Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have had Soil Master select in my tanks for several years, and yes, by being consistent with the fertilizer for a while it does build up a reserve. Depends on how much extra fertilizer you are adding compared to how much the plants are using for how fast it builds up. If the plants are pretty much using it all up as you add it then very little is going into the reserve.
However, these materials do remove the KH from the water, and this is easily tested and remedied. I added baking soda to one tank for about a year before it slowed down in removing it from the water. This suggests to me that these materials have a very high CEC and you could (if you wanted to) add LOTS of fertilizer, perhaps while doing a fishless cycle, and turn it into a very rich substrate. Another possibility might be to add some powdered ferts at the bottom of the tank during set up, then being very careful not to disturb them. As they get wet they will dissolve and the fertilizer will get picked up by the substrate at first, then by any roots that have made it down that far. In a new set up of course there are not that many roots so widespread, so the substrate will have first claim on the fertilizers. By keeping the ferts on the bottom of the tank there is very little water movement to lift the ferts up into the water column. I think I would still be conservative about adding too much ferts this way. Maybe a month's worth of EI?? Too much and the roots could burn if the substrate cannot adsorb it.
Then there is the removal side: If you have a lot of heavy root feeders they will be removing the fertilizers really fast, so again, the substrate will not be holding it very long, if at all, even if you are feeding pretty heavily.
In another tank I mixed coral sand with Turface and the KH never dropped in that tank. This suggests that when the elements or molecules are closer to the substrate the substrate will take in those, and more or less ignore the material that is in the water column. Another clue that perhaps pre-fertilizing a high CEC material may be a good way to go.