A novel dielectric tensiometer enabling precision irrigation control of polytunnel-grown strawberries in coir

The benefits of closed-loop irrigation control have been demonstrated in grower trials, which show the potential for improved crop yields and resource usage. Managing water use, by controlling irrigation in response to soil or substrate moisture changes, to meet crop water demands is a popular approach but requires substrate specific moisture sensor calibrations and knowledge of the moisture levels that result in water deficit stress or over-watering. The use of water tension sensors removes the need for substrate specific calibration and enables a more direct relationship with hydraulic conductivity. In this paper, we present a novel dielectric tensiometer that has been designed specifically for use in soil-free substrates such as coir, peat and rockwool with a water tension measurement range of -0.7 to -2.5 kPa. This new sensor design has also been integrated with a precision PID-based (drip) irrigation controller in a small scale coir substrate strawberry growing trial: 32 strawberry plants in 4 coir growbags under a polytunnel. The data illustrate that excellent regulation of water tension in coir can be achieved, which delivers robust and precise irrigation control - matching water delivery to the demands of the plants. During a 30-d growing period, vapour pressure deficit (VPD) and daily water use data were collected and the irrigation controller set to maintain coir water tension at the following levels: -0.90, -0.95 and -1 kPa for at least 7 consecutive days at each level. For each set-point, the coir water tension was maintained by the irrigation controller to within ±0.05 kPa. Meanwhile, the polytunnel VPD varied diurnally from 0 to maximum of 5 kPa over the trial period. Furthermore, the combination of the dielectric tensiometer and the method of PID-based irrigation control resulted in a linear relationship between average daily VPD and daily water use over 9 d during the cropping period.