SOIL LOOSENING AND DRAINAGE EFFICIENCY OF SILT SOILS

Abstract

An experiment was set up to investigate the effects of soil loosening on the drainage efficiency of a structurally unstable silty clay loam in Devon. Such soils are common to the Palaeozoic slates of South West England, and in many parts of Wales and Scotland. The dominance of the fine earth fraction these soils by silt sized particles and the associated low clay fractions ( 10-35%) gives rise to weak structures that are unsuitable f or mole drainage. Six drains were laid with permeable fill at a 20 m spacing on a gently sloping site at Seale-Hayne College Farm. A year after installation half the site was loosened to a depth of 0.4 m. The whole site was then cultivated and winter wheat sown. The study concentrated on the modification of the soil physical environment and water transmission routes following loosening operations for autumn re-wetting, the winter period and spring drying. Data collection involved the analysis of drainflow hydrographs and the measurement of soil water physical parameters. Results from an intensive twelve month monitoring period indicate that the modification of subsoil to improve drainage does not necessarily give the results expected . In this study it has been shown that loosening of an unstable silty clay loam did not enhance drainage, in fact the reverse was true which can have dire consequences in terms of trafficability and plant root growth. Loosening lead to an increased porosity which resulted in a greater, albeit temporary, retention of water within the disturbed zone. This means that loosened soils were wetter prior to a rainstorm and remained so for a longer period after rainfall. This response, in turn lead to significant reductions in soil shear strength, which has important implications for successful crop husbandry in the autumn and spring periods when field operations are necessary. Further work is required to assess the temporal persistence of soil loosening on soil physical conditions and crop responses.