Gas concentration measurement in horticultural growing media

The objective of this study was to test a simple, accurate and inexpensive system for measuring the composition of oxygen (O-2), carbon-dioxide (CO2) and ethylene (C2H4) in horticultural growing media. The model system consisted of a cylinder of 25 cm height, filled with 3.5 1 growing medium. Porous gas-sampling cells of 3.8 ml were inserted at 3 heights. The cell was closed with a septum so that a syringe could be used to take 2.5 ml gas-samples from the cell. CO2, O-2 and C2H4 were analysed using gas chromatography (GC). To test this system a trial with cucumber on 4 growing media was performed. In addition to gas sampling, water content measurements were made every hour using TDR (Time Domain Reflectometry). The following media were used: peat (frozen black peat), fine perlite, coarse perlite and rockwool (slabs). For rockwool the gas-sampling cells were inserted at two heights. O-2 levels as low as 5.4 %, CO2 as high as 8.7 % and C2H4 as high as 8.3 10(-5) % were measured. From the top to the bottom the O-2 concentration decreased and the CO2 and C2H4 concentrations increased. The gradients for O-2 and CO2 concentrations over the height of the media were for most media less then 0-0.6 %, except for rockwool where an average difference of 4 % O-2 was found between top and bottom. For C2H4, the differences between top and bottom were relatively more extreme (up to 0.9, 10(-5) % C2H4), particularly for rockwool (2.5 to 0.9 10(-5) % C2H4). O-2 contents were lowest and the CO2 and C2H4 were highest, for frozen black peat and the bottom layer of rockwool. There was a lot of variation between replicates as found especially at the bottom layer of rockwool. There appears to be a relation between O-2 concentration and the physical characteristics, since peat and the bottom of rockwool with a low air content show low O-2 concentrations. The levels of ADH (Alcohol dehydrogenase) as measured in the roots are low and no aboveground effects were found. Therefore probably it may be concluded that oxygen deficient conditions did not occur under the conditions of the experiments.