Characterization of ion-selective electrodes for an on-field soil nutrient analysis system

Abstract

There is an established need for more careful application of soil nutrients and fertilizer to maximize crop yield for an ever-growing population. This study focuses on the manufacturing and characterization of ion-selective electrodes (ISEs) to further study if they could be used to reliably measure soil markers like nitrate, phosphate, potassium, and pH for farming applications. Research into a low-cost design for a soil nutrient analysis system to characterize the viability of farming soil has already begun through proof-of-concept prototypes and testing. This study builds upon such early-stage testing. The goal of this investigation was to build an understanding of the accuracy of these ion-selective electrodes in soil samples. To arrive at the end goal of this investigation, we divided this project into three main stages for these nitrate, phosphate, potassium, and pH ISEs.

Stage one focused on characterizing the performance of ISEs cured in environments with two different oxygen levels, in effort to describe the variation in slope and detection limit contributable to curing environment. This study found that ISEs cured in lower oxygen settings (-10 ppm) were more reliable. Stage two focused on characterizing the selectivity of ISEs for the target ion over interfering ions commonly found in soil, in effort to describe the magnitude of error in a soil measurement due to interfering ions with the ranges found in soils. This study found that the nitrate electrodes tested performed with reasonable selectivity for the interfering phosphate, sulfate, and carbonate ions. Stage three focused on benchmarking the accuracy of the ISEs against standard lab techniques for a library of soil samples. The soil concentrations calculated from the potentials measured by the ISEs were reasonable for some of the soil sample ranges, but not all of them.

The results from these three stages of testing imply that the manufacturing process needs to be updated to include conditioning the ISEs in strontium chloride in effort to improve the reliability and stability of the ISEs.