The biological properties of growing media (GM) in soilless cultivation have been considered minor traits for decades. However, there is a growing demand for alternative, cheaper GM constituents, characterized by organic origin. A better understanding of the biological processes of GM is a key factor in developing improved functional properties in the next generation of “living” GM. This chapter reviews current knowledge about the biological properties of GM for soilless cultivations, paying attention on their importance for plant nutrition, and the influence exerted by weeds, pests/pathogens and beneficial microorganisms to promote crop performance and improve the ability of GM to suppress pests and diseases. More understanding of the relationship between GM, microbial communities and plants increases opportunities to develop sustainable strategies for disease control, fertilization efficiency and horticulture performance.