These substrates can be readily added to the aquarium. They contain trace elements to nourish aquarium plants. Some substrate have special type of soil which resemble a specific location in the wild. Some substrates can resemble the soil of the Amazon or the soil in an ocean reef zone.
These Fertilizers can be mixed with the substrate. Laterite is an example. Laterite provides a source of iron for plants. It should be placed below the substrate where the roots of the plant can reach.
Keeping it above the substrate causes it to leach into the water and cause a spike in algae.