ARS | Publication request: Evaluation of Ion-Selective Membranes for Real-Time Soil Macronutrients Sensing

Submitted to: Asae Annual International Meeting
Publication Acceptance Date: July 3, 2004
Publication Date: August 1, 2004
Citation: Kim, H., Hummel, J.W., Birrell, S. 2004. Evaluation Of Ion-Selective Membranes For Real-Time Soil Macronutrients Sensing. Asae Annual International Meeting. [cd-Rom]. American Society Of Agricultural Engineers. Paper No. 041044.

Interpretive Summary: Conventional soil sample collection and analysis is too costly and time consuming for real-time sensing in variable-rate fertilizer management systems. A sensor, based on ion-selective membrane technology, is being developed that can rapidly measure nutrient levels in soil extracts. The ion-selective membrane must be compatible with the extracting solution, and adoption of automated, on-the-go sensing of soil nutrients would be enhanced if a single extracting solution could be used for all soil macronutrients. In this paper, we summarized and updated previous work on nitrate membranes, and reported on sensitivities and selectivities of several potassium membranes. In these tests, we compared four standard soil extractants (deionized water, Bray P1, Mehlich III, and Kelowna) and investigated three valinomycin-based membrane compositions. The Kelowna extracting solution lowered the sensitivities and selectivities and raised the lower detection limit of the membranes for sensing potassium. Nonetheless, two membrane compositions were identified that might be used with the Kelowna extracting solution for potassium sensing. Additional tests are planned to evaluate the use of the Kelowna extracting solution with ion-selective membranes for phosphorus. On-the-go real-time sensors might be used to direct additional fertilizer to sub-field areas where it will be beneficial; in other areas, fertilizer may have no economic benefit and could result in environmental degradation. Lower food production costs and reduced environmental impact will provide benefits to both producers and consumers.

Technical Abstract: 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+, Mg++,Ca++,Al+++, and Li+, as well as providing a consistent sensitivity when DI water, Kelowna, or Bray P1 solutions were used as base solutions. The TDDA-based 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.