Subirrigation Controlled by Capacitance Sensors for Citrus Rootstock Production

Brazil is the world's largest orange producer. To produce high quality plants and to reduce pests and diseases spread, seedlings are produced in completely closed nurseries. However, little attention has been paid to irrigation practices (sprinklers or manual), resulting in improper disposal of water with nutrients and pesticides into the soil. Subirrigation can reduce water and nutrients losses, allows nutrient solution (NS) recycling and reuse, decreases fertilizer use, and reduces the release of nutrients into the environment, while also reducing labor costs. Our objectives were to automate a subirrigation system using capacitance moisture sensors to monitor and control substrate volumetric water content (VWC), and to establish subirrigation water and nutrient management guidelines for Rangpur Lime production in cone-tainers with pine bark substrate. We tested four VWC thresholds as irrigation triggers (0.12, 0.24, 0.36, and 0.48 m³∙m^-3), three NS concentrations (25%, 50%, and 75% of recommended fertilization) and a control with manual irrigation and 100% of the recommended fertilizer rate (200 mg∙L^-1 N). We used 56×70×6 cm ebb-and-flow benches with individual NS tanks and submersible pumps in a commercial nursery. The automation was accomplished by connecting capacitance sensors to a multiplexer, a datalogger, and relay drivers. Automated subirrigation worked properly throughout the experimental period: the substrate gradually dried out until the threshold for a specific treatment was reached, at which time the irrigation was performed automatically. Lower irrigation thresholds resulted in less frequent irrigation and reduced stomatal conductance, photosynthesis, water use efficiency, leaf area index, plant height, stem diameter, total leaf area, and shoot and root dry mass (P < 0.001 in all cases). The 50% NS treatment resulted in the best growth. There were no incidence of disease (especially Phytophthora nicotianae var. parasitica and Xanthomonas axonopodis pv. citri), and the occasional appearance of leaf miners and fungus gnats. Capacitance sensors were effective for monitoring and controlling subirrigation. Manual irrigation caused large VWC fluctuations (from 0.16 to 0.43 m³∙m^-3) and resulted in smaller plants (24.55 cm tall) than the 0.36 and 0.48 m³∙m^-3 VWC treatments. 50% of the recommended fertilizer and 0.36 m³∙m^-3 VWC reduced the number of irrigations and resulted in taller plants (44.63 cm) with greater stem diameter (3.46 mm) than control plants. Management of VWC and NS concentration gives growers better control over plant growth and can be used to produce Rangpur Lime rootstocks faster than with conventional methods.