Geography Reference
In-Depth Information
sometimes lead to large gullies forming, making it difficult to farm.
The sediment lost through erosion moves downstream and can
often silt-up roads, reservoirs and water courses. Some agricultural
chemicals are also bound to the sediment and so this pollutes water
courses. Erosion control often involves reducing grazing intensity,
planting strips of vegetation to form barriers to reduce the wind
and catch sediment, use of cover crops when the surface would
normally be left bare, more careful ploughing and building terraces
to reduce the slope and catch any sediment (so the landscape looks
like a series of steps coming downhill).
Soil pH has been reduced by the burning of fossil fuels which
has resulted in rainwater becoming more acidic. Also the harvest-
ing of crops and overuse use of nitrogen fertilisers causes acidifica-
tion. Soil acidification increases the solubility of heavy metals in
the soil which can be toxic to plants, reducing growth rates or
altering the types of plants for which the soil is suitable (e.g. forest
decline in central Europe). The soil organisms may also be affected
with species changing towards those which are more tolerant of
acidic conditions, resulting in a slower rate of litter decomposition.
Pollutants such as heavy metals, pesticides and fertilisers can
damage soil and may result in water contamination. Heavy metals
(metallic elements with a density greater than six grams per cubic
centimetre) such as copper, lead, zinc and mercury are present nat-
urally in soil, but atmospheric pollution and application of sewage
sludge, farm wastes and leaching from landfill sites can add heavy
metals to soil. The worst areas for heavy metal pollution are those
around industrialised regions such as in north-west Europe.
Mining, smelting, energy generation, agriculture and the wear of
vehicles and other machinery are sources of heavy metal pollution
to soils. Heavy metals build up in soil as they become bound to
organic matter and clay minerals and are generally not taken up by
plants. However, if the soil becomes more acidic this can result in
heavy metals being released into soil water and becoming available
for plants to take up, or for leaching into rivers, lakes and ground-
water. This is the crucial phase because if this happens then the
crops we eat may have toxic levels of heavy metals, or the water
becomes dangerous to drink.
Modern agriculture relies on pesticides for crop protection and
disease control, and fertilisers to provide additional nutrients such
