Acid Rain

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.