Pore rigidity as an undervalued process in soil structure development

Soil structure development can be described with tensile and shear processes as well as the further stabilization of interparticle bonds by hydraulic, chemical, biological, and physicochemical processes. The related shrink, swell or stress strain processes, as well as organic bindings and biological glueing processes, however, define the rigidity limits of soil structure and soil functions, which also coincide with defined boundaries that can be applied in modelling approaches. Aggregate formation due to volume separation occurs in soils depending on these interactions and undergo further strengthening or weakening processes with consequences for their rigidity. The goal of this review is to document these processes with corresponding results and to discuss some consequences for global change impacts on, e.g., plant growth and yield or mechanical strength. It is obvious that the hydraulic and mechanical processes have become neglected to some extent in the study of soil structure formation and aggregation, which caused remaining research gaps identified in this review. Consequently, there is an urgent need for a more precise determination of the rigidity limits of soils under various land use and climatic conditions to better predict or model climatic impacts but also the effect of soil management changes or amelioration impacts. • Aggregate formation undergoes intense changes during pedo- and anthropogenesis. • Hydraulic, mechanical, and thermal processes affect internal soil strength. • Structural shrinkage and precompression stress define the rigidity of the pore system. • The rigidity limit is the boundary for the predictability of soil functions • Tensorial hydraulic, stress strain, and thermal functions vary beyond the rigidity limit.