|Sudduth, Kenneth - Ken|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2007
Publication Date: 10/30/2007
Citation: Kim, H.J., Hummel, J., Sudduth, K.A., Motavalli, P.P. 2007. Simultaneous analysis of soil macronutrients using ion-selective electrodes. Soil Science Society of America Journal. 71:1867-1877. 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 soil nitrate, phosphate, and potassium ion-selective electrode (ISE) sensors that, when used with the Kelowna multiple-element extracting solution, accurately quantified nutrient levels in laboratory solutions. The goal of this research was to evaluate the accuracy of the sensors for measuring nitrate, phosphate, and potassium in soils. We used thirty-seven soils from Missouri and Illinois as our test samples and compared the nutrient levels with our sensors to those obtained with standard laboratory methods. ISE-measured nitrate concentrations were very similar to those from standard methods. ISE-measured phosphate and potassium concentrations were lower than those from standard methods, but the difference could be easily corrected with a calibration equation. The results show that the ISE sensors could measure nitrate, phosphate, and potassium concentrations at levels typical in agricultural soils. An ISE nutrient sensing system 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 concentrations are already sufficient. Such a system could provide lower production costs and reduce environmental impacts, benefiting both producers and consumers.
Technical Abstract: Automated sensing of soil macronutrients would be useful in mapping soil nutrient variability for variable-rate nutrient management. Ion-selective electrodes (ISEs) are a promising approach because of their small size, rapid response, and ability to directly measure the analyte. This study reports on the laboratory evaluation of a sensor array including three different ISEs, based on TDDA-NPOE and valinomycin-DOS membranes, and cobalt rod, for the simultaneous determination of nitrate-nitrogen, available potassium, and available phosphorus in soil extracts. Thirty-seven Illinois and Missouri soils were extracted using the Kelowna soil extractant (0.25M CH3COOH + 0.015M NH4F). The response of each electrode type in mixed solutions of nitrate, potassium and phosphate ions was modeled based on the Nikolskii-Eisenman equation with all coefficients of determination (r2) greater than or less than 0.97 (P < 0.001). In soil extracts, the nitrate ISEs provided concentrations similar to those obtained with standard laboratory methods (r2 = 0.89, P < 0.001). Concentrations obtained with the K ISEs were about 50% lower than those obtained with standard methods due to lower K extraction by the Kelowna solution (r2 = 0.85, P < 0.001). The P ISEs provided concentrations about 66% lower than those obtained with standard methods due to a combination of decreased P estimates in soil extracts and lower P extraction by the Kelowna solution; however, there was a strong linear relationship (r2 = 0.82, P < 0.001). Although P and K concentrations were low in comparison to standard laboratory procedures, a calibration factor could address this issue. These results show that ion-selective electrode technology can be implemented successfully for nitrate-nitrogen, potassium, and available phosphorus measurement with the Kelowna extractant.