Evaluation of Ion-Selective Membranes for Real-Time Soil Macronutrients Sensing

Abstract

On-the-go, real-time soil nutrient analysis would be useful in the site-specific management of soilfertility. 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-selectivemicroelectrode technology is an ion-selective membrane that responds selectively to one analyte in thepresence of other ions in a solution. This paper describes the evaluation of nitrate and potassium ion-selectivemembranes and the investigation of the interaction between the ion-selective membranes and soil extractingsolutions to identify membranes and extracting solutions that are compatible for use with a real-time ISFETsensor to measure nitrate and potassium ions in soil. The responses of the nitrate membranes withtetradodecylammonium nitrate (TDDA) or methlytridodecylammonium chloride (MTDA) and potassiummembranes with valinomycin were affected by both membrane type and soil extractant. A TDDA-based nitratemembrane would be capable of detecting low concentrations in soils to about 10-5 mole NO3-/L. Thevalinomycin-based potassium membranes prepared showed satisfactory selectivity performance in measuringpotassium in the presence of interfering cations such as Na+, Mg2+,Ca2+,Al3+, and Li+ as well as providing aconsistent sensitivity when DI water, Kelowna, or Bray P1 solutions were used as base solutions. The TDDAbasednitrate membrane and the valinomycin-based potassium membrane, used in conjunction with Kelownaextracting solution, would allow determination of nitrate and potassium levels, respectively, for site-specificcontrol of fertilizer application.