World’s Soil Resources

Estimated dominant topsoil pH
< 4.5 4.5 - 5.5 5.5 - 7.2 7.2 - 8.5 > 8.5 Water Rocks outcrops, glaciers, salt flat
Figure 6.8 Estimated dominant topsoil pH. Source: FAO/IIASA/ISRIC/ISS-CAS/JRC, 2009.
The main causes of soil acidification vary by region:
• Regions where soil acidification occurs because of soil texture – parts of North America, Southeast,
East and South Asia (Aherne and Posch, 2013; Eswaran et al., 1996; Hicks et al. 2008; Shamshuddin et
al., 2014; Ouimet et al., 2006)
• Regions where proximity to deltas and coastal plains is a primary cause – parts of West Africa (Bullock
et al., 1996),
• Regions where weather conditions are a main cause – parts of Africa and Asia (Breuning-Madsen and
Awadzi, 2005; Drees, Manu and Wilding, 1993; Eswaran et al., 1996; Kottek et al., 2006; Wilke, Duke and
Jimoh, 1984),
• Regions where acid deposition is an important factor – parts of East Asia and North America (Aherne
and Posch, 2013; Quinn, 1989; Wolt, 1981)
• Regions where the massive application of ammonium-based fertilizers plays an important role – parts
of East and South Asia (Guo et al., 2010; Wang, Zhang and Zhang, 2010).
In Europe, soil acidification is an issue only in some highly urbanised and industrialized hotspots (EEA,
2010; Kopáček et al., 2004; Menz and Seip, 2004; Moser and Hohensinn, 1983). In the Southwest Pacific, soil
acidification is of concern only in intensively farmed areas (Brennan, Bolland and Bowden, 2004; Hartemink,
1998; Xu et al., 2002; Lockwood et al., 2003; NLWRA, 2001). Thus soil acidification affects all regions to some
extent, but it is of main concern in poor and developing countries which are growing rapidly but are unable to
buffer the decrease in soil pH through conventional means.
6.5 | Global status of soil salinization and sodification
6.5.1 | Status and extent
Salt-affected soils occur in more than 100 countries and their worldwide extent is estimated at about 1
billion ha. Salt-affected soils include those affected by salinity, where the electrical conductivity of the soil
exceeds 4dSm–1; and those affected by sodicity, where the exchangeable sodium percentage (ESP) exceeds 6
(Ghassemi, Jakeman and Nix, 1995). Saline soils contain excessive soluble salts, mainly sodium chloride (NaCl)
and sodium sulphate (Na 2
SO 4
) or other neutral salts. These salts increase osmotic pressure, diminish water
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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