Before the soil is poured into the tank, you can optionally add a substrate fertiliser like e.g. Powersand by Aqua Design Amano as lowermost layer. This substrate fertiliser consists of pumice or lava granules as carrier material and is used to introduce additional nutrients to the tank. Furthermore, this basis made of coarse porous material under the soil considerably improves water circulation in the substrate.
Useful bacteria use the pumice or lava as growth substrate and find the best conditions for settling and spreading there.
When substrate fertilisers are used, you can significantly reduce the amount of fertiliser you add to the water column. Your aquatic plants will primarily use the abundant nutrient sources they find in the substrate to cover their needs of essential micro- and macronutrients. Especially in well-planted tanks or aquascapes, a substrate fertiliser is an important component for healthy plant growth.