There is increasing interest toward in situ solution monitoring of soil chemicals for agricultural, industrial, and ecological purposes. Rather than extracting soil solution, a series of laboratory experiments was conducted to evaluate a diffusion equilibration technique, providing for real time in situ soil solution nitrate concentration via UV absorption spectroscopy. Experiments allowed for diffusion of nitrate from an outside reservoir into a porous cup. The experimental data were compared with model predictions using three different analytical solutions: (i) an exact solution (EX model) assuming radial symmetry that includes a porous cup with a porous wall and a finite size outside reservoir, (ii) an approximate solution (RC model) that assumes an infinitely large supply reservoir with a porous cup wall that does not influence ion diffusion, and (iii) and a model based on an electric circuit analog (ECA model). The results indicate that the ECA solution provides the best match overall to the experimental data.