Fresh aquasoil (such as ADA aquasoil) often contain significant amounts of ammonia. Plants preferentially uptake ammonia rather than nitrates as a nitrogen source as it is easier to assimilate. However, aquasoils do deplete their ammonia content over time (weeks for fast growth tanks, many months for slow growth tanks).
Spiking the substrate with ammonia rich root fertilizers such as osmocote is an easy way to boost plant growth in planted aquarium. Ammonia in the water column is toxic to both fish (and plants too at higher levels). However, providing it in the substrate is very effective. The key point about osmocote is to not to use too much (such as in gel caps). It provides great bang for the buck if used correctly. I prefer inserting individual balls (one ball every inch square), once every 3 months, in heavily planted areas. Insert them deep, and use them sparsely. If the balls are are not inserted deeply enough, the result ammonia spikes in the water column often give rise to heavy green dust algae.
When choosing osmocote, it is generally better to go for those with no trace elements (meaning use those that only contain NPK) as terrestrial fertilizer trace compositions may not be completely compatible for aquatic environment usage. Some are overloaded with heavy metals that can induce toxicity buildup in the soil over long term.
This method might not work as well for inert substrates, especially those of coarse grain size as ammonia binds to soil but not inert material. Using them in coarse pea gravel for example, just means that you are slow releasing the osmocote into the water column.
Feeding plants through the substrate zone is a good method for those less keen on dosing their water column heavily. It also allows one to feed a particular group of plants, but not others (whereas water column feeding feeds all plants). This allows us to control the relative growth rates of different plant groups in the tank.