Susceptibility of a sandy loam soil to preferential flow as affected by tillage

Abstract

Flow patterns for water and solutes in structured soils are often heterogeneous. Understanding the spatial variability of flow is essential in solute transport studies and for management of chemical movement. We hypothesized the occurrence of effects of alternative tillage operations for winter wheat on flow patterns and that such effects could be revealed by dye tracing. Tracer studies using the anionic dye Brilliant Blue FCF were conducted on a sandy loam soil (Agrudalf) subjected to four different tillage treatments: (T1) — harrowing two times with a springtine harrow, drilling; (T2) — direct drilling; (T3) — ploughing, light subsurface compaction, one harrowing with a PTO-driven rotary harrow, drilling; (T4) — ploughing, one harrowing with a springtine harrow, drilling. Studies were conducted (i) in the autumn after plant emergence 4 weeks after tillage and planting, and (ii) in the spring 7 months after tillage and planting. Twenty-five millimetres of water containing 4.0 g l −1 of the dye was applied uniformly to the soil surface within 1 h. Plots were excavated 1 day after dye application. Stained flow patterns on 22 vertical 1.00 m 2 soil profiles from each treatment were photographed and subjected to image analysis. Deeply penetrating flow paths were found for all treatments both in the autumn and in the spring. The number of individually bounded, stained flow pathways per metre profile length averaged over depth in the 30–100 cm soil layer (NP 30–100) and the degree of coverage of profile faces with dye in the 0–20 cm layer (DC 0–20) were significantly affected by tillage treatment, both in the spring and in the autumn ( P<0.001). Averaged for the 22 profiles per treatment, NP 30–100 equalled to 2.0, 3.5, 0.7, and 1.8 m −1 for T1, T2, T3 and T4, respectively, while DC 0–20 equalled to 57, 44, 64 and 65%, respectively. Horizon boundaries and other observable structural features related to soil tillage and structural development appeared to be very important for the initiation of preferential flow.