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post #4 of (permalink) Old 11-15-2012, 02:11 AM
Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Posts: 11,721
Soils of several sorts can hold onto fertilizers in a way that the plants can get them but the ferts are not released into the water column. This is referred to as Cationic Exchange Capacity. Mostly it is extremely fine substrates that can do this. Clay and well decomposed organic matter. The clay can be clumped together like ADA products and others and still have this ability.

The sports field products and the 'sorbant products like kitty litter, Oil Dri and related products are very good at this, too.
Soil Master Select was one of the earlier products used like this, but is no longer available.
Turface is one that is still available, but the colors are not great looking in an aquarium. Sort of bright.
Safe-T-Sorb is one of the oil absorbing materials that looks really good in an aquarium, being soft greys and browns like a natural river bed. It seems to hold up well, too.
Oil Dri and most other oil absorbent materials and kitty litter are usually white or off white, and may fall apart too fast in an aquarium.

These materials are all somewhat lighter weight than most aquarium substrates. Plants do not stay down so well when they are first planted, though they will root well and run through the substrate quickly if they can stay down long enough to get going.
No matter how much fertilizer a substrate may have when you first install it you need to supply more fertilizer over time.

Think of the substrate as your plates and bowls. The fertilizer is like the food. Keep on using the plates and bowls, adding food for each meal.
The substrate should last for years, but you need to keep adding fertilizer.
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