Development of a Sensor to Measure Water Content and Electrical Conductivity of Soilless Substrates

More and more irrigation scheduling research is showing that fertilizer management in an integral part of a complete irrigation scheduling regime.  By monitoring both electrical conductivity (EC) and water content of substrates, growers can make better decisions on the quantity and timing of irrigation water AND fertilizer. While measuring water content and EC in greenhouse substrates is not new, the ability to continuously monitor in multiple locations is rarely possible because of the cost of accurate sensors.  Data interpretation is further complicated by the fact that the available sensors on the market measure bulk electrical conductivity while growers need to know the value of pore water electrical conductivity.  The goal of our research was to develop a sensor that could be used to help growers better manage fertilizer and irrigation.  Specifically, our objectives were to: 1) Create a sensor that accurately measures the water content and bulk electrical conductivity of soilless substrates such as peats, perlites, and rockwool; 2) Improve upon bulk EC measurements with the future goal of improving pore water EC models; 3) Develop consistent and standardized “best practice” methodologies for usage so that data comparisons and recommendations are universal. Several prototype sensors were constructed and optimized for accurate water content and bulk EC response in soilless substrates.   Preliminary calibrations of the sensor in potting soil, rockwool, perlite, and other non-soil media demonstrated accurate prediction of volumetric water content that is insensitive to electrical conductivity up to 8 dS/m. These results suggest that the new sensor is a viable tool for both researchers and growers to measure water content and bulk EC in soilless substrates.