Simultaneous Analysis of Soil Macronutrients Using Ion-Selective Electrodes

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 Co rod, for the simultaneous determination of NO 3 -N, available K, and available P in soil extracts. Thirty-seven Illinois and Missouri soils were extracted using the Kelowna soil extractant (0.25 mol L -1 CH 3 COOH + 0.015 mol L -1 NH 4 F). The response of each electrode type in mixed solutions of NO 3 , K, and P ions was modeled based on the Nikolskii-Eisenman equation with all coefficients of determination (r 2 ) ≥0.95 (P < 0.001). In soil extracts, the NO 3 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 (r 2 = 0.85, P< 0.001). The P ISEs provided concentrations about 64% 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 = 0.81, 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 ISE technology can be implemented successfully for NO 3 -N, available K, and available P measurement with the Kelowna extractant.