Calibration Repeatability of Soil Moisture Sensors in Biochar Amendments under Changing Salinity

Soil moisture sensors play a key role in the efficient management of irrigation systems, contributing to the optimization of water. They often require initial calibration, and their accuracy over time is a valid concern, considering sensor drift and aging, in particular for low-cost sensors. This paper evaluates two types of soil moisture sensors - reflective and capacitive - after different calibrations at different times, using soil amended with biochar and different salinity levels. Sensor readings were compared against a direct measurement method, and accuracy and repeatability were assessed using three statistical criteria. Results show that the calibration procedure needs to consider the material composition of the soil to ensure reliable performance. Reflective sensors demonstrated better stability, maintaining errors below 10% while capacitive sensors showed errors approaching 30%, as biochar and salinity increased.