Organic compounds and micro-organisms in closed, hydroponic culture:: Occurrence and effects on plant growth and mineral nutrition

Organic compounds in closed hydroponic cultivation systems may originate from the incoming water, root exudates, substrate and microbial activity. Most of the organic compounds are probably released from plant roots. In a closed hydroponic system, there is a close interrelationship between the cultivated plant, its physical and chemical environment and the establishment of the microflora. Presence of organic compounds, such as phenolic acids, may affect the nutrient uptake processes in the roots as well as root and shoot development. With respect to composition and dynamics of the microflora, stimulatory as well as inhibitory effects on the plants may be expressed. In liquid hydroponic systems, high concentrations of organic compounds are probably prevented by microbial degradation. DOC (dissolved organic carbon) values of 40 ppm and viable counts of bacteria of 10(7) cfu/ml are generally not exceeded. In hydroponic systems with solid substrate other conditions can prevail in the root environment resulting in higher DOC concentrations.