Mechanical subsoiling: Mitigation of recompaction by light traffic and on-land ploughing

Summary

Plough pans have been shown to severely hamper root development, limit rooting depth and reduce crop yield. However, mechanically loosening of compacted subsoils often gives discouraging results. This has in many cases been related to a quick recompaction of the subsoil after loosening, as loosened subsoils are highly prone to recompaction. We evaluated the degree of recompaction of a mechanically loosened sandy loam. In addition, we studied the effect of plough pan compaction and subsoiling on root and yield response for a winter wheat crop. Plots were mechanically loosened to a depth of 35 cm in 1997 and again in 1998. Perennial grass/clover was grown with limited traffic intensity in 1999-2000. The perennial grass/clover was ploughed under in spring 2001 and oats established. Winter wheat was established in the autumn about a month after the harvest of oats. A recompaction experiment was conducted in 2001 and 2002 using the plots from the subsoiling experiment. On-land ploughing was compared with traditional mouldboard ploughing. In addition, the loosened plots were either heavy trafficked (>6 Mg axle load and high pressure tires) or light trafficked (<6 Mg axle load and <100 kPa inflation pressure). In this paper results from the combination of on-land ploughing and light traffic (NINV-OL) and the combination of traditional ploughing and heavy traffic (NINV-TH) will be presented. The loosened soils were referenced by a conventional ploughing-harrowing tillage system (CONV). The CONV soil was treated with on-land ploughing and light traffic from 2001 and experienced traditional ploughing for the last time in 1998. Penetration resistance was recorded in the field. Undisturbed soil cores were taken in 1999 and 2002 from the 18-27 and 25-30 cm layer and used for measuring porosity, air permeability, Ka, and pore continuity, PO, at –100 hPa matric potential. Root growth of winter wheat was followed during the growing season applying the minirhizotron technique and supplemented with core sampling at around anthesis (June 11). Grain yield was also determined. The results showed that the combination of on-land ploughing and light traffic mitigated recompaction of the upper subsoil. It was further found that the structural condition of the CONV plough pan was improved from 1999 to 2002. Ka,100 almost doubled for the 18-27 and 25-30 cm layers from 1999 to 2002. This suggest that the CONV plough pan had been biologically tilled resulting from high biological activity in the two years of grass ley (1999 and 2000) and a shift from traditional tillage and traffic to on-land ploughing and light traffic when growing cereals in 2001 and 2002. The CONV soil with a biologically ameliorated but still evident plough pan performed similar or better than the plough pan loosened and moderately recompacted NINV-OL soil. The CONV soil facilitated higher root intensity and produced similar yield. For instance, at around anthesis in 2002 (June 16), the fraction of grid frames at 94-104 cm depth with roots were 51, 31 and 14% for CONV, NINV-OL and NINV-TH, respectively. The deep rooting in CONV occurred despite root diameter data indicating that root growth was hampered in the plough pan layer of CONV and NINV-TH. Our results indicate that mechanical subsoiling may create even more constraints than benefits to crop development. Biological amelioration induced by appropriate changes in cropping system as well as tillage and traffic intensity comprise a favourable alternative to mechanical subsoiling for sandy loams with plough pan compaction.