Laboratory evaluation of dual-frequency multisensor capacitance probes to monitor soil water and salinity

Real-time information on salinity levels and transport of fertilizers are generally missing from soil profile knowledge bases. A dual-frequency multisensor capacitance probe (MCP) is now commercially available, for sandy soils, to simultaneously monitor volumetric soil water content (VWC) measured as a percentage and salinity as a unitless volumetric ion content (VIC). The objectives of this research were to assess the relationship of salinity and water content with these dual-frequency MCPs under laboratory conditions, and assess its potential for field use in sandy soils of the mid-Atlantic region of the US. Water and salinity studies were conducted in two sand-filled PVC columns, 1.2 m long by 0.25 m ID. Each column was instrumented with ten dual-frequency capacitance sensors and two thermocouple temperature sensors. Four salinity levels were studied in the two columns using 0.5, 1, 2, and 4 dSm(-1) NH4NO3 solutions. Water, salinity, and temperature readings were continuously recorded at 1-min intervals. The VIC values were found to be primarily qualitative, but combined with real-time VWC measures the probe could still be an important fertigation management tool to provide near-continuous real-time information on fertilizer penetration, spread and subsequent changes during crop growth.