Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Land and container gardening has a lot in common with aquatic gardening, but there are some important differences.
Land plants are not especially noted as being capable of adding oxygen to the soil.
Aquatic plants do this a lot.
The natural soil that aquatic plants grow in (a lake, for example) is already compacted, anaerobic goo. We do want something a bit better in our tanks, because we are also keeping fish, and the anaerobic substrate creates conditions that can kill fish.
The natural soil that land plants live is is usually not compacted, and their roots are not capable of handling those conditions (poor air and water movement and related issues).
In containers you are watering a lot, because they dry out a lot. This is like forced aging, or mineralizing the soil. Every few years it is common to unpot the plants, scrape off any loose soil and repot using as much new soil as possible. This is supposed to replenish whatever has happened to the old soil because of this forced aging.
This does not happen so much in aquariums. A good substrate should not break down in an aquarium.
Soil, for land plants or aquariums is most basically a mineral component, and an organic matter component.
The mineral component is a combination of particle sizes called sand, silt and clay. There may be larger particles, gravel, pebbles, rocks... call it what you want. Not soil particles.
The organic fraction is dead matter from plants and animals, in various stages of decomposition. Fallen leaves, manure, grass clippings and all sorts of other things.
For our gardens many of us make a compost pile and break down organic matter that way.
In nature composting takes time, and organic matter in soils can be in many particle sizes. The smaller particles interact with the smallest mineral particles (clay) and the clay soils become better textured when there is plenty of organic matter blended with them.
As the organic matter in our garden soils and containers breaks down you have to keep adding more. Mulch, compost...
In an aquarium, or in a lake there is a constant rain of organic matter. Fish food, falling leaves, fish poop, dead bugs...
Decomposition happens under water, too, but perhaps at a different rate than on the land. The microorganisms that break down organic matter work better with more oxygen. Higher oxygen levels on land, at the surface of the soil (think fallen leaves on the forest floor) allow the material to decompose very fast. So there is a constant trickle of decomposing organic matter carried into the soil by worms, gophers and other animals.
In an aquarium or lake there is less oxygen in the substrate. The decomposers work slower. Unless you have MTS or other diggers the organic matter is not constantly getting blended with the lower levels of substrate. In many aquariums (non-planted especially) this is getting vacuumed out frequently.
In an aquarium, too much organic matter is generally not good. As it breaks down there is ammonia and CO2 added to the water. Sounds great for plants, but this can be overdone.
A more mineral based substrate is better in an aquarium. Clay sized particles have high CEC no matter if they are clumped together or separated. A baked clay substrate that is handled gently so it does not fall apart is a very good substrate.
Clay soil in the garden can be very good, too, but is hard to work. It must not be compacted, or it loses its great texture. It takes time to add enough organic matter to make a poor clay soil into a good soil. Then you need to keep adding organic matter. Clay particles are especially difficult to work with in containers, so mostly people avoid it by using a sandy loam with lots of compost in containers, or even a soil-less mix of perlite and compost. The organic component still needs to be replaced in these soil-less mixes.