The hydraulic properties of growing media - from laboratory measurements to greenhouse management

The knowledge of hydraulic properties of growing media is essential for appropriate management of water-air relationship, nutrients and oxygen transport and their availability to plant roots. In general, the only hydraulic property that is directly measured is the θ -h relationship of the growing medium. The other hydraulic relationships are indirectly estimated from the measured θ -h relationship. The θ -h relationship for a given growing medium is measured in the laboratory by measuring the water content of the sample, θ, at a given tension head, h, when equilibrium has reached (time scale of days, weeks). A question arises whether such θ -h relationships adequately describe the transient θ -h relationships in the growing medium during wetting, drainage or water extraction by roots when significant changes take place during short time periods (minutes, hours). Are the in situ θ -h relationships depend on irrigation frequency, plant-roots spatial distribution, momentary atmospheric demand, etc. This study looks into some of these questions by comparing laboratory-measured and in situ-measured θ -h relationship. The later were obtained by frequent and independent measurement of θ and h variation with time at two heights in the growing container. Horizontally-installed electro-tensiometers and vertically-installed TDR probes were used to measure h and θ,respectively in a growing container filled with tuff (volcanic ash, scoria) in which roses were grown. Overall, the in situ θ -h relationships differed from the laboratory-measured θ -h relationships. Higher moisture contents were measured in situ for given tension heads compared to those measured in the laboratory. It was found that the deviations between the in situ- and the laboratory-measured θ -h relationships depend on the irrigation frequency and spatial roots distribution. The sources of these deviations and their implication to water availability to the plant roots and irrigation management were analyzed and discussed.