Remediation of subsoil compaction by subsoiling and deep rooting crops

Soil compaction forms a major threat to the well-functioning of agricultural soils. By reducing the pore volume and continuity both crop growth and ecological services, like water infiltration and storage, can be negatively impacted. It is often most severe at the interface between topsoil and subsoil, just out of reach of regular tillage operations. At this depth a plough pan can be formed, restricting interaction between top- and subsoil for roots, and gas and water transport. In this study we looked at a combination of mechanical and biological remediation to alleviate this problem. The experiment was performed on a sandy loam field near Ghent, Belgium with a highly compacted plough pan, which almost completely restricted roots to reach the subsoil and was practically impermeable for gases. Subsoiling was performed once in three different maize-based cropping systems: forage maize in monoculture, a ley-arable crop rotation with two years of alfalfa and a maize-winter cereal rotation.

The mechanical remediation (subsoiling) clearly helped to break open the restricting plough pan. Rootablility and air permeability clearly increased, leading to a significant increase in maize yield. On a longer time scale, however, we observed that this loosened soil was very prone to recompaction. In the second year after the subsoiling the highly compacted plough pan returned. This same year also showed no difference between the subsoiled and untreated control in maize yield.

To see if deep rooting crops can help stabilize the loosened soil after subsoiling, this study included treatments with fodder radish and alfalfa as (cover) crops. These crops showed a high rooting density in the subsoil, especially where the tines of the subsoiler had passed. Although this did not seem to improve the overall physical soil quality, it did protect the soil from complete recompaction. The penetration resistance did not markedly increase after standard agricultural practice.