The sensors were calibrated for a soil-less substrate by adding different amounts of water to 3.5-litre containers of a commercially-available substrate containing a slow-release fertiliser (Osmocote Plus 14-14-14). The correlation between the water content determined by the sensor and that measured by weighing was very good.
Begonia seedlings were then transplanted into 3.5-litre containers of the same substrate and six different soil moisture contents were maintained. Five containers were linked together in each replicate row and their irrigation was controlled by a TDT sensor placed in every fifth container.
The output of the sensor was scanned by a commercially-available irrigation controller and when the moisture content of the substrate fell below the required threshold value, a solenoid valve opened and approximately 8.3ml of water was delivered to each container.
The system was able to control moisture content very accurately and it also provided information about evapo-transpiration. Although the sensor looks promising, its length precludes it from being used in containers shorter than 20 cm in height.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
A Calibrated Time Domain Transmissometry Soil Moisture Sensor Can be Used for Precise Automated Irrigation of Container-Grown Plants by Miralles-Crespo and van Iersel (2011). HortScience 46 (6): 889-894. ISHS members can view HortScience from the website www.ishs.org.