Summary: This paper uses real option theory to assess the value of agricultural land that can be seeded with crops. We consider one- and two-factor models for the evolution of crop prices through time and derive a partial differential equation (PDE) for the land value. We model the potential selling decision of the land owner as a put option and incorporate it as a boundary condition in the PDE for the land price. We solve this equation numerically and show that theoretical prices are close to market land prices and that the value of the put option accounts for, at least, 25\% of the total land value.