Soil water retention and hydraulic conductivity dynamics following tillage

Soil bulk density (ρ_b) may be purposely reduced in agricultural fields using tillage to improve hydraulic properties. However, tillage alters the soil structure, resulting in unstable soils. As the soil stabilizes, ρ_b increases over time. While this is known, studies on soil hydraulic properties in tilled soils, including comparisons between tilled and non-tilled soils, commonly assume a rigid soil structure. This study presents changes in soil water retention and saturated hydraulic conductivity (K_sat) as ρ_b increased dynamically with time following tillage at a loam-textured field site. Over the summer of 2015, soil cores were collected at several depths below the surface following precipitation events. Soil water retention curves and K_sat were determined using pressure cells and the constant head method, respectively. Tillage reduced ρ_b to 0.94 g cm⁻³. Changes in ρ_b increased with depth, reaching a ρ_b of 1.11 g cm⁻³ in the 0–5 cm layer, and a ρ_b of 1.42 g cm⁻³ at the deepest tilled layer. Soil water retention curves were markedly steeper for samples with higher ρ_b, indicating an overall increase in water retained at a soil matric potential (Ψ) of −33 kPa. Evaluation of two modeling approaches for water retention as a function ρ_b indicated that changes in water retention with increases in ρ_b could be reasonably estimated if a matching point was used. No clear relationship between K_sat and ρ_b was obvious for ρ_b < 1.06 cm³ cm⁻³, but for ρ_b > 1.06 cm³ cm⁻³, K_sat decreased markedly (order of magnitude) as ρ_b increased. Hydraulic properties varied strongly depending on time since tillage and soil depth, and results have implications for models of tilled soils, as well as for studies comparing tilled and non-tilled soils.