Spatio-temporal variation of anisotropy of saturated hydraulic conductivity in a tilled sandy loam soil

Knowledge on anisotropy of saturated hydraulic conductivity can improve the understanding of transport phenomena in soil. We hypothesized that saturated hydraulic conductivity (Ks) in the upper part of the root zone of an agricultural sandy loam soil was anisotropic at different soil depths and times after tillage. Ks was measured on undisturbed 100 cm3 core samples taken in the horizontal and vertical directions in up to four soil layers (Surf: surface layer (0–5 cm); Top: topsoil (10–15 cm); Trans: transition layer between topsoil and subsoil; Sub: subsoil (40–60 cm)) 1, 8 and 32 months, respectively, after mouldboard ploughing and drilling. The ratio between estimated geometric mean values for Ks in the vertical and the horizontal directions (Kms,v/Kms,h) was used to test the hypotheses. A total of 669 soil samples were analysed. Kms,h varied with time after tillage and between soil layers. One month after ploughing, Kms,v/Kms,h was <0.23 (P = 0.975) in the Trans layer with an average value of 0.084, i.e. Kms,h was 12 times larger than Kms,v. Anisotropy could not be documented in this layer 8 or 32 months after ploughing, i.e. Kms,v/Kms,h was not significantly different from 1.0. For the Surf and Top layers 32 months after ploughing, Kms,v/Kms,h was in the intervals 1.4–50 and 3.1–77, respectively, (P = 0.95) with average values of 8.4 and 15, respectively. Thus, Kms,v was 8.4 respectively 15 times larger than Kms,h in the two layers. Anisotropy was not found in these layers 1 or 8 months after tillage. Strong anisotropy was found in the Sub layer with Kms,v/Kms,h averaging to 14 and 32, respectively, 8 and 32 months after tillage. Kms,v and Kms,h generally decreased with time in the Surf, Top and Trans layers, except in the vertical direction in the Top layer between 8 and 32 months after ploughing, and in the Trans layer between 1 and 8 months after ploughing. Overall, the geometric means of Ks varied between 10−4.0 and 10−7.1 m s−1. sults may reflect systematic effects of soil settlement and drying/wetting phenomena coupled with biological activity and the existence of stable, vertically oriented biopores in the subsoil. It appears to be necessary to consider anisotropy of Ks and its variation in the analysis and modelling of water flow and chemical transport in agricultural soils, particularly to explain heterogeneous flow phenomena at the plot and field scales.