Mitigation of subsoil recompaction by light traffic and on-land ploughing

Plough pans have been shown to severely hamper root development, limit rooting depth and reduce crop yields. We evaluated the effect of plough pan re-compaction on root and yield response for winter wheat in a field trial conducted in two neighbouring fields on a sandy loam. Plots were mechanically loosened by a subsoiler to a depth of 35 cm in 1997 and 1998. In 2 years following the loosening operation, perennial grass/clover was grown with limited traffic intensity. Subsequently oats were established and followed by winter wheat. On-land ploughing was compared with traditional mouldboard ploughing. In addition, the plots were either heavy-trafficked (10-18 Mg axle load and ∼200 kPa inflation pressure) or light-trafficked (<6 Mg axle load and <100 kPa inflation pressure). The loosened treatments were referenced by non-loosened soil. Root growth of winter wheat was followed applying the minirhizotron technique. In one of the fields, these measurements were supplemented with core sampling for root length determination approximately at anthesis. Soil water content was followed in one of the fields using time domain reflectometry (TDR). Grain yield and nitrogen content in grain were determined. The adjoining study showed that the combination of heavy traffic and traditional ploughing caused strong recompaction of loosened soil, whereas the combination of light traffic and on-land ploughing produced moderate recompaction. For the loosened plots in one field, the strongly recompacted soil produced 7% lower yield than moderately recompacted soil, whereas no clear difference was found for the other field. No clear difference between the loosened treatments on root growth was observed. Surprisingly, the non-loosened soil performed similar or even better than the loosened and moderately compacted soil. The non-loosened soil facilitated higher root intensity at depth and produced similar yield and N-uptake. Our results suggest that mechanical subsoil loosening of humid sandy loams only is recommendable in case of very severe subsoil compaction. Natural alleviation of subsoil structure induced by changes in soil management may comprise a favourable alternative to mechanical subsoil loosening.