Adding nutrients to an existing sediment, the simple way
In advice to someone recently:
Osmocoat seems like a wise idea for DIY root tab solutions that has seen some revisiting lately(been used for the last 30 years by aquarist). Slow release, small etc, ice cubes filled with Osmocoat also would make adding it in spots very easy too. This is similar to soil mud ice cubes that can be added then melt in place without making a mess trying to insert it deep into the sediment.
Some folks had thought to use gel caps for pills to do this, but ice cubes are even easier. Different ice shapes can be made to add as much or smaller amounts as needed. Fertcicles