|KIM, HAK-JIN - UNIV OF MISSOURI
|BIRRELL, STUART - IOWA STATE UNIV
Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2006
Publication Date: 3/16/2006
Citation: Kim, H., Hummel, J.W., Birrell, S.J. 2006. Evaluation of nitrate and potassium ion-selective membranes for soil macronutrient sensing. Transactions of the ASAE. 49(3):597-606.
Interpretive Summary: Conventional soil sample collection and analysis are too costly and time consuming for real-time sensing in variable-rate fertilizer management systems. Sensor technology is being developed that can rapidly measure nutrient levels in soil extracts. The sensor's active element 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 sensing elements and reported on several potassium sensing elements. In these tests, we compared four standard soil extractants and investigated three nitrate and three potassium sensing elements. The Kelowna extracting solution affected both the nitrate and potassium sensing elements, but some of the sensing elements tested were still active at the nutrient concentration levels found in soil extracts. Other extractants included in the tests did not perform well for simultaneous sensing of nitrate and potassium. Additional tests are planned to evaluate the use of the Kelowna extracting solution with sensor elements 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. Ion-selective microelectrode technology requires an ion-selective membrane that responds selectively to one analyte in the presence of other ions in a solution. This paper describes 1) the evaluation of nitrate and potassium ion-selective membranes and 2) 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**2+, Ca**2+, Al**3+, and Li**+ as well as providing a consistent sensitivity when deionized 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.