Modeling post-tillage soil structural dynamics: a review

Tillage modifies the soil structure to create conditions favorable for plant growth. However, the resulting loose structure is susceptible to collapse by internal capillary forces and external compactive stresses with concurrent changes in soil hydraulic properties. Presently, limited understanding of these complex processes often leads to consideration of the soil plow-layer as a static porous medium. Our objective is to provide a review of recent progress in modeling soil structural dynamics at the pore-scale, based on soil mechanical and rheological properties. The basic geometrical framework of the models was a cubic arrangement of monosized spherical aggregates (other arrangements are discussed). The process of soil aggregate rejoining by capillary forces was modeled by considering the rate of energy dissipation due to viscous deformation of wet soil, and corresponding energy release due to reconfiguration of water capillary menisci. The model was complemented by independent rheological characterization of soil that provides control on the rate as well as the onset and termination of aggregate coalescence. The model was also adapted for consideration of steady stress (such as overburden) acting upon the unit cells. Unlike steady stress, transient stress (such as traffic) is applied for too short of a period to allow for total energy dissipation by viscous deformation. Hence, a portion of the deformation is elastic (with a recoverable portion of the applied energy). Rheological characterization under transient (oscillatory) stress provided coupled elastic and viscous properties under several loading frequencies. Effects of transient stresses on the geometrical model were modeled by considering a combination of (i) Hertzian-type elastic strain and (ii) viscous flow of soil at the contacts. Application of the models is demonstrated using illustrative examples and rheological measurements of Millville silt loam soil. Finally, we provide an outlook for upscaling the unit cell results to an aggregate bed scale.