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The soils at Wilton and East Knoyle were not as erosion vulnerable as at Pewsey
probably due to the more stable soil structure resulting from their higher clay, organic
matter and/or calcium carbonate contents. The key factors influencing run-off rates
on the chalkland site were timeliness of cultivation and the presence of tramlines
running up-and-down slope, which concentrated the flow. The comparatively low
run-off volumes and export of SS and P at East Knoyle reflect the beneficial effects
of subsoiling, and drilling across-slope, even though the site was relatively flat. The
consistent beneficial effects of non-inversion cultivation methods in reducing run-off,
and/or SS and P mobilisation under heavy rain at Pewsey and Wilton support earlier
work (Carter, 1998). These benefits reflect both better crop cover and improved
resilience to trafficking after drilling. The lack of any effect of reduced cultivation on
the heavier clay soil at East Knoyle can be related to the lack of crop residue cover
afforded by maize stubble.
In our study, run-off volumes were increased by up to 65% and SS and TP loads
were increased up to five- and fourfold, respectively, where tramlines were present.
Earlier initiation of run-off, the lack of any crop cover to protect the soil and the
channelling effect created by the depth and pattern of indentation left by the lugs
of the tractor tyre were key contributory factors. Variation in the impact of tramlines
will depend on the extent to which these key factors are represented. These data
strongly suggest that the timing of tramline establishment in relation to antecedent
soil moisture conditions can have an important influence on the risk of increased runoff
and erosion from these soils.
ACKNOWLEDGEMENTS
This work was jointly funded by the European Commission (DESPRAL project EVK1-
CT-1999-00007), Defra and the Environment Agency (EA). The authors also thank
Mr. A. Drake, Mr. T. Goodman and Mr R. Butler for hosting the demonstration plots
on their land, and to Mr. C. Westcott (EA) for organising catchment stakeholder
discussion.
REFERENCES
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Chambers BJ, Garwood TWD and Unwin RJ (2000). Controlling soil water erosion and
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Edwards AC and Withers PJA (1998). Soil phosphorus management and water
quality: a UK perspective. Soil Use and Management 14, 124–130.
Huggins R (1999). The Landcare project. In: T Petchey, B D’Arcy and A Frost (eds.)
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