In situ monitoring of soil solution nitrate in saturated and unsaturated sandy soil

Abstract

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 a UV fiber-optic sensor and Ion Selective Electrode (ISE). The measurement technique does not require soil solution extraction, but is based on insitu soil solution equilibration by diffusion between the porous cup and the surrounding medium. The technique is presented for nitrate solution with sandy soil using new designed of solution samplers. Analytical solutions are presented to evaluate solute diffusion coefficients, as controlled by a variety of soil water contents. The principles of operation are demonstrated for diffusion a saturated and unsaturated Oso Flaco Sand, illustrating the potential application solution samplers in a soil environment.