Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Cationic exchange capacity is the ability of the soil to attract charged particles. The soil needs to be fine enough to hold a charge, and the material that it attracts needs to have a charge. So both need to be really small. Molecular in size. Clay that has bound together in special ways is still clay, and each particle may hold some fertilizer. For example, a material like Turface. Organic matter does something similar, which is one reason to add a little peat moss in the bottom of the tank.
Sand and gravel do not do this. They may help keep the soil open for good water flow, but are not directly involved with CEC.
Plants can remove these charged particles from the soil as they need them.
There is no one answer to how much the soil can hold (depends on the soil).
or how fast plants remove the various things (depends on how fast they need them).
or how fast it regenerates (Depends on how much fertilizer you are adding).
If you add fertilizer to the water column, and the ferts are in their molecular form (KNO3, KH2PO4, K2SO4, and so on) then some of the fertilizers will get used imediately by the plants, and some will keep circulating in the water until the fertilizer gets near a bit of soil with high CEC. The fertilizer has to get very close. (Good water circulation in the substrate) Then it may be attracted to the soil particle, and will stay there until the plant uses it, or until something else (a different fertilizer or mineral) comes close enough and is more attracted to the clay particle. Then the two might trade places.
In a low tech tank very low dosing, or just fish food and water changes will usually be enough to keep a good soil supplied with the fertilizers as the plants remove them. The fish food needs to decompose (digested by fish, acted on by bacteria) before the fertilizers are small enough to become part of the CEC reactions in the soil. This is OK, the fish food they ate last week is probably fertilizer by now, and todays' meals will become fertilizer in a few days or a week. It is a continuous cycle. I would start with a good soil.
With a bit more light and added CO2 the plants will grow faster, and remove the fertilizer from the soil and the water column faster. Starting with a good soil, I would start dosing right away, but at a low level, and watch the plants to see how they are doing. Use the Estimative Index or other method at its lower level and see how things go.
In a high tech tank, starting with good soil, I would dose more, because the plants will take most of it out of the water right away, leaving very little to add back to the soil. This is where the EI method is really good. By making sure there is unlimited levels of fertilizers the soil will keep up its supply. If you were skimping on the fertilizer and saying "but I started with a rich soil" then the plants will remove all the fertilizer from the soil pretty fast, and take the ferts out of the water column as fast as you add them. The soil will not get ahead under these conditions.
So: Start with a rich soil, or dose heavily enough to charge the soil as the tank establishes itself.
Keep up the fertilizers at levels that the plants keep on growing and not showing any deficiencies.
Provide light and carbon in levels that help the plants to grow well.
Then the soil will stay fertile. Plants that prefer getting certain minerals from the soil can do so. Plants that get their nutrients from the water can do so. Better balance all around.
Starting with a good supply of fertilizers in the soil is not an excuse not to dose, but is a better way to keep things balanced. An additional buffer.