Growth and establishment of plants and, therefore, the success of soil bioengineering, depend decisively on soil development, particularly on soil aggregation and nutrient accumulation. The natural responsibility of the germ patch assembly is mainly borne by soil micro-organisms. Within them, mycorrhizal fungi are playing a partieular role. As symbiotic partners, these fungi provide the nutrition of the plants. Tests with mycorrhizal Dryas octopetala L. yielded in higher biomass above ground as well as subterranean compared with non-mycorrhizal plants. Furthermore, symbiotic fungi belong to the architects of aggregates and, therefore, perform the development of stable soil structures. The addition of mycelia of mycorrhizal fungi nearly doubled the stability of a structureless substrate mixture. Furthermore, rain experiments demonstrated that soil added with mycorrhizal fungi is more resistant against erosion processes. Results obtained by geotechnical analyses of naturally developed soil confirmed the data of the laboratory experiments. A functional mycorrhizal foundation is a necessary requirement in every step of plant development and, therefore, for soil bioengineering.