sponge in the substrate?

[red devil]

Would it be useful/beneficial to put some kind of sponge material under the lowest level of substrate of a planted tank...to hold a supply of fertilizer that would be replenished by a kind of hypodermic needle?

[TwoTankAmin]

No

[geofied]

If you have a root friendly substrate such as Flourite, Eco-complete, Schultz Aquatic, etc. it holds nutrients pretty well (like kitty litter) for extended use.

Using a sponge is an interesting approach, and could work if the sponge actually relinquishes the fertilizer as hoped. I use sponges on my filter intakes and once they get gunked up, I have to squish 'em under the faucet or in a bucket to get the stuff out. You would likely want the roots to go through the sponge or at least bump up against it at some point so they can "draw" from it.

[mellowvision]

catfish bouncing all over the place...