Pore Size Distribution Derived from Soil–Water Retention Characteristic Curve as Affected by Tillage Intensity

Abstract

Tillage practices can influence the pore size distribution (PSD) of the soil, affecting soil physical and hydraulic properties as well as processes that are essential for plant growth, soil hydrology, environmental studies and modeling. A study was conducted to assess the effect of no-tillage (NT) and conventional tillage (CT) on PSD derived from soil–water retention curves (SWRCs) using the van Genuchten’s equation (vG) at 0–15 cm and 15–30 cm depths in a sandy loam soil. Values of PSD or slopes (C(h)) were calculated from the SWRCs by differentiating the vG equation. Soil water retention curves under both tillage systems and within two depths were determined using the evaporation HYPROP method. The vG equation was well fitted to measured soil water retention data. The diameter (D) of soil pores retaining water at various matric suctions (|h|) of water in soils was calculated by the capillary equation. A significant effect of tillage on soil PSD was observed in the macro-pore (D > 1000 μm, at |h| < 3 hPa) and meso-pore (D between 10 and 1000 μm, at |h| between 300 and 3 hPa) size classes, while the micro-pores size class (D < 10 μm, at |h| > 300 hPa) was unaffected at the 0–15 and 15–30 cm depths. Larger values of C(h) or PSD in CT were associated with greater soil loosening induced by the CT operations and greater proportion of large pores (structural porosity) occurred in soils under CT compared to soils under NT. Macro-pore and meso-pore proportions were significantly greater in soils under CT than in soils under NT within both soil depths. The hydraulic parameters of the vG equation and its derivative function can be used to compare soil–water retention curves and pore size distributions between soils under untilled and tilled conditions.