Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Other
Publication Acceptance Date: 10/3/2003
Publication Date: 10/3/2003
Citation: Kim, H., Hummel, J.W., Birrell, S.J. 2003. Evaluation of ion-selective membranes for real-time soil nutrient sensing. American Society of Agricultural Engineers. Paper No. 03-1075.
Interpretive Summary: Conventional soil sample collection and analysis are costly and time consuming. Intensive sampling, using these techniques, is impractical for variable-rate nitrogen fertilizer management systems. Rapid, on-the-go extraction of nutrients from samples is needed for real-time sensing. A sensor, based on ion-selective membrane technology, is being developed that can rapidly measure soil nitrate levels in soil extracts. The extracting solution must be compatible with the ion-selective membrane, 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 macro nutrients. In these tests, we compared two standard nitrate extracting solutions (deionized water and copper sulphate) with a universal extracting solution (Kelowna), and we investigated several nitrate membrane compositions. The Kelowna extracting solution lowered the sensitivities and raised the lower detection limit of the membranes for sensing nitrate. Nonetheless, one membrane composition was identified that might be used with the Kelowna extracting solution for nitrate sensing. Additional tests will evaluate the use of the Kelowna extracting solution with ion-selective membranes for potassium and 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.
Technical Abstract: A key to developing a real time, automated soil nutrient sensor depends on the ability to effectively extract soil nutrients from a soil sample and precisely detect them in a very short time period. An ion-selective field effect transistor (ISFET) chip has proven to be a good candidate for use in real-time soil nutrient sensing because of its rapid response and low sample volume. This paper describes the evaluation of nitrate ion-selective membranes and the investigation of the interaction between the ion-selective membranes and soil extracting solutions. The response characteristics of the membranes and their suitability for use for real-time soil nutrient sensing were investigated through evaluation of their sensitivity and selectivity using 16 ion selective electrodes (ISEs) in a computer-controlled system. All membranes showed an approximately linear Nernstian response when nitrate concentrations were above 10-3 M, irrespective of extracting solution type. At low nitrate concentrations, below 10-4 M, both the extracting solution and ligand types significantly affected the sensitivity and selectivity of each membrane. A TDDA-NPOE membrane showed greater sensitivity to nitrate than did the MTDA-NPOE and MTDA-TOTM membranes. In addition, the selectivity of the TDDA membrane was superior to that of the MTDA membranes when bicarbonate, chloride, and bromide were considered as the interfering ions.