- The biofilter constitutes the water reservoir, to which is added about a quarter of the volume in clay beads. These serve as a culture medium for bacteria that will allow the transformation of natural inputs (urine, compost juice, etc.) into nutrients that can be assimilated by plants. In particular, the transformation of ammonia into nitrite and then nitrate, essential for foliar development (leaf development). Bacteria develop naturally after 6 weeks or can be purchased in culture at hydroponics sites.