This paper develops a basic dynamic economic model that can be used for theoretical and numerical analysis of optimal soil management practices. A dynamic biophysical/economic optimal control model is developed in a multi-disciplinary framework, treating soil as a multi-pool portfolio of a particular limiting mobile nutrient (e.g. nitrogen). This specification allows for fertilizer to directly enter the active pool, while tillage initially affects the decadal pool, reflecting the realities of agricultural production. We examine the properties of the steady-state and the time paths of the optimal solutions. In addition, alternative sustainability criteria of farm-level agricultural practices are presented, and the optimal solution of the problem is evaluated to determine if it meets any or all of the definitions of sustainability.