In Situ Monitoring of Nitrate in Soil Solution.

A lack of in-situ instrumentation limits continuous monitoring of soil solution concentration to evaluate environmental (contaminants) and agricultural management (plant nutrients) practices. We developed a prototype soil solution monitoring technique, to measure long-term in-situ nitrate concentration, consisting of an in-situ stainless-steel porous cup, with real time concentration measurements using an Ion Selective Electrode (ISE) and a UV 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 is presented for nitrate solution with 2 different stainless steel collection cups. Analytical solutions are presented to evaluate solute diffusion coefficients, as controlled by a variety of experimental conditions. The principles of operation are demonstrated for diffusion in liquid solution and a saturated sand, illustrating the potential application in a soil environment.