World’s Soil Resources

Recently some countries have implemented policies and programmes to encourage waste minimization.
These programmes of ‘Extended Producer Responsibility’ make producers responsible for the costs of
managing their products at the end of their life. This approach is expected to encourage the manufacture of
more environmentally-friendly electronic products (UNEP DEWA/GRID-Europe, 2005).
6.4 | Soil acidification status and trends
6.4.1 | Processes and causes of acidification
Soil acidity increases with the build-up of hydrogen (H+) and aluminium (Al 3
+) cations in the soil or when
base cations such as potassium (K+), calcium (Ca 2
+), magnesium (Mg 2
+) and sodium (Na+) are leached and
replaced by hydrogen or aluminium (Bolan, Hedley and White, 1991; Helyar and Porter, 1989; von Uexküll and
Mutert, 1995). The main causes of soil acidification are: (1) long term rainfall that results in on-site leaching
of base cations; (2) draining of potentially acid sulphate soils; (3) acid deposition when urbanization,
industrialization, mining, construction or dredging release acid substances into the air or water, causing offsite
acidification; (4) excessive application of ammonium-based fertilizers (e.g. ammonium sulphate) as part
of intensive agriculture cropping practices; and (5) deforestation and other land use practices that remove all
harvested materials, often resulting in a drop of the pH in the topsoil. Only the first of these five causes is a
natural phenomenon; all others are human-induced.
In natural ecosystems, soils become more acid with time. Consequently old soils, particularly in humid
climates or those developed from acidic rocks, are more weathered and acidic than younger soils or soils of dry
climates or those developed from more basic rocks (Helyar and Porter, 1989; von Uexküll and Mutert, 1995).
Soil acidification is of the greatest concern in soils that have a low capacity to buffer the decrease in pH and in
soils that already have a low pH, such as acid soils in highly weathered tropical areas (Harter, 2007; Johnson,
Turner and Kelly, 1982). Soil texture and soil organic matter content play an important role in the buffering
capacity of a soil and hence in determining how prone a soil is to acidification (Helyar and Porter, 1989; Steiner
et al., 2007). Light sandy soils poor in organic matter are the least buffered against acidification.
Acid sulphate soils contain metal sulphides which, when exposed to oxidation, produce sulphuric acid.
Inland, acid sulphate soils form naturally in aquatic ecosystems and also as a consequence of human-induced
changes to land use and hydrology. Structures regulating water flow such as dams, weirs and locks prevent
flushing of metals, salts and organic matter, and promote the build-up of acid sulphate soils. Acid sulphate
soils also form in coastal areas and are common in mangrove forests, saltmarsh, floodplains, and salt- and
freshwater wetlands (Lin and Melville, 1994; Pons, van Breemen and Driessen, 1982; Pannier, 1979). Due to the
abundance of metal sulphides in rocks, mining activities also foster the formation of acid sulphate soils (Dent,
1986).
The atmospheric deposition of sulphur dioxide (SO 2
), nitrogen oxides (NOx) and ammonia (NH 3
) leads to
acid deposition. This can affect not only areas near to the urban, industrial and mining sites where the oxides
are produced and released into the environment, but also sites located far away (Fanning et al., 2004; Menz
and Seip, 2004; Mylona, 1996; Orndorff and Daniels, 2004). The term ‘acid deposition’ includes both wet and
dry (gaseous) precipitation, usually in the form of acid rain or fog. Besides affecting the chemistry of soil and
water resources, acid deposition directly harms plants and fish. Acid deposition is currently a major concern in
fast-developing countries such as China (Chen, 2007).
Land use and soil management play a crucial role in determining the chemical characteristics of the soil.
Intensive farming practices that employ large inputs of nitrogen fertilizers and remove large quantities of
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