Does organic agriculture reduce soil erodibility? The results of a long-term field study on loess in Switzerland

In a long-term field trial in northwestern Switzerland, the effects of organic and conventional land-use management on earthworm populations and on soil erodibility were investigated. A silt loam soil which had developed in deep deposits of alluvial loess characterised the study site. Three methods were applied to analyse soil erodibility, at three different periods between autumn 1992 and 1993: aggregate stability (measured in the laboratory by a high energy rainfall simulation and by percolation) and soil particle detachment (measured in the field by splash erosion). Earthworm biomass and density, as well as the population diversity, were significantly greater on the organic plots than on the conventional plots. Likewise, the aggregate stability of the organic plots, when determined by means of percolation, was significantly better. Therefore, erosion susceptibility is greater on plots farmed conventionally. On the other hand, splash erosion monitoring and simulated rainfall experiments only partially highlight differences in erodibility between the two main land-management methods. Future comparisons between the farming systems should also include farmer managed fields with greater differentiation in crop rotations and cultural practices like tillage, fertilisation and pesticide use.