Join Date: Apr 2005
Location: Brentwood, CA, USA
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| | Re: Suitable soils for the Walstad method
I just finished a careful reading and re-reading of “Ecology of the Planted Aquarium”, the section about substrates. I think I learned a lot, and understand a lot more about the things I have seen going on in my aquarium. So, I decided to try to make a summary of that section, covering most of the most important parts. I'm doing this primarily to help me remember it, and so I can re-read this if I forget!
The two basic substrates are “gravel” and “dirt, with a gravel cap”. Gravel, whether it is pool filter sand, Flourite, or a higher cost material, relies on the water for most of the nutrients the plants need, and the plant roots have limited access to nutrients. “Dirt, with a gravel cap” provides a concentrated supply of nutrients, available to all of the rooted plants. It keeps the water relatively free of nutrients, and the substrate nutrients can last for years. Those nutrients are isolated from algae, making avoiding algae easier.
“Dirt”, better called topsoil, is made up of: mineral particles, most of which are primarily silicon, aluminum and iron, which are abundant in most topsoils; clay, which is also primarily aluminum, and silicon, in the form of tiny sheets of aluminum silicate; organic matter or humus, which is the final state of organic matter; precipitated inorganic matter, like calcium silicate, phosphates and carbonate, from shells of micro and other organisms; and microorganisms, like bacteria (mostly), protozoa, fungi, algae and yeast.
One positive feature of topsoil is that the humus particles have strong negative electric charges, which makes them bind to cations, like iron, potassium, calcium, etc. The anions associated with those cations tend to remain attached to their cations, so they, too, are bound to the humus particles. This means the nutrient ions remain in the soil and nutrient ions in the water tend to be adsorbed by the soil, making them also less available to algae.
The organic matter in the topsoil will decompose over time, producing CO2 as a byproduct. That CO2 seeps into the water and is available as a nutrient for the plants.
A good “dirt” substrate will consist of about an inch of topsoil, leveled, and covered with about a half inch of inert fine gravel, like pool filter sand, or Flourite, etc. Too deep a gravel cap can become too anaerobic and begin forming hydrogen sulfide, which is toxic. If we use topsoil from flowerbeds, yards, gardens, etc. it might have residual chemicals in it, that are possibly harmful to the fish or plants. This can be removed by using activated charcoal in the filter for a few weeks before planting the tank or adding fish - something rarely necessary.
The only things that should be added to the topsoil might be dolomite, for calcium and magnesium, and possibly phosphates. No other “fertilizers” should be added. The soil shouldn’t contain a lot of peat, because that tends to lower the pH of the soil excessively. No sulfates should be added, because they may be converted to hydrogen sulfate.
This substrate should be good for several years.