The objective of this study was to get more information about the root zone, mainly the gas composition in substrates, which are used in closed hydroponic systems.

For analyses of carbon dioxide (CO₂), ethylene (C₂H₄) and oxygen (O₂) different methods were proofed and used. For analyses of carbon dioxide and ethylene, a gas sampling system was used to get gas samples from the root zone. CO₂  gas samples of 20 ml were analyzed with an IR sensor by air diffusion into a measuring cuvette. C₂H₄ gas samples of 3 ml were analyzed using gas chromatography (GC). Gas sample cells were inserted into the substrates at 2 or 3 heights. Dissolved oxygen was determined with a membrane covered galvanic sensor, in the air, in the in the nutrient solution, and in the drain solution. To analyze the root zone, a hydroponic system with chrysanthemum cv. 'Snow' on 3 substrates was performed. The following substrates were used: Sawagrow (polyester fiber), polyester fleece (thin layer of fleece), and UC-mix (peat substrate).  

Gas sampling at 2 or 3 heights from root zone did function well, results ranged  from 350 ppm to 7700 ppm CO₂, and  from 0.0088 ppm to 0.1147 ppm of C₂H₄. The organic substrates UC-mix contains more CO₂, from top to the bottom the CO₂ level increased. The CO₂ level is influenced by microbial and root respiration. The determined CO₂ concentration is no limiting growth factor.

Results of C₂H₄ shown an influence by the substrates and a gradient from the top to the bottom. But highest level was determined in the middle and at the bottom of the substrates. The time course shows during the day pattern according the photoperiod. The trend of C₂H₄ level was almost constant during the experiment.

The dissolved O₂ level in solution was about 57-82 % and in drain solution

78 % - 99 %.  O₂ level of solution and drain can be an indicator for oxygen deficiency, but it is no direct measurement in root zone or roots. The O₂ content in air between 10.5 % to 20.4 %. During the experiment the O₂ concentration decreased; for CO₂ the opposite trend was found.