Osmocote Plus is a slow release fertilizer sold for garden use.
It can be used in aquariums at the rate of 5 grams per square foot of substrate.
In the initial set up you can simply sprinkle it on the floor of the tank, then cover it with the substrate. Careful when you plant not to bring the Osmocote up.
In an established tank it is better to find some way to push the Osmocote deep into the substrate. Some people use a capsule sold in health food stores, size 00. I freeze small amounts of Osmocote in just enough water to hide the beads, then put the "ferti-cicles" where I want in the tank. I use small amounts so I can push them into the substrate as close as 6" apart.