Changes in the soil structure of two clay soils under contrasting systems of management
This study aimed to investigate soil structural changes in two clay soils under different forms of cultivation and different crops, and to identify the main processes responsible. The range of definitions, classifications, methods of determination of structure and the factors and processes which affect soil structure are reviewed. At Cruden Bay, Aberdeenshire, three sites were chosen at different stages in a seven year rotation and sampled throughout two growing seasons. At Compton Beauchamp, Oxfordshire, the effects of direct drilling, ploughing and permanent grazing on the soil structure were compared over one growing season.Rainfall and soil water content were recorded at both locations. Infiltration rates were recorded at Cruden Bay but variability in soil properties (texture, horizon thickness, structure) and climatic conditions at the three sites affected infiltration results so that its true value was difficult to assess. Plaster of Paris was used to preserve the seedbed for sampling, and to help identify field cracking and earthworm channels. Impregnated blocks were used to study structure. Macroporosity (pores >350 Jim) was measured by image analysis. Thin sections were prepared where necessary.At both locations differences in cropping and water extraction led to different field cracking patterns. At Compton Beauchamp pore continuity and earthworm activity were greater in the direct drilled and grass sites than in the ploughed site.The surface structure formed after seedbed preparation was composite subangular blocky/granular. The effects of shrinking was to create macropores, and the effects of swelling was to close macropores. Earthworm activity led to the development of channels, crescentic pores and vermicular structure. Since many of these processes are cyclic, the resulting structural states showed a cyclic recurrence.The results indicate the need for the concepts of incomplete and complete structure, composite structure and cyclical structural change.