Soil quality in organic farming - effects of crop rotation, animal manure and soil compaction

Summary

The positive effects on soil tilth from cropping and fertilisation are prone to destructive forces from traffic and tillage. This presentation provides results from a project aiming to quantify the relative importance of crop rotation, fertilisation and traffic/tillage and their interactions.
The investigation includes a field trial established 1986 on loamy sand and another initiated 1997 on a sandy loam. All 1986-trial plots are grown with annual cash crops. Annual incorporation of animal slurry and straw residues is compared to reference plots receiving no organic matter. In the 1997-trial, a diversified crop rotation including grass ley and catch crops is compared to a sequence of annual cash crops excluding catch crops. The latter treatment is performed with and without annual application of animal slurry. In both trials, half of all plots were mechanically compacted by tractor the year prior to investigation. The 1986-trial also included part-plots intensively tilled with a rotary cultivator.
Results obtained so far in the 1997-trial indicate that soil organic carbon, microbial biomass, hyphal lengths and soil pore volume are significantly increased by manure application but especially by the diverse crop rotation. In contrast, clay dispersibility and wet aggregate stability were not significantly affected. Hence, biological soil attributes may serve as early indicators of soil quality changes. In the 1986-trial, significant effects of the organic matter treatment were apparent for some biological (microbial biomass), chemical (total and hot-water extractable organic C) and physical (bulk soil strength and friability) attributes. The mechanical disturbance by compaction as well as by intensive tillage caused a significant reduction in soil friability though partly alleviated by the organic matter-treatment.