Direct measurement of soil chemical properties on-the-go using ion-selective electrodes

One of the main objectives of precision agriculture is site-specific management of agricultural inputs to increase profitability of crop production, improve product quality, and protect the environment. Information about the variability of different soil attributes within a field is essential to the decision-making process. Various soil sensors have been developed to map physical and chemical soil properties on-the-go. A method of direct soil measurement (DSM) using ion-selective electrodes (ISEs) has been applied in a commercial implement for on-the-go mapping of soil pH. This study describes a quantitative analysis of the potential for using a similar measurement procedure to simultaneously map soil pH, available potassium, nitrate-nitrogen and sodium contents. Response, calibration, precision and accuracy of the DSM were analyzed based on the results of a laboratory test. Precision of eight tested electrodes was assessed through the root mean squared error (RMSE) and ranged from 0.11 to 0.26pX (negative base 10 logarithm of specified ion activity). The accuracy of electrodes was assessed while comparing test results against reference measurements conducted in commercial soil laboratories (RMSE ranged from 0.18 to 0.37pX). The coefficients of determination (R^2) of regressions between average DSMs and corresponding reference measurements were equal to 0.93-0.96 (soil pH), 0.61-0.62 (potassium), 0.41-0.51 (nitrate-nitrogen), and 0.11 (sodium). Although DSM of pH represents a promising method for precision agriculture, the potential applicability of other ion-selective electrodes based on the results obtained declined in the order: potassium>nitrate>sodium. The primary reason for this decline was the difference in the level of variability of these soil properties in the set of soil samples used in this experiment rather than electrode errors. Performance of sodium electrodes remains questionable due the fact that only one probe was selected. Further research on integrated on-the-go mapping of soil chemical attributes using ion-selective electrodes is needed to investigate suitable application approaches.