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SOIL AND CROP MANAGEMENT EFFECTS ON
SEDIMENT AND PHOSPHORUS CONCENTRATIONS IN
RUN-OFF FROM AGRICULTURAL LAND
PJA Withers, RA Hodgkinson and A Bates
Catchment Management Group, ADAS Gleadthorpe, Meden Vale, Mansfield,
Nottinghamshire NG20 9PF, UK, E-mail: paul.withers@adas.co.uk
SUMMARY
To encourage the adoption of best management practices in a priority catchment
(Hampshire Avon) in south-west England suffering diffuse pollution, surface runoff
from field demonstration plots on different soil types and land management
treatments was monitored over two winter seasons. Late drilling increased run-off
up to fivefold, and mobilisation of sediment and phosphorus (P) by up to an order
of magnitude, compared with early drilling using traditional cultivation techniques.
Tramlines running up-and-down slope generated significantly greater run-off, and
often increased run-off sediment and P concentrations, than where tramlines were
absent, or running across-slope. Adopting reduced cultivations (minimum tillage)
either decreased run-off, the mobilisation of sediment and P in run-off, or lessened
the impact of tramlines. The results support the uptake of early drilling, timeliness of
cultivations to avoid soil compaction, better tramline management and non-inversion
cultivation techniques to help reduce agriculture’s impact on water quality.
INTRODUCTION
Sediment and phosphorus (P) concentrations in land run-off, and loads of these
diffuse pollutants entering surface waters, are a major environmental concern and
need to be controlled to acceptable levels in order to preserve or improve good water
quality. Much of the agriculturally-derived P entering waterbodies via surface and subsurface
run-off is associated with fine topsoil particles that have been enriched with
P from previous P fertilisation (Edwards and Withers, 1998). Combinations of erosion
vulnerable soils, over-cultivation presence of tramlines and lack of crop cover during
storm events significantly increase the risk of sediment and P transport, and off-site
impacts (Chambers et al., 2000). Control over run-off initiation and mobilisation of
sediment is therefore an important part of the integrated approach to catchment
management required for diffuse pollution control.
In 1999, a catchment management initiative called Landcare was started in the
Hampshire Avon river basin upstream of Salisbury, England to help reduce the
agriculturally-derived loads of pollutants, particularly sediment and P entering the
major tributaries (Huggins, 1999). As part of this initiative, farmer demonstration
plots organised by the Environment Agency (EA) for farmers and their advisers were
established at field sites on the three major lithologies that dominate the catchment:
Upper Chalk (Wilton), Upper Greensand (Pewsey) and Kimmeridge Clay (East Knoyle).
To supplement this demonstration activity, the plots were monitored over two winter
periods (2002/03 and 2003/04) to provide supporting data that would encourage the
adoption of more sensitive land management practices.
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