An Automatic Soil Pore-Water Salinity Sensor Based on a Wetting-Front Detector

The recent development of wetting-front detectors has provided a low-energy method of collecting transient samples of soil water under irrigation and rainfall conditions. A simple four-electrode conductivity sensor is presented for the automatic logging of soil water salinity extracted from the wetting front during that part of the irrigation cycle when accumulated salts in a crop root-zone are being mobilized under gravitational flows. The conductance of the platinum-on-ceramic cell is measured with an ac square-wave driven by a pair of micropower operational amplifiers whose rectified ground current acts as the dc signal proportional to electrical conductivity (EC). A 1:200 current mirror reflects this signal into a simple charge-balance 16-b analog-to-digital converter (ADC) bridge formed by a single op-amp acting in conjunction with the internal comparator of a low-cost microcontroller. Similarly, the platinum temperature sensor on the conductivity cell forms an integral part of a second 16-b charge-balance ADC in conjunction with a 1-mA current source. No instrumentation amplifier is required. The temperature coefficients of both the saline solution and the sensor circuit are picked up in the calibration process to produce an accurate temperature-corrected EC digital output that can be collected automatically by a data logger via the SDI-12 environmental data bus. Field data for a two-month period are presented in comparison to an automated vacuum sampling system, with reasonable agreement.