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ARS Home » Midwest Area » Columbia, Missouri » Cropping Systems and Water Quality Research » Research » Publications at this Location » Publication #207198

Title: Sensing Nitrate and Potassium Ions in Soil Extracts Using Ion-selective Electrodes

item Sudduth, Kenneth - Ken
item Hummel, John

Submitted to: Journal of Biosystems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2006
Publication Date: 12/31/2006
Citation: Kim, H., Sudduth, K.A., Hummel, J.W. 2006. Sensing nitrate and potassium ions in soil extracts using ion-selective electrodes. Journal of Biosystems Engineering. 31(6):463-473.

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. The sensing elements must be compatible with the extracting solution, and adoption would be enhanced if a single extracting solution could be used for all soil macronutrients. In previous work, we identified soil nitrate and potassium ion-selective electrode (ISE) sensors that, when used with the Kelowna multiple-element extracting solution developed in Canada, accurately quantified nutrient levels in laboratory solutions. The goal of this research was to evaluate the accuracy of the sensors for measuring nitrate and potassium in soils. We used seven soils from Missouri and Illinois as our test samples, and compared the nutrient levels with our sensors to those obtained with standard laboratory methods. ISE-measured nitrate levels were very similar to those from standard methods. ISE-measured potassium levels were lower than those from standard methods, but the difference could be easily corrected by use of a calibration equation. The results show that the ISE sensors could measure the nitrate and potassium levels in typical 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 levels are already sufficient. Such a system could provide lower food production costs and reduced environmental impacts, benefiting both producers and consumers.

Technical Abstract: Automated sensing of soil macronutrients would allow more efficient mapping of soil nutrient variability for variable-rate nutrient management. The capabilities of ion-selective electrodes for sensing macronutrients in soil extracts can be affected by the presence of other ions in the soil itself as well as by high concentrations of ions in soil extractants. Adoption of automated, on-the-go sensing of soil nutrients would be enhanced if a single extracting solution could be used for the concurrent extraction of multiple soil macronutrients. This paper reports on the ability of the Kelowna extractant to extract macronutrients (N, P, and K) from US Corn Belt soils and whether previously developed PVC-based nitrate and potassium ion-selective electrodes could determine the nitrate and potassium concentrations in soil extracts obtained using the Kelowna extractant. The extraction efficiencies of nitrate-N and phosphorus obtained with the Kelowna solution for seven US Corn Belt soils were comparable to those obtained with 1M KCl and Mehlich III solutions when measured with automated ion and ICP analyzers, respectively. However, the potassium levels extracted with the Kelowna extractant were, on average, 42% less than those obtained with the Mehlich III solution. Nevertheless, it was expected that Kelowna could extract proportional amounts of potassium ion due to a strong linear relationship (r2 = 0.96). The use of the PVC-based nitrate and potassium ion-selective electrodes proved to be feasible in measuring nitrate-N and potassium ions in Kelowna - soil extracts with almost 1:1 relationships and high coefficients of determination (r2 > 0.9) between the levels of nitrate-N and potassium obtained with the ion-selective electrodes and standard analytical instruments.