrial garden amending purposes. The acidification that accompanies decomposed peat use diminishes bacterial activity, the anaerobiosis lowering substratum pH.
3) Mineral particles By size, clays, sands, silts of inorganic make-up may be useful from several standpoints. Useful forms that have been employed include carbonaceous sands and gravels, broken flower pots, "oil-absorber" clays,
So now you know the theoretical size and properties of the best mix of aquarium plant soil to utilize; how or where do you get the real thing? Well, you can either buy it, make it, or dig it up per the following:
A) Commercial Aquatic Plant Soil Products: Are available through aquarium supply outlets. In the west we have two stalwart companies, Tetra and Dupla who offer complete wet horticulture lines; around the world there are also other excellent manufacturers. All earnest would-be underwater green thumbs should read these companies' products circulars.
B) Terrestrial Plant Potting Mixes: Can you imagine writing these statements and opinions? Just what is a "standard" potting mix? It's obvious, there is no such thing. A typical mix involves "dark natural soil" with possibly vermiculite, peat moss, and more added to it.
I myself avoid, and encourage you to shun vermiculite, perlite, animal manure, blood meal, urea, other plant food additives; but not peat, in particular, the more decayed peat called humus. Aquatic gardeners are either "pro or agin" using decomposed vegetable material in their soil mixes; I am strongly for it for the stated functions: peat acidifies, prevents packing and channeling, and aids in stabilizing other biochemical reactions (aiding microbial production of mineral salts from fish fertilizer). I include up to 50% peat in my soil; most of the times utilizing a "african violet" type house plant mix for the rest of the remainder, and a little laterite. The clay in the last two are key as inorganic catalysts, facilitating nutrient transfer through the "holding" of charged particles.
C) Garden Diggings: and other novel material can be had by rooting around in the great outdoors, backyard or any freshwater body. Is this such a good idea? What about pesticides, unwanted "critters", chemical fertilizers, herbicides? And perhaps worst; what if you discover "the greatest mix in the cosmos" and are unable to replicate your results down the road? Not to totally dissuade you, but like a good scientist, I'd set up more than one tank to try my results out (if manipulating more than one variable especially).
The decided best and most popular approach to getting the soil to stay where you want it (under the substrate) is to soak it until waterlogged (you may have to gently boil peat), toss the still-floating components, mix the mud with some of the sand/gravel and in turn bury that with a few inches more gravel/sand. Variations on this theme include the use of an anaerobic plenum, a dead space made by foam, screen, undergravel plate... Maybe you're using heater cables; don't forget to put them in first!
How much soil? I use about a four ounces dry weight per square foot of bottom.
And a note re sloping gravel. If yours is intended to be deeper in the back, do terrace it with drift- or petrified wood, glass, what have you; and don't try to slant your soil mix; i.e. leave it alone as the bottom same-depth layer.
Carefully filling the tank with finesse takes naught but a dinner plate or tray to prevent stirring the substrate and soil.
The next day or two, a large, make that almost total water change may be called for to clear the water to your specifications.
Some aquariculturists hold off planting for a couple of weeks, some partially plant and wait out bacterial and algal blooms. I go whole-hog and plant the system pretty much entirely once it has cleared, generally within a few days. Other non-plant life is added weeks later.
Modes of Action:
As the water and soil interact in your system, many changes take place. The lowered oxygen and pH (maybe not-using undergravel filters makes more sense to you now?) promotes solubility of iron and manganese (these chemical species may otherwise exist in less soluble valent forms). Where will your plants roots get their needed oxygen if the soil lacks it? Through transport from their stems and leaves above.
This is not the whole story either, as biological microbes further act on chemical nutrients to render them more and less useable to vascular plants. This interplay between solubility and insolubility, aerobic and anaerobic processes defines the allowed life of a system. The chemical, physical and biological conditions presented by the use of soil in aquarium substrates.
