According to the online lexicon Wikipedia, additives are part of a mixture. You also could call additives adjuvants. Additives are essential to make plant cultivation under artificial light in closed rooms a success. We differ mineral additives from organic additives. The role they have in the cultivation process depends on their properties. For example: Perlites and Vermiculites are used to increase the soil structure, air capacity and water capacity. Compost or worm humus on the other hand is mostly added in bio- or peat-reduced soil.