| Agricultural Uses | Acid rain is rain that contains excessive concentration of
acidic radicals like nitrate and sulphate. It is a major
pollutant in Canada, Norway, Sweden, the United States
and many highly industrialized countries. Millions of
tons of sulphur dioxide and nitrogen oxide are discharged
into the atmosphere as gaseous particles which are
oxidized to sulphuric and nitric acids. When these oxides
of sulphur and nitrogen get dissolved in atmospheric
moisture, they cause acid rain. Similarly, sulphur dioxide
and nitrogen oxides emitted in the atmosphere by forest
fires and the burning of fossil fuels are converted into the
respective acids and precipitated as acid rains.
Usually, the pH of rain water is 5.6 to 5.7, but that of
acid rain can be as low as 2.0. The amount of dissolved
sulphur during annual rainfall ranges from 10 kglha (in
less industrial areas) to as much as 100 kg/ha (in highly
industrialized areas).
Acid rain affects plant growth. It also damages
foliage, impairs photosynthesis, destroys fish in lakes
and rivers, and affects marble, limestone and buildings
(especially stone masonry constructions). Lichens are
particularly sensitive to changes in pH and are used as
indicators of acid pollution. In many developing
countries, acid rain is caused by excessive burning of
wood, coal and petroleum products.
Though oxides of sulphur and nitrogen are principal
culprits in causing acid rain, emission of ammonia from
livestock waste, fertilizer applications and industrial
processes also contribute considerably to the formation
of acid rain. Certain volatile organic compounds can also
cause acid rain through photochemical reactions.
Acid rain is also known to lead to the dissolution of
toxic metals like cadmium, copper and aluminum which
otherwise remain adsorbed onto sesquioxides and
humus. |