Objective  Investigating the influence of horizontal ridge and vertical ridges on water characteristics curve of different slope positions would provide a basis for choosing direction of ridge tillage of red sloping farmland.

  Method  The soil moisture characteristic curve of tillage layer in different slope positions (upper, middle, and lower) of two ridge directions were measured by pressure membrane instrument and simulated by Van Genuchten (VG) model. Water retention characteristics and influencing factors of different slope positions of red soil after horizontal ridge and vertical ridge were analyzed and compared by VG model parameters.

  Result  The results showed that the volume moisture contents of tillage layer of different slope positions of red soil corresponding 0 to 1000 kPa water suction were 0.36 to 0.66 cm³ cm⁻³, and the VG model could better express the quantitative relationship between them. The saturation moisture contents (θs) of upper, middle and lower slopes position of the VG model parameters of vertical ridge were 0.59, 0.59 and 0.65 cm³ cm⁻³, respectively, and the residual water contents (θr) of them were 0.37, 0.38 and 0.41 cm³ cm⁻³, respectively. They increased as the slope positions decreased. Saturation moisture contents (θs) of upper, middle and lower slopes position of the VG model parameters of horizontal ridges were 0.58, 0.56 and 0.59 cm³ cm⁻³, respectively, and residual water contents (θr) were 0.37, 0.35 and 0.38 cm³ cm⁻³, respectively. The trend of them decreased with the slope positions at first, then increased. VG model parameters d, n were 0.11 and 0.92. With changes of ridge direction and slope position, the changes of d and n were not obvious.

  Conclusion  The results of the principal component analysis showed that total porosity and sand grain content were the leading factors affecting the physical and chemical properties of tillage layer of red soil. The water retention capacities of the upper and middle slope position of tillage layers of vertical ridge were not as good as those of the lower slope position, while the water retention capacities of the upper, central and lower slope position of the horizontal ridge were little difference. The water release capacity of the horizontal ridge of tillage layer was better than that of vertical ridge compared with the same slope position, indicating that the horizontal ridge could improve the soil water supply capacity.