Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Soil is composed of many different things, but can be simplified:
The mineral fraction is sand, silt and clay. They were never alive. These are minerals of different particle sizes. Only clay is small enough to have cationic exchange capacity. Sand, silt and clay from some sources keeps on breaking down in the tank, but most soils do not. If you start with a certain ratio of sand:silt:clay, then you end up with that ratio as the tank matures.
The organic fraction is stuff that came from plants or animals, such as garden compost. As these materials are broken down to very fine material they ultimately get so small they have some cationic exchange capacity. If you put organic matter under water it continues to decompose, unless the conditions are really extreme (peat bogs, for example). The decomposing organic matter releases many things into the water. The most common things are tannic and other organic acids (makes the water yellow to brown colored), CO2, and ammonia.
To make a substrate:
I would start with soil that has very little organic matter, and what it has is so fine you can no longer identify it. Compost that is as fine as dust, or pretty much that size.
Lots of sand and silt, some clay.
If you have access to garden soil that has not had pesticides (herbicides, insecticides, other) here is a test that may help:
Put some soil in a straight sided jar. Put a piece of masking tape on the side of the jar and mark how much soil is in the jar.
Add water and a bit of dish washer detergent. Not much, just a few drops.
Shake a LOT.
Set the jar down and time how fast the particles fall. Mark on the tape:
30 seconds. This is sand.
2 minutes. This is silt.
2 hours. This is the coarser clay.
overnight. This is a finer clay.
See how cloudy the water still is.
Here is how to interpret the results:
as much as 50% sand is pretty good.
As much as 30% silt is pretty good.
Up to 10% clay is OK, if it falls out of the water in 2 hours.
If it takes overnight, you may still be able to work with this soil, but I would mineralize it first. These fine clay particles will cloud the water too easily. Mineralizing the soil may help them cling together and act more like larger particles.
If the water in the jar is still cloudy after sitting overnight do not use this soil. It has colloidal clay. Soil particles that are so fine Brownian motion keeps them suspended. The aquarium would be hazy all the time.
If there are floating particles this is organic matter. If the stuff is fine enough that you cannot tell where it came from (leaf, stick...) then that is best. You can screen the soil to remove larger particles, or pick them out by hand.
I have heard of people starting a tank with garden compost. Well composted, to the point that it is still a bit fluffy, but you cannot tell what things started out as.