Examining the Role of Substrate Moisture Sensor Placement in Container Volumetric Water Content Measurement Accuracy

Substrate moisture sensors permit growers to utilize precise and dynamic irrigation scheduling that improves water use efficiency and reduces overall water use while maintaining or improving plant quality and production schedules compared to conventional, static timer-based irrigation practices.  Measurement accuracy influences the efficacy of sensor-based systems.  Sensor placement, both orientation to the surface and depth in substrate, may influence the ability to generate accurate substrate moisture measurements.  Yet, sensor placement is a variable that is often not investigated.  Experiments were initiated to determine which of five EC-5 capacitance sensor placements best estimates whole container volumetric water content (VWC) for 11.4 L containers filled with a pine bark substrate amended with 15% sphagnum peat moss and planted with Hibiscus moscheutos.  Five sensor placements were tested; two placements were inserted into the substrate surface either vertically or diagonally and the remaining three sensors were horizontally inserted through the container sidewall at 5 cm, 10 cm, and 15 cm from the base of the container.  All placements showed a strong linear relationship (r²> 0.92) with VWC determined gravimetrically and converted to a volume basis, indicating they all are appropriate indicators of container substrate moisture.  Sensors in the vertical placement had a more accurate slope than 15 cm from the base, but the y-intercept was further from zero.  Sensors placed horizontally at 15 cm above the base had a y-intercept closer to zero than horizontally placed sensors at 5 cm from the base and vertical and diagonal sensors, but the slope was less accurate (P-value 0.0148) than the vertical placement.  For this species and container size, no probe placement was universally preferable to the other placements, but using surface installed placements (vertical and diagonal) may be most practical for sensor calibration and ease of installation and removal at harvest, while choosing a horizontal placement may be most practical for hand weeding, pruning, and agrichemical applications and may be more protected from dislodging.