Soil mixing and redistribution by strategic deep tillage in a sandy soil

Strategic deep tillage (SDT) is proposed as a single or occasional practice to help sustain the long-term productivity of the no-till system. Soil water repellence, herbicide resistant weeds, subsoil acidity and compaction are constraints that can evolve in long-term no-till systems and SDT may provide an effective solution for these. While the effect of SDT on the lateral and vertical movement of soil is understood, soil heterogeneity and mixing caused by SDT is not. Soil mixing may be an important factor for the availability of soil nutrients after SDT or for the effectiveness of soil amendments. Soil mixing and redistribution were quantified using coloured soil tracers and digital image analysis. Trenches of synthetic coloured soil were installed at four depths at a field site and four tillage treatments were applied; offset disc harrow and deep ripping (DHDR), disc plough (DP), mouldboard plough (MP) and rotary spader (RS). Digital images were acquired for soil pit faces at 5 cm intervals to capture the location of the coloured soil after SDT, these images were segmented and the 3-dimensional distribution of the coloured soil tracers was reconstructed. The mixing indices for the tillage treatments were low level (0.04-0.12). The vertical movement of soil differed between treatments; approximately 80, 60, 60 and 10% of soil from 0 to 10 cm was redistributed to deeper soil layers by MP, DP, RS and DHDR respectively. The soil profiles after the SDT treatments were composed of layered, or rotated soil that had been translocated within the working depth of the implement; however, there was some mixing at the interface between layers, or seams of soil that originated from different depths. The impact of a change in the spatial distribution of soil nutrients created by SDT on crop growth is an important knowledge gap.