The Use of Gravimetric Data for Real-Time Irrigation Monitoring and Control In Greenhouse Crop Production

Real-time precise monitoring of substrate water status allows growers to control irrigation more precisely so that adequate water is applied to the plant while avoiding excess leachate that requires resources and space to store and/or treat. Electrical probe moisture sensors that measure certain electrical properties within the soil in order to quantify volumetric water content have gained popularity. Sensors must be calibrated for individual substrates and may need to be recalibrated often throughout the production of a single crop. Monitoring and controlling substrate water status using gravimetric data is an effective method that requires less technical expertise and equipment than dielectric moisture sensors, crop modeling, or photosynthetic rate measuring equipment. An automated irrigation system that allows irrigation events to be scheduled according to substrate gravimetric water content and that allows various irrigation regimes to be applied simultaneously was built in 2010.

 Fifteen DigiTOL® 8213 Bench Scales were leveled on a greenhouse bench. Each scale was connected to a laptop personal computer running Windows XP via a DB-9 serial cable. A solid state relay module with five solid state relays was also connected to the computer via a DB-9 serial cable. A macro was written within Microsoft Excel 2007 using Visual Basic programming language that intercepts data from the individual scales using WinWedge 3.0, a software program that allows Windows to communicate with electronic devices. Containers are filled with a uniform mass of substrate at potting and placed on the scales. An oven-dried sample of substrate taken at potting allows gravimetric water content to be calculated according to weight. The macro logs weight data from each scale at a set frequency. The weight at which an irrigation event is triggered and stopped can be set accordingly for each scale.  When an irrigation event is triggered for a specific scale, the macro sends a signal via WinWedge to the solid state relay module that closes a relay switch and activates a solenoid valve exclusive to the specified scale. During an irrigation event, the macro logs the weight according to the frequency set by the user. When the weight reading of that scale reaches the specified point, the solenoid valve is deactivated. The macro checks the weight once more after a specified amount of time has passed. If channeling has occurred and the target weight has not been reached, another irrigation event is triggered.