Abstract: A method for the hydroponic cultivation of plants which includes passing a nutrient liquid through a filter which has been impregnated with one or more cultures of microorganisms capable of controlling the growth or germination of pathogenic fungi. The impregnated filter releases a portion of the microorganisms into the nutrient liquid as the liquid passes through the filter. The nutrient liquid containing the microorganisms is then contacted with at least the roots of the plants, or parts thereof. The invention is also directed to the filter unit which includes a pre-filter for removing particulate matter, an assembly containing fiberous material impregnated with the biomaterial, a flow means for transporting the nutrient liquid through the fiberous material so that the biomaterial is released in a continuous, controlled manner into the nutrient liquid, and a discharge means to discharge the nutrient liquid containing a portion of the biomaterial from the filter unit.