I've suggested this for "soil mud cubes" for many years now.
Ferts will work, not sure what you hope to gain from it, the ferts will diffuse into the water column rapidly.
They always do.
Plants roots will leach them as well.
Plenty of experimental evidence shows this.
Soil is bound and slowly breaks down over time and tends to hold better, more organic matter is prefereable.
It's also......"dirt" cheap.