Validation Testing of a Soil Macronutrient Sensing System

Rapid on-site measurements of soil macronutrients, i.e., nitrogen (N), phosphorus (P), and potassium (K), are needed for site-specific crop management, where fertilizer nutrient application rates are adjusted spatially based on local requirements. This study reports on validation testing of a previously developed ion-selective electrode (ISE) based soil macronutrient sensing system using 36 soil samples from a single site, the Northern Illinois Agronomy Research Center (NIARC), and previously developed calibration models. Objectives were to (1) validate calibration models with a new array of membranes and electrodes, and (2) evaluate the ability of the system to estimate variations in soil NO3-N, P, and K within a single test site. Soil extract samples were obtained using the Kelowna extractant. Electrode responses were measured with five ISEs for each of NO3-N, P, and K and were normalized using the baseline correction and two-point normalization methods developed in our previous work. The array of ISEs fabricated with new membranes and cobalt rod, in conjunction with the previously developed normalization methods and calibration models, accurately estimated NO3-N, P, and K in solution without need to recalibrate the ISE system through standard laboratory analysis of soil samples from the new test site. ISE-measured NO3-N, P, and K concentrations in Kelowna-based soil extracts were similar to those determined by standard instruments, validating the ability of the system to identify within-field macronutrient differences. The use of a calibration factor to adjust ISE measurements for the difference in extraction efficiency between Kelowna and standard extractants resulted in a slope near unity between soil NO3-N, P, and K concentrations determined by ISEs and standard methods. However, a relatively large offset in soil P concentration between calibrated ISEs and standard methods will require further investigation to identify the cause. This study showed that it was possible to transfer existing calibration equations to new membranes and electrodes through application of the baseline correction and two-point normalization methods and an adjustment for differences in extraction efficiency. This finding enhances the applicability of the ISE-based soil macronutrient sensing system and methodology for rapid soil analysis.