Objective  Shear strength can reflect the stability of the cultivated soil structure as well as its suitability for work. When soil overburden pressure exceeds shear strength, soil structure is severely damaged, resulting in decreased farmland productivity. Shajiang black soil has a high soil bulk density, a high clay content and swelling clay minerals, and is classified as a Vertisol in USDA soil classification. It’s typically characterized by structural obstacles like poor workability, which are closely related to soil shear strength. The key influencing factors of shear strength were investigated for establishing its pedo-transfer function to improve soil structure and workability given various undesirable traits of Shajiang black soil.

  Method  Typical Shajiang black soil was selected to study the two key factors that affect the soil shear strength: the soil bulk density (ρ) was set at 1.2, 1.4 or 1.6 g cm⁻³ and soil water content (θ) was set at 100%, 85%, 70%, 55%, or 40% of field capacity, respectively. The Mohr-Coulomb formula was used to calculate shear strength parameters such as cohesive force (c) and internal friction angle (φ) using a direct shear apparatus. The effect of soil bulk density and water content on soil shear characteristics were investigated, and the pedo-transfer function of soil shear strength was established by multiple regression fitting .

  Results  Our results indicate that: (1) The cohesive force (c) of Shajiang black soil decreased as the water content increased, but increased as the bulk density increased. The shrinkage and swelling properties of Shajiang black soil have been considered sufficiently, and the wet bulk density (ρ') is used as one of the independent variables to establish Pedo-transfer functions. The relationship was well fitted by the regression equation c=a_1 e^\left(b_1 \rho^\prime-c_1 \theta\right) (R² = 0.984, P < 0.001). (2) With increasing water content, the internal friction angle (φ) decreased at first and then stabilized. It showed that bulk density had a linear relationship with it. The relationship between φ-θ-ρ' was well fitted with equation of \varphi=a_2 e^\left(b_2 \rho^\prime / \theta\right) (R² = 0.856, P < 0.001). (3) Combined with the Mohr-Coulomb formula, the soil shear strength pedo-transfer function was finally established with \tau=a_1 e^\left(b_1 \rho^\prime+c_1 \theta\right)+\sigma \tan \;\left(a_2 e^\left(b_2 \rho^\prime(\theta)\right.\right). This function could predict the shear strength of Shajiang black soil effectively (R² = 0.870, P < 0.001).

  Conclusion  It can be concluded that soil water content is the most important factor influencing Shajiang black soil shear strength, and cohesive force has a greater impact on shear strength than the internal friction angle. The findings can be used as a guide for improving the structural stability and workability of Shajiang black soil.