Geoscience Reference
In-Depth Information
making soils at risk, and soil organic matter gives them
more stability. Unfortunately intensive cultivation of cash
crops of cereals, oilseed rape, sugar beet, potatoes and
vegetables depletes organic matter, and there is a need to
develop systems of crop and soil management to minimize
this loss.
Prehistorically and historically, highest rates of erosion
have followed woodland clearance and the onset of
cultivation. The chalklands of Wessex, the East Anglian
Breckland and the South Downs were mostly cleared of
woodland by 3,500 years BP by Neolithic and Bronze Age
cultivators. Erosion was most widespread at times of
maximum population pressure when rates of woodland
clearance were at their highest. In upland areas beyond the
limits of cultivation, erosion rates have speeded up from
the mid-eighteenth century onwards due to increasing
numbers of sheep, the burning of moorland for grouse
moors and the destruction of vegetation by industrial
pollution.
Since the Second World War British agriculture has
undergone an unprecedented intensification under
subsidy support policies from first the UK and then the
EU governments. The main trends in agriculture have
been:
has made extensive studies of erosion risk on calcareous
silt loams and silty clay loams on the chalkland of the
South Downs. Processes of slaking, disaggregation and
capping occur very rapidly on silty soils in rainstorms.
Most eroded soil is moved in rills and gullies, with sheet
erosion being of minimal significance.
Soil erosion continues to be a matter of concern for
government, land managers and the public. It would be
satisfying to report that conditions have improved in
More Economically Developed Countries (MEDCs), but
this remains doubtful according to Boardman and Poesen
(2006). The crisis is worsening in Less Economically
Developed Countries (LEDCs), where population growth
continues to fuel land clearance and continuing pressure
on marginal land (
Plate 19.8
).
It is difficult to quantify erosion rates and their negative
impacts on crop yields, and many of the estimates derive
from models rather than from field measurements. The
most widely used model is the
Universal Soil Loss Equation
(USLE) developed by the US Department of Agriculture
to predict sheet and rill erosion. The USLE is given by:
A = RKLSCP
where
A
= soil loss (kilograms per square metre per
annum),
R
= rainfall erosivity,
K
= soil erodibility,
L
=
slope length,
S
= slope gradient,
C
= cropping and
P
=
conservation practices. Numerical values for each factor
can be calculated from standard tables. USLE predicts only
the amount of soil moved, and not the amount of soil
moved out of a field or drainage basin. The latter is
estimated by another model, the
Sediment Delivery Ratio
(SDR). Predictions from SDRs are usually much lower
than predictions from USLEs, because much of the soil
moved is deposited downslope in fields, along fences and
hedgerows, as colluvium and as stream alluvium. Where
there is severe gully erosion, however, the reverse can be
the case. Reservoirs built for water supply or flood control
can become infilled quickly by eroded soils, and have
been used for estimating erosion rates.
1
Expansion of farming into new areas of marginal land,
cultivated for the first time.
2
Loss of field hedges.
3
Use of heavier farm machinery.
4
Use of power harrows to produce fine soil tilths for
seeding.
5
Change to continuous cropping systems at the expense
of rotation.
6
Change to the cultivation of winter cereals, with
consequent decline in winter stubble, thus putting
arable land at risk in the wet autumn and winter
months.
7
Increased stocking of sheep on the uplands, allied to
the use of fire and human trampling.
Erosion of cropland in the United Kingdom depends
crucially on three key factors: land management, soil
properties and rainfall. The widespread adoption of
winter cereals since about 1980 has meant that fields are
at risk in most years between October and January. Fine
seedbeds are produced, and rainfall events on these fine
surfaces give erosion rates that are an order of magnitude
greater than on land in a rough ploughed condition or
covered in crops. The cultivation of spring cereals, with
stubble in the fields throughout the winter, minimizes
erosion particularly at high-risk sites. Boardman (1991)
CONCLUSION
The properties of soils are important for determining soil
fertility. The health and welfare of the human race depend
upon the ability of soils to provide a sustainable yield of
good-quality food. The capacity of soils to do so reflects
their physical, chemical and biological properties.
Physical properties depend upon soil texture or
particle size, and soil structure or aggregation. These two