In a closed system as a small aquarium, ultimately something necessary to plant growth runs out, becomes zero-rate limiting. How to avoid this nutrient deficiency dead end? In small, uncrowded, non-boosted systems, fish feeding and water changes might well suffice. More crowded, and in particular light and carbon dioxide boosted systems tend to deplete soluble nutrients in short order. Without replacing or adding to the soil-nutrient base, spindly, yellowy growth followed by possible fouling occurs.
Offsetting this trend can be done by direct chemical supplementation (potassium, phosphorus, sulfur, calcium, magnesium, iron, manganese, boron, zinc, copper, molybdenum) on a regular basis (usually in concert with water changes) and/or adding to, or changing the soil in the substrate. The former involves a complete "tear down" of some (as in use of trays) or all of the system; a total clean out, with the water, gravel, livestock, everything being removed to "clean" the tank. Adding to the nutrient depleted soil is much easier. This is accomplished with store bought or home-made "plugs" or "sheets" of prepared material that stay whole long enough to dunk in and under the gravel/sand substrate.
How often to fertilize? To dump and change, or add to the systems soil? Good question. Definitely as your plants leaves yellow or slow their growth too much. Soil in substrates and pots may last for six months to a couple of years; your plants may become root-bound before this time.
Alternatives to Soil Augmentation:
I always find it illustrative to point out the opportunity cost of any situation, that is, the ultimate question of what else you might do, have done, including nothing. If you had a hankering to husband aquatic plants, and didn't want to introduce soil to the substrate, what else could you, should you do instead? Maybe nada, zippo, zilch. Considering that the system is set up, conditioned and maintained properly, regular "fish fertilizing" (wastes from feeding your fishes) will most likely provide all sufficient major and minor nutrients.
Want more growth, flowering, the plants pushing your tanks top off? Adding chemical fertilizers can get you there, as can more light and suffusing excess carbon dioxide into the water. Are these other adjuncts within your budget? Maybe not; but a decent dose of soil additive is. It may well be out just outside in your backyard or shed.
As another possibility, there are commercial plugs, pots and planting strips of planting media; many of these have proven to work well.
Much has been investigated and written on the use of various organic and/or inoganic additives to aquarium substrates for promoting aquatic plant growth. Westerners would do well to try out this simple, natural way of adding to the overall health and maintenance of captive freshwater systems.
Plants do unsurprisingly poorly just rooted in washed aquarium gravel. The chemical and biological conditions established and sustained by soil use provide natural nutrient availability.
Pushak, Steve. Precautions for the use of soil substrates. http://home.infinet.net/soil.precautions.html
Andreason, Dick. 1991. More ideas on substrates. The Aquatic Gardener 4(2):3,4/91.
Anon. 1978. The aquarium plant plug. FAMA product test. FAMA 1/78.
Hahm, Robert W. 1996. Topsoil in the substrate. Compuserve's Fishnet (tm) forum Aquatic Plants file created 14-Jul-1996.
Kelly, Jim. 1996. Notes on key soil characteristics for aquarists. The Aquatic Gardener 9(4):7,8/96.
Krumbholz, Paul. 1994. Mineral nutrition of aquatic plants, part 2; Underwater soils and nutrient availability. The Aquatic Gardener 7(4):7,8/94.
Lango, William. 1979. An aquarists technique for cultivating an underwater garden. FAMA 1/79.
Lowe, Charles. 1981. Aquarium plants, one hobbyist's views. Aquariums Australia 2(1):81.
Marquis, Thomas E. 1991. Here's mud in your tank, parts 1 & 2.
The Aquatic Gardener 4(6), 5(1):11/12/91 & 1,2/92.
Martin, Larry W. 1975. Nutrient requirements of aquatic plant. Bits and Pisces 2/75.
Raven, Peter H., Evert, Ray F. & Helena Curtis. 1976. Biology of Plants, 2nd ed.. Worth Publishers, NY. 685 pp.