Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2012
Publication Date: 3/26/2013
Citation: Kim, H., Sudduth, K.A., Hummel, J.W., Drummond, S.T. 2013. Validation testing of a soil macronutrient sensing system. Transactions of the ASABE. 56(1):23-31. Interpretive Summary: The conventional practice of soil sample collection and analysis is costly and time consuming when applied at the intensity needed in variable-rate fertilizer management systems. A more efficient approach would be to sense soil macronutrient (nitrogen, potassium, and phosphorus) status in real time as a machine moves across a field. This approach requires a system that can extract nutrients from the soil, coupled with sensors that can rapidly measure nutrient levels in the soil extracts. In previous work, we identified soil nitrate, phosphate, and potassium ion-selective electrode (ISE) sensors that, when used with the Kelowna multiple-element extracting solution, accurately quantified nutrient levels in a set of soil samples from Missouri and Illinois. The goal of this research was to see if the methods and calibration equations developed in that previous study could be applied to a new set of ISE sensors for estimating nutrient level variations in soils from a single site. The previous calibration equations worked well with the new sensors, an important finding because development of new calibration equations is time consuming and difficult. Also, soil macronutrient levels estimated with the ISE sensors were generally quite similar to those measured by established laboratory methods. This shows that the ISE sensors could measure nitrate, phosphate, and potassium concentrations at levels typical in agricultural soils. An ISE nutrient sensing system might be used to target fertilizer to sub-field areas where it would be beneficial, and to reduce fertilizer application in sub-field areas where nutrient concentrations are already sufficient. Such a system could provide lower production costs and reduce environmental impacts, benefiting both producers and consumers.
Technical Abstract: Rapid on-site measurements of soil macronutrients (i.e., nitrogen, phosphorus, and potassium) 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-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 (i) validate calibration models with a new array of membranes and electrodes, and (ii) 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 EMF data 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 electrodes 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 system. 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 baseline correction and two-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.