by Robert Paul Hudson

Soil in the aquarium has become more popular again in recent years as a low tech approach that often includes the use of minimal lighting and no added C02. Diana Walstad wrote a book called Ecology of the Planted Aquarium around this method.

Soil is used to provide either macro nutrients or trace minerals, or both.  Nitrogen is the chief macro nutrient and is provided in the form of nitrate or ammonia.  Nitrogen is derived from decomposing organic matter, which in soil is a combination of leaf compost and manure. Mineral elements come from decomposed rock. Top soil contains high amounts of organic material while sub soil has higher concentrations of minerals and sand. Garden top soil often also contains sticks and bark, while subsoil is usually pretty clean. “Potting” soil is simply garden top soil without the sticks and often has fertilizer and Perlite added.  Perlite is beads of white foam like material that break up the soil allowing exchange of oxygen and also absorbs nutrients. How beneficial they are in the aquarium is uncertain, but they float in water.

Which is better sub soil or top soil?
Top soil has been avoided in many circles for fear of what large amounts of decaying organic material will do in the aquarium. There are two main areas of concern: algae control and anaerobic substrate. Soil heavy in organics may release large amounts of ammonia into the water, which certainly could cause an outbreak of algae and have an adverse affect on the fish and animal population temporarily until it is under control. A more long term problem is anaerobic areas of the substrate. As organic material breaks down and decays, the process depletes oxygen that is surrounding the material and eventually creates methane gas. Lack of oxygen creates dead spots in the substrate, plant roots in the anaerobic areas will turn black, and if the plant’s roots cannot reach outside the affected area to draw oxygen, the whole plant may die.

The Ideal Compromise

Because of the potential anaerobic problems, for many years people focused on using sub soil, “sandy loam”. Because of being very low in organic material, it was considered much safer and it primarily offered minerals. Clay was also used for the same reason. Laterite or clay additives are used as a thin layer in the bottom of the substrate to provide minerals. Later companies developed clay gravels for the same purpose: a substrate medium that was inert, provided an endless supply of oxidized minerals, and had good cation exchange capacity, (ability to absorb nutrients from the water.) Diana Walstad’s book and others put forth the benefit of having organics in the substrate in small controlled amounts and people began considering top soils again.

Ideally, the best of both worlds would be a top soil that is finely pulverized without any leafy chunks or fresh manure. It should also be free of any fertilizer additives. Twigs and bark should be screened out. They do NOT decompose in the substrate. They are just small pieces of wood. Does wood decompose in your tank? Not really.

Brands of top soil vary across the country, and even the same brand can vary in content depending on where it was processed. If you cannot get a sample from the bag before buying it, go to a nursery and describe to them the type of soil you want: a mix of top and sub soil without any chunks of leaf, bark, twigs, or manure, and they may be able to bag something just for you.

Suggestions from my friend Jane:

Perlite – while not a bad thing for houseplant soil, this stuff “lightens” heavy soils, especially those that tend to be high in clay. It provides porosity and helps to allow container plants’ roots access to air from the small spaces between particles (very important for terrestrials). For the aquarium keeper, this stuff floats, and is a royal pain in the @$$, as the little white bits will float up for months and cling to anything in the water surface.

Vermiculite – also generally a good ingredient in houseplant soil, this is a mineral that has been expanded by exposing it to great heat. While it also “lightens” soils to some extent, it also provides a lot of surface area for water to cling to, and helps absorb and retain water, while not staying thoroughly WET. It evens out the wet/dry cycle. For the aquarium keeper, this isn’t as annoying a floating component as perlite, and may help against compaction. (*aside – I’ve actually added some vermiculite to a soil underlayer as an experiment, with very good results to date).

Wetting Agents. These are surfactants. I’ve personally had a very bad experience with these when trying to pot up some very rare terrestrial plant cuttings. My bad experience was with Martha Stewart’s potting soil from KMart. Shredded sphagnum peat has an annoying habit of being difficult to wet once it gets very dry. It actually repels water to some extent. A wetting agent, or surfactant, gets the water to make contact with the other materials, and increases absorbability. But, it also increases the moisture retaining time. For me, this kept the precious cuttings too wet for too long, and caused rot. I’m not positive what the effect would be in an aquarium, but my sense is that it would not be good for the creature or plants. Plants have a very thin natural cuticle to protect them, and I’d guess this would not “play nicely” with that. Who knows what the long term effects on fish would be.

Take a look at some of the cheaper soils. Those tend to NOT have wetting agents, fertilizers or other questionable amendments. I’ve used “Hyponex” and “Jolly Gardener” but found the contents of both brands to vary widely depending on where the bag was purchased, and at what time of year. Try a small bag of a few different types. Anything you don’t use in the aquarium would probably be fine for houseplants.

Here is a VERY general assessment technique: Moisten it, play with it. When moderately moist (like a wrung out sponge), a small handful should have a bit of give when you squeeze it in your fist. Now open your hand flat. Does it crumble apart? That indicates its high in sand. Does it stay compressed, like a hard lump? High in clay. Does it look shiny or slick, or very muddy? That indicates a lot of organics.

Ideally, it will fluff back out a little bit (like when baking a cake – touch the top to see if its done – it should sping back when lightly touched) as you’ve released the compression. It should generally keep its shape, perhaps fracturing in one or two places. If you push on it, it should then break apart rather easily.

Now smell it. It should not smell moldy or astringent or bitter. It should smell pleasantly earthy, and the smell should not be noticable unless you have your nose right up to it.  Thanks Jane! 

Preparing the soil

You do not need to soak or wash the soil. All you will accomplish is making a mess. You can however sift out any twigs or bark. Use a wire mesh or screen strainer. If you spread out the soil and let it sit under the sun for a few hours, (12 hours), until the soil is completely dry, that will neutralize excess ammonia. Another trend currently is “mineralizing” the soil with powdered minerals. The advantage of this is the minerals in the soil would then be immediately available to the plants from day one instead of having to first bind with organic acids to become water soluble. It is a messy, time consuming process that I am not convinced is worth the effort. You can read about it here: http://gwapa.org/wordpress/articles/mineralized-soil-substrate/

Moderation is the key

The key to success is to use any kind of soil in moderation.  Anaerobic conditions are most dangerous when in large amounts. Isolated anaerobic spots are normal in any aquarium and are not too much of a concern. In a healthy, growing aquarium the release of nitrogen into the water is of no concern and does not mean an algae outbreak will be the end result unless it is a significantly large amount that the tank ecosystem cannot handle. Diana Walstad states: “I use about 1 gal of soil/sq. foot for a 1″ layer of moist soil.” She also goes on to say that soil should be covered with no more than an inch of gravel.  She explains the reason is that the deeper the soil is in the substrate, the harder it is for oxygen to reach it and the area will become severely anaerobic.  The only problem I have with that is the fact an mere inch deep covering can be easily uncovered every time you move a plant, want to re-arrange ANYTHING, or do anything to disturb the gravel. I would go a little deeper.