Controlling Water Content and Electrical Conductivity in Soilless Substrates Using in Situ Sensors

Efficient use of water and fertilizer is not only important for more sustainable production of greenhouse crops, but can have financial benefits as well.  We have shown previously that irrigation can be controlled using soil moisture sensors and now are trying to control both irrigation and fertilizer applications using a sensor that can measure both substrate water content and bulk electrical conductivity or EC (5TE sensors, Decagon, Pullman WA).  We grew ornamental cabbages (Brassica oleracea) in 15 cm-pots.  A datalogger connected to 5TE sensors measured and recorded substrate water content and bulk EC in 16 pots.  Pore water EC was calculated using the Hilhorst model, and plants were irrigated or fertigated based on water content and calculated pore water EC.  The datalogger was programmed to maintain the substrate water content at 0.25, 0.30, 0.35, or 0.40 m³/m³ and the pore water EC at 1, 2, 3, or 4 dS/m in a factorial arrangement.  Control of substrate water content was good, generally within 0.03 m³/m³ of the set point.  Control of pore water EC was more difficult, and there were large fluctuations in pore water EC, especially during the first 20 days of the experiment. There are multiple reasons for these fluctuations: 1) pore water EC has to be calculated from substrate water content and bulk EC, and it is not clear how accurate the Hilhorst model is, 2) good contact between the small electrodes that serve as the EC sensor and the substrate is difficult to maintain, 3) The calculated pore water EC depends on the VWC of the substrate, resulting in rapid changes in pore water EC following an irrigation or fertigation event.  In addition, as the substrate dries out following irrigation, the pore water EC increases because the solution becomes more concentrated as the amount of water decreases.  We also noticed that fertilizer moves more slowly through the substrate than water and thus takes longer to reach the sensor. Both VWC and pore water EC set points affected the growth of the plants.  Shoot dry weight decreased as the set point for pore water EC increased.  Shoot dry weight also was reduced by low set points for VWC, while shoot dry weights were similar with set points of 0.35 and 0.4 m³/m³.  Such effects of VWC on plant growth are consistent with earlier findings.