Five definitions: soil erosion, wetlands, primary air pollutants, eutrophication, compost

Five of the seven terms are defined, as the question allows: (i), (iii), (iv), (vi) and (vii).

(i) Soil erosion

Soil erosion is the movement of the surface litter and topsoil from one place to another by wind or flowing water. It is a natural process, but farming, construction, overgrazing, burning of grass cover and deforestation speed it up greatly. Losing topsoil makes land less fertile and less able to hold water, and the soil washed away muddies and clogs lakes and rivers. Gullies in the Chambal ravines show what unchecked erosion does.

(iii) Wetlands

Wetlands are ecosystems in which the water level rises and falls sharply with the seasons. They have stretches of shallow water with aquatic plants and are ideal habitats for fish, crustaceans and water birds; when they are drained, the rivers linked to them tend to flood. Chilika lake in Odisha and the Keoladeo marshes in Rajasthan are Indian examples.

(iv) Primary air pollutants

Primary air pollutants are those emitted directly from identifiable sources, both natural (dust storms, volcanic eruptions) and human (vehicles and industries). Five of them, carbon oxides (CO and CO2), nitrogen oxides, sulphur oxides, volatile organic compounds (mostly hydrocarbons) and suspended particulate matter, together make up about 90 per cent of global air pollution. Secondary pollutants such as ozone, by contrast, form later in the air from them.

(vi) Eutrophication

Eutrophication is the over-enrichment of a lake or river with nutrients, from sewage or from fertilisers washed off fields. The nutrients cause heavy growth of algae and plants; when these decay, the oxygen in the water falls sharply, fish and crustaceans die, a foul smell arises and the natural life of the water body is gradually destroyed. Lakes in many Indian cities choked with weeds show this process.

(vii) Compost

Compost is the dark, crumbly manure produced by composting, the controlled decomposition of organic matter such as kitchen and garden waste by micro-organisms; in aerobic composting the breakdown takes place in the presence of oxygen. It returns nutrients to the soil and keeps wet waste out of landfills; a compost pit in a college garden is a simple example.