Evolution of Micro-pore structure in saline agricultural soil driven by sand addition of specific particle sizes and its regulation mechanism on water and salt transport.

Due to the dense structure and low permeability of saline-alkali soils with high clay content, the vertical migration of water and salt within the soil profile is severely restricted, posing considerable difficulties for saline-alkali land amelioration. To address the aforementioned deficiencies of shallow saline-alkali soils, this study proposes directly incorporating river sand with a specific particle size gradation into the shallow layer of saline soil to reconstruct the soil skeleton and improve pore connectivity. Through laboratory soil column leaching tests and direct shear tests, the incorporation ratio of river sand was systematically optimized, and the evolution of the microstructure before and after amelioration was characterized by scanning electron microscopy. The results indicate that incorporating river sand with a particle size of 0.075-1.0 mm at an incorporation rate of 20%-30% can significantly shorten the leaching duration and effectively promote the vertical transport of water and salt. Furthermore, compared with unamended saline soil, sand-amended saline soil exhibits improved pore structure and permeability, with the pore network becoming coarser and progressively more open, thereby forming continuous and efficient drainage and salt-discharge channels. Notably, an incorporation rate of approximately 30% represents a critical threshold for the evolution of cohesion; once this threshold is exceeded, the system response mechanism undergoes a transition, and cohesion enters a recovery phase. The technical approach proposed in this study is economical, practical, and broadly applicable, demonstrating promising application prospects not only in the fields of saline-alkali land remediation and ecological restoration, but also providing a reliable scientific basis and an efficient solution for relevant engineering practice.