Since root growth has been correlated to oxygen diffusion rate as measured by the platinum microelectrode, diffusion calculations based upon the boundary conditions of the electrode can be used to get orders of magnitude of water thickness around a root which will be sufficient to limit oxygen supply. The thickness is dependent upon the soil porosity around the root and the oxygen concentration in the pores. The thickness ranges from 0.08 to 0.35 mm. between porosities of 0.2 to 0.5 when the oxygen concentration in the pores is at equilibrium with atmospheric oxygen.