Surface measurements of the hydraulic properties of a tilled and untilled soil

The surface characteristics of soil can have a profound effect on the hydrology of tilled land. Apposite measurements of the surface hydraulic properties of Plainfield sand (Wisconsin, U.S.A.), a Typic Udipsamment, were used to assess the hydrologic impact of 5 years tillage by either moldboard plow or no-till. The crop was always corn (Zea mays L.). The “mean” pore size (λ_m), weighted in a way relevant to the flow of water through the soil surface, was computed here from saturated and unsaturated measurements of sorptivity (S₀) and hydraulic conductivity (K₀). Disc permeameters of dissimilar radii were used at two unsaturated supply-potential heads of ψ₀=−100 mm and −20 mm to find S₀ (ψ₀) and K₀ (ψ₀). At saturation (ψ₀=0), infiltration rings of contrasting radii were employed. The saturated and unsaturated values for S₀ and K₀ of the plowed soil were either the same as, or greater than the corresponding values for the no-till soil. Combination of the values for the saturated S₀ and K₀ showed that the no-till soil had a λ_m=1.34 (±0.67) mm, while in the plowed soil the “mean” pore size during saturated flow was only 0.19 (±0.18) mm. The large λ_m, and the high coefficient of variation, for the no-till soil was presumed to be related to the macropore network associated with the decay of crop residue in the less-sorptive matrix. The small homogeneous λ_m of both the saturated and unsaturated plowed soil reflects the annual pulverization of the soil surface by tillage.