Soil structure, organic matter and earthworm activity in a comparison of cropping systems with contrasting tillage, rotations, fertilizer levels and manure use

In order to assess long-term effects on productivity, environmental impacts and soil fertility of contrasting farming practices, six cropping systems, ranging from conventional arable without livestock to organic mixed dairy farming with few arable crops, have been compared since 1989 on a loam soil. A decline in soil structure quality was found in a conventional arable system with annual ploughing with no rotational grass. This system had higher bulk density and mean aggregate size than other systems, and lower levels of plant available water and aggregate stability. Opposite trends were related to the proportion of grass leys in the other systems and to their levels of soil organic matter. The latter declined markedly over 15 years in the conventional arable system, and there were smaller declines in most other systems. In an arable system without ploughing, but with rotary tillage in spring, organic matter was maintained and high structural stability was found. This system had high bulk density, but the proportion of small aggregates equalled that found in systems with ley. There were overall increases in earthworm density (84%), earthworm biomass (80%) and the density of earthworm channels (132%) in the topsoil between 1994 and 2004. Most of these increases were found in systems with 1–3 years of ley in the 4-year rotation. Low values of all earthworm parameters, and only minor changes over the period studied, were found in two non-organic arable systems without ley, indicating high pressure on soil fauna. Lower earthworm activity was found in the non-organic arable system with reduced tillage as compared to the non-organic arable system with annual ploughing. Thus, 50% leys in the rotation appeared desirable for the maintenance of satisfactory soil structure and earthworm activity. Though the deterioration of soil structure in the conventional arable system was not extreme, as the soil was well structured initially, the results may have implications for the sustainability of stockless arable systems on soils with a less favourable initial structure.