Martin S Goodchild, Delta-T Devices Ltd., Cambridge, UNITED KINGDOM and Malcolm D Jenkins, Delta-T Devices Ltd, Cambridge, United Kingdom
In this study we present a method for the real-time observation of the soil-plant-atmosphere-continuum (SPAC) using vapour pressure deficit (VPD) and substrate moisture measurements. The experimental arrangement consisted of 8 raspberry plants (Glen Ample) grown in 4 pots of coir under a polytunnel where forced ventilation is provided by an electric fan when the polytunnel reaches a defined air temperature. Coir substrate specific sources of measurement variance have been addressed by using a substrate temperature correction algorithm for capacitance-based moisture measurements. Precision irrigation, using substrate moisture closed-loop control, was employed to maintain ‘stable’ substrate water status conditions. The coir moisture level was set to avoid plant water stress and run-off conditions. By using temperature triggered forced ventilation, it was possible to see three regimes of water-use, namely: overnight, early morning high-humidity limited transpiration, and significantly higher water-use under forced ventilation, the latter requiring additional irrigations events to meet demand. Data collected from air temperature, relative humidity and temperature corrected substrate moisture sensors illustrated that substrate moisture responses occurred within minutes of the fan being switched on. The forced ventilation events resulted in a rapid decrease in relative humidity with a reduction in the increase in air temperature, this combination of responses produced a sharp increase in VPD and resulted in significantly higher water-use.