|Kim, Hak-Jin - NATL INST OF AG ENG KOREA|
|Birrell, Stuart - IOWA STATE UNIVERSITY|
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2007
Publication Date: April 20, 2007
Citation: Kim, H., Hummel, J.W., Sudduth, K.A., Birrell, S.J. 2007. Evaluation of phosphate ion-selective membranes and cobalt-based electrodes for soil nutrient sensing. Transactions of the ASABE. 50(2):415-425. Interpretive Summary: The conventional practice of soil sample collection and analysis is costly and time consuming when applied at the intensity needed in variable-rate fertilizer management systems. A more efficient approach would be to sense soil macronutrient (nitrogen, potassium, and phosphorus) status in real time as a machine moves across a field. This approach requires a system that can extract nutrients from the soil, coupled with sensors that can rapidly measure nutrient levels in the soil extracts. The sensing elements must be compatible with the extracting solution, and adoption would be enhanced if a single extracting solution could be used for all soil macronutrients. In previous work, we identified the Kelowna multiple-element extracting solution as usable with soil nitrate and potassium sensors. The goal of this research was to identify and evaluate sensors for phosphorous, where the technology is considerably less mature than for nitrate and potassium sensors. Three candidate phosphorus sensing electrodes were tested with the Kelowna extracting solution. Two of the electrodes used ion-selective membranes, while the third was fabricated from cobalt rod. Of the three, only the cobalt rod electrode gave satisfactory results. This electrode was sensitive over the typical range of soil phosphorus concentrations measured in agricultural fields. A nutrient sensing system using the cobalt-rod phosphorus electrodes, combined with the Kelowna extracting solution and nitrogen and potassium electrodes identified in our previous research, might be used to target fertilizer to sub-field areas where it would be beneficial, and to reduce fertilizer application in sub-field areas where nutrient levels are already sufficient. Such a system could provide lower food production costs and reduced environmental impacts, benefiting both producers and consumers.
Technical Abstract: A real-time soil nutrient sensor would allow efficient collection of data with a fine spatial resolution to accurately characterize within-field variability for site-specific nutrient application. Ion-selective electrodes are a promising approach because they have rapid response, directly measure the analyte, and are small and portable. Our goal was to investigate the ability of three different phosphate ion-selective electrodes – two fabricated with organotin compound-based PVC membranes and one fabricated from a cobalt rod – used in conjunction with Kelowna soil extractant to determine phosphorus over the typical range of soil concentrations. Electrodes using organotin compound-based PVC membranes containing bis(p-chlorobenzyl)tin dichloride as an ionophore exhibited sensitive responses to HPO42- over a range of 10-4 to 10-1 mol/L in Tris buffer at pH 7. They were nearly insensitive to phosphate when using Kelowna soil extractant as the base solution, perhaps because of the high concentration of fluoride (0.015 mol/L) in the Kelowna solution. In addition, the life of the membranes was less than 14 days. Electrodes using another tin-compound-based PVC membrane containing tributyltin chloride as an ionophore also provided unsatisfactory results, showing much less sensitivity to H2PO4- than previously reported. The cobalt rod-based electrodes exhibited sensitive responses to H2PO4- over a range of 10-5 to 10-1 mol/L total phosphate concentration with a detection limit of 10-5 mol/L in the Kelowna solution. This detection range would encompass the typical range of soil phosphorus concentrations measured in agricultural fields. The selectivity of the cobalt electrodes was satisfactory for measuring phosphates in the presence of each of six interfering ions, i.e., HCO3-, Cl-, Br-, NO3-, Ac-, and F-, with the electrodes being 47 to 1072 times more responsive to phosphate than to the tested interfering ions.