Effects of Industry

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, releasedby 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 leachessalts 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 settlementand 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 rateit will have disappeared completely in 50 years.

Forest removalhas 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 undisturbedunder 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 lossis 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.

Overgrazing

Soil erosion is not confined to cropland, but also takes place on grazing land. If pasturesare over­stocked, or if grazing is uncontrolled, then the vegetation is adversely affected. Such reduction of plant cover occurs especially in the semi-arid areas where many of the world's pasture lands are located. If the vegetation cover is reduced, then the soil surface is less protected and erosion is accelerated. As population pressure increases, there is a tendency for agriculturists to move into more marginal areas and pastoralists are in turn displaced into more arid areas. In the Sahel of Africa, for example, which extends from Senegal in the west to Ethiopia in the east, overstocking severely degraded huge areas of land and contributed to the spread of the Sahara. In addition, the slash-and-burn agriculture practiced on the margins reduced the vegetation cover and a severe drought in that area further aggravated the problem.