Objective To address the issues of heavy texture and poor water availability in the albic layer of the albic soils, this study investigated the changes in soil hydraulic properties when mixing soils in the albic and illuvial layers at varied proportions as affected by soil bulk density and shrinkage behavior, which would provide theoretical support for the mechanical amelioration of albic soils.

Method An indoor simulation experiment was conducted, in which albic and illuvial layers were mixed at volume ratios of 3: 1, 3: 2, and 3: 3, with two bulk densities of 1.25 g cm⁻³ and 1.50 g cm⁻³. The soil water characteristic curve, shrinkage behavior and related soil hydraulic parameters were measured.

Result The linear extensibility coefficients of the soils in the albic and illuvial layers were 0.093 and 0.155, indicating they had large shrinkage capacity. Soil shrinkage significantly increased the field capacity and wilting point by 15.79% and 37.51%, while air capacity decreased significantly by 44.79%. The porosities of macro-, meso-, and effective pores significantly decreased, whereas the micro-porosity increased. When soil shrinkage was considered, compared to the albic layer soil, the mixed treatments exhibited improved water retention and water release capability at low suction. Under low bulk density, the mixed treatments significantly enhanced the wilting point and air capacity by 13.39% - 18.41% and 22.37% - 29.37%, while increasing macro- and micro- porosities by 245.45% - 336.36% and 13.95% - 18.59%. The 3: 2 mixing treatment exhibits higher available water content and effective porosity than other mixing treatments.

Conclusion The high soil shrinkage capacity of the albic and illuvial layers significantly affects soil hydraulic properties and pore size distribution. Mixing treatments significantly improve soil aeration in the albic layer, with more pronounced enhancements in water retention and macro- and micro-porosities under higher bulk density. The 3: 2 mixing ratio demonstrates the most effective improvement in hydraulic properties.