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The pollution of the air from industrial plants (such as metal smelters) can cause considerable damage to plant life and soil. Some gases, such as sulfur dioxide, released by factories dissolve in rain to form a weak acid, which is extremely harmful to the environment and to human health. Quarrying and mining, particularly open-cast mining, leave soil unprotected and accelerate erosion. Attempts are made in some places to refill open mined areas with topsoil and to plant vegetation on it to reduce the damage. But this is not always done and erosion follows. In addition, rain leaches salts out of the exposed soil, which increase its acidity and the acid run-off sterilizes the soil down slope. Mining also leads to subsidence of the land - which damages buildings and roads - and heaps of waste material mar the landscape. Deforestation
To create new land for agriculture and settlement and to satisfy the need for timber the world's tropical rainforest is being felled and burned at a rate of about 110,000sq km a year, at this rate it will have disappeared completely in 50 years.
Forest removal has been practiced for centuries, but the onslaught on the tropical forest is especially drastic because its complex of soils and vegetation is so fragile. The chief source of plant nutrients is in the vegetation rather than in the soil; if the vegetation is removed, much of the land's fertility is lost. Once a forest is cleared, a greater degree of rainfall runs off the soil surface directly, and soil erosion is increased. The erosion of topsoil makes the land less productive and also causes land and waterways down-stream to silt up. In the Himalayas, for example, farmers and peasants in the past gathering fuel denuded hills, thus increasing erosion and flooding in the lower plains. In the 1930s, the United States suffered badly from soil erosion. The North American plains had been deforested and the exposed topsoil was blown away, leaving a barren, extensive dustbowl. Soil usually develops very slowly, even when undisturbed under natural vegetation. It may take from 100 to 400 or more years to generate 1cm of topsoil. On cultivated land the soil is usually unprotected by crops or vegetation for part of the year, and the rate of soil loss is almost always higher than the rate of soil formation. It is estimated, for example, that the amount of soil lost in one year from maize-growing land in Iowa, in the United States, is equivalent to the amount that forms naturally in eleven years. Soil erosion may result in a loss of both nutrients and soil-rooting depth. Whereas it may be possible, at a price, to replace nutrients by the application of fertilizers, the loss of rooting depth is irreplaceable.
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