Abstract: A substrate of limited dimensions consisting of mineral fibers for out-of-ground cultivation is used for the cultivation phase corresponding to the development of seedlings. This substrate, of general parallelepipedic shape, exhibits small surface protuberances on its lower face which separate it from the ground on which it rests. The protuberances can be feet disposed on the lower face of the substrate, or edges formed by providing the substrate with a concave lower face. The substrate according to the invention makes it possible to improve out-of-ground cultivation conditions.