Fertilized Substrates are used for Planted Aquariums. While common substrates such as gravel and sand can be used for planted aquariums, some hobbyists who seek a more rich growth of plants for their aquarium prefer Fertilized Substrates.
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.