Automated ebb-and-flow subirrigation for citrus liners production. I. Plant growth

Ebb-and-flow subirrigation is a closed system that applies water to the bottom of the containers, reducing water and nutrient losses due to recirculation of fertilizer solution (FS). The technology can improve plant growth and eliminate the improper disposal of salts into the environment. Subirrigation is widely used in the ornamental industry, and sensor-based ebb-and-flow benches can be used by the citrus nursery industry. There is a need for establishing water and fertilizer guidelines to produce citrus rootstocks. The objectives of this study were: 1) design and build an ebb-and-flow subirrigation equipment, 2) automate the system operation using soil moisture sensors, 3) evaluate the system performance on plant growth, and 4) determine the water and fertilizer guidelines to produce Rangpur lime (Citrus×limonia) liners in 56-cm3 cone-shaped containers. The treatments were four volumetric water content (VWC) to trigger subirrigation (0.12, 0.24, 0.36 and 0.48m3m−3), three FS concentrations (25%, 50% and 75% of the fertilizer recommendation) and a control (nursery manual overhead irrigation using breaker nozzles), arranged in a completely randomized 4×3+1 factorial plus design, with three replications. The system was automated by 39 capacitance sensors connected to a data logger, multiplexer and relay drivers, which controlled independent submersible pumps. Subirrigation was turned on when VWC dropped below the set thresholds. Sensors effectively monitored the substrate moisture and controlled subirrigation. Treatments with highest VWC had higher substrate moisture and number of irrigations over time (p<0.0001). The volume of water applied doubled from treatments 0.12–0.48m3m−3 (p<0.0001). Subirrigation increased shoot height, stem diameter, dry weight, leaf area, physiological parameters (leaf intracellular concentration of CO2, transpiration, stomatal conductance, and net photosynthesis), and water use efficiency in response to increase in VWC and FS (p<0.0001). Subirrigation shortened the crop cycle in 30 days, anticipating the liners transplant for grafting, allowing another cultivation cycle in the nursery during the year. VWC of 0.48m3m−3 and FS concentration of 50% of the recommended value for overhead irrigation are indicated for Rangpur lime liner production in 56-cm3 containers.