ARS | Publication request: Laboratory evaluation of dual-frequency multisensor capacitance probes to monitor soil water and salinity

Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 12, 2009
Publication Date: N/A

Interpretive Summary: Farmers often apply fertilizers such as nitrogen with irrigation water. Over-application of fertilizer can result in economic loss through waste of fertilizer and potential ground-water pollution as unused fertilizer moves past the plant roots in the soil. On the other hand under-application of fertilizer can result in yield losses. Therefore, the ability to monitor the depth of fertilizer application is of great benefit to agricultural managers. We tested an easy to use soil water measuring tool that also measures soil salinity, an estimate of fertilizer content in soil. In laboratory experiments, the instrument was found to provide good estimates of fertilizer or content of the soil after excess water drained from the soil profile. This information will be useful to agricultural managers and scientists who apply fertilizer with irrigation water.

Technical Abstract: 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 that simultaneously monitor volumetric soil water content (VWC, ') and salinity as volumetric ion content (VIC). The VIC is a numerical value that relates to soil electrical conductivity (EC), but it is not directly interchangeable with it. The objective for this research was to assess the relationship of salinity and water content with these dual-frequency MCPs under laboratory conditions and use the results to make recommendations for field use. 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 10 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. The columns were saturated bottom-up with each saline solution, and then allowed to drain for about four days. Water, salinity, and temperature readings were continuously recorded at one-minute intervals. The best linear correlation (r2 = 0.99) between drained-VIC and salinity treatment was found at asymptotic ' values < 11.5%. Although asymptotic low ' field conditions may not be achievable under many field conditions, these results suggest that qualitative measures of water and salinity may be a powerful tool to assess and manage fertigation penetration depths and subsequent losses due to leaching and/or crop uptake.