World’s Soil Resources

7.2.2 | Soil sealing
Soil sealing is most commonly associated with the expansion of urban areas and leads to a permanent,
non-reversible loss of agricultural land. Yields are eliminated, not just reduced and the soil, if completely
sealed, becomes effectively non-soil. Urbanization of agricultural land should thus be considered as a threat
to future food production, not only for the loss of good quality agricultural land but also because of the risk
of soil pollution through waste disposal and acid deposition from urban air pollution (Chen, 2007; Hubacek
et al., 2009; Clavero, Villero and Brotons, 2011). Blum and Nortcliff (2013) provide a very rough estimate of
daily losses of soil due to sealing at the global scale of 250-300 km 2 , and suggest this rate could increase due
to continuing migration of rural dwellers to urban areas. Thus, new policies that favour sustainable rural
development, oriented to avoid rural-urban migration as well as to support the return to rural areas of people
living in the cities, could avoid soil degradation and promote food security.
7.2.3 | Soil contamination
Soil contamination reduces food security both by reducing yields of crops due to toxic levels of contaminants
and by causing the crops that are produced to be unsafe to consume. As summarized in Chapter 6 (Section
6.3), there are worldwide tens of thousands of known contaminated sites due to local or point-source
contamination. In regions with a long-standing industrial base, the expansion of contamination is limited, but
in countries undergoing rapid industrialization or resource development the potential for the further spread
of contamination is great. The tremendous expansion of industry in China is one example of this: 20 million
ha of China’s farmland (approximately one fifth of China’s total farmland) is estimated to be contaminated
by heavy metals, and this may lead to a significant reduction in food availability (see also Section 6.3 above).
Contamination is also severe due to point sources such as Cs pollution from the Fukushima Dai-ichi nuclear
power plant and the Chernobyl disaster of 1983. Diffuse soil contamination occurs in many regions (Blum
and Nortcliff, 2013), but is more commonly linked with concerns about food safety rather than significant
decreases in crop yields.
7.2.4 | Acidification
Acidification of agricultural soils is primarily associated with the net removal of base cations (e.g. product
removal without replacement with ameliorants such as lime) and the direct addition of acidifying inputs (e.g.
ammonium-based N fertilizer) to inherently low-pH soils, which have a low capacity to buffer added acidity. It
is most prevalent on ancient, highly weathered soils. Acidification is a significant regional threat in countries
such as Australia and Vietnam (see Chapters 10 and 15). Liming is an effective response to control acidity of
surface horizons, but rates of lime addition lag behind required levels even in developed countries like Australia
(SOE, 2011) and continuing loss of yield occurs.
7.2.5 | Salinization
Salinization in a soil progressively reduces crop yields; beyond a certain crop-specific threshold, growth of
a given crop may fail entirely. The regional summaries in Chapters 9 to 16 illustrate how difficult it often is to
separate the causes of salinization: whether the saline soils are naturally occurring (primary salinization) or
the salinization has been caused by inappropriate management, which is often the case with poorly executed
irrigation programmes (secondary salinization). Estimates from the 1990s place the land area affected by
primary salinization at approximately one billion ha, and the area of land with secondary salinization at 77
million ha (Ghassemi, Jakeman and Nix, 1995).
Salinization is typically associated with arid and semi-arid areas, and may be exacerbated by climate
change (see also Section 6.5). An increase in irrigated land is commonly suggested as a means to increase
food production, but poorly designed and implemented irrigation schemes can readily cause an increase in
Status of the World’s Soil Resources | Main Report The impact of soil change on ecosystem services
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