Evaluation of Ion-Selective Membranes for Real-Time Soil Macronutrients Sensing
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On-the-go, real-time soil nutrient analysis would be useful in the site-specific management of soil fertility. The rapid response and low sample volume associated with ion-selective field effect transistors (ISFETs) make them good soil fertility sensor candidates. An important component of the ion-selective microelectrode technology is an ion-selective membrane that responds selectively to one analyte in the presence of other ions in a solution. This paper describes the evaluation of nitrate and potassium ion-selective membranes and the investigation of the interaction between the ion-selective membranes and soil extracting solutions to identify membranes and extracting solutions that are compatible for use with a real-time ISFET sensor to measure nitrate and potassium ions in soil. The responses of the nitrate membranes with tetradodecylammonium nitrate (TDDA) or methlytridodecylammonium chloride (MTDA) and potassium membranes with valinomycin were affected by both membrane type and soil extractant. A TDDA-based nitrate membrane would be capable of detecting low concentrations in soils to about 10-5 mole NO3 - /L. The valinomycin-based potassium membranes prepared showed satisfactory selectivity performance in measuring potassium in the presence of interfering cations such as Na+, Mg2+,Ca2+,Al3+, and Li+ as well as providing a consistent sensitivity when DI water, Kelowna, or Bray P1 solutions were used as base solutions. The TDDAbased nitrate membrane and the valinomycin-based potassium membrane, used in conjunction with Kelowna extracting solution, would allow determination of nitrate and potassium levels, respectively, for site-specific control of fertilizer application.
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