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SOIL PHYSICS

In Situ Monitoring of Soil Solution Nitrate: Proof of Concept

^a Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616
^b Dep. of Mechanical and Aeronautical Engineering, Univ. of California, Davis, CA 95616

^* Corresponding author (jwhopmans{at}ucdavis.edu).

A lack of in situ instrumentation limits continuous monitoring of contaminants and plant nutrients in soil solution to evaluate environmental and agricultural management practices. We developed a prototype soil solution monitoring technique to measure long-term in situ NO₃^––N concentration, consisting of an in situ stainless steel (SS) porous cup, with real time concentration measurements using an ion selective electrode and an ultraviolet fiber-optic sensor. The measurement technique does not require soil solution extraction, but is based on in situ soil solution equilibration by diffusion between the porous cup and the surrounding medium. The technique was tested on a NO₃^––N solution with two different SS collection cups. Analytical solutions evaluated solute diffusion coefficients, as controlled by a variety of experimental conditions. The principles of operation were developed for diffusion in a liquid solution and a saturated sand, illustrating the potential application in a soil environment.

Abbreviations: ECA, electric circuit analog • ISE, ion selective electrode • RC, Riga and Charpentier (1998) • SS, stainless steel • UV, ultraviolet