Pore structure characteristics and soil workability along a clay gradient

Clay is a basic soil constituent that governs many soil properties including pore characteristics, which in turn control a range of crucial soil properties and functions. This study explored the relationships between aggregate strength and soil pore structure characteristics for a range of clay contents along a natural gradient, and investigated the influence of clay content and matric potential on soil workability. Soil samples (100 cm³ soil cores and bulk soil) were sampled from the 5–15 cm depth in four locations in an arable field near Lerbjerg, Denmark, which ranged in clay content from 0.119 to 0.446 kg kg⁻¹. The soil cores were drained to five matric potentials in the range −10 to −1000 hPa to obtain volumetric water content and air-filled porosity (ε_a). We measured air permeability and gas diffusivity and calculated tortuosity (τ) and pore organization at −100 hPa. Soil aggregates were obtained from the bulk soil and their tensile strength (Y) and specific rupture energy (E_sp) were determined at −100, −300, −1000 hPa and at air-dry state. Y and E_sp increased with decreasing matric potential and with increasing clay content, which consequently affected soil workability. ε_a at −100 hPa or air-filled macroporosity were negatively related to Y and E_sp, and positively to the range of water contents for tillage (Δθ_RANGE). This indicates the importance of air-filled macroporosity for soil crumbling during tillage. The findings of the study suggest that management practices that increase soil macroporosity can potentially decrease aggregate strength and increase Δθ_RANGE and hence improve soil workability. We suggest that in fields with highly variable soils operations should be scheduled at periods when the range of water contents for tillage are suitable for the whole field to reduce the risk of soil structural damage induced by tillage.