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SOIL FERTILITY & PLANT NUTRITION

Simultaneous Analysis of Soil Macronutrients Using Ion-Selective Electrodes

^a National Institute of Agricultural Engineering (NIAE), Suwon, South Korea 441-100
^b USDA-ARS Cropping Systems and Water Quality Research Unit, Columbia, MO 65211
^c Dep. of Soil Environmental and Atmospheric Sciences, Univ. of Missouri, Columbia, MO 65211

^* Corresponding author (kimhj69{at}rda.go.kr).

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 NO₃–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 CH₃COOH + 0.015 mol L^–1 NH₄F). The response of each electrode type in mixed solutions of NO₃, K, and P ions was modeled based on the Nikolskii–Eisenman equation with all coefficients of determination (r²) 0.95 (P < 0.001). In soil extracts, the NO₃ 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 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 NO₃–N, available K, and available P measurement with the Kelowna extractant.

Abbreviations: EMF, electromotive force • ICP, inductively coupled argon plasma • ISE, ion-selective electrode