World’s Soil Resources

products increase soil acidity (Barak et al., 1997; Bolan, Hedley and White, 1991). Indeed, the conversion of
ammonium to nitrate releases hydrogen ions (H+) into the soil solution that can potentially lower the soil pH.
This is a problem in soils with low ability to buffer the increase in H+ such as those poor in lime and negatively
charged organic matter and clay. Harvesting has the potential to increase soil acidity by removing base cations
from the soil. This is an issue in both agricultural and forested areas wherever large amounts of biomass are
removed by crop harvesting and deforestation (Cavelier et al., 1999; von Uexküll and Mutert, 1995).
6.4.2 | Impact of soil acidification
On acid soils (pH < 5.5), crops and pastures suffer from the resulting increased phytotoxicity (Al, Fe, Mn, etc.),
from the reduced availability of nutrients, and from decreased microbiological activity (Cronan and Grigal,
1995; Robson and Abbott, 1989; Slattery and Hollier, 2002; Sverdrup and Warfvinge, 1993; Whitfield et al., 2010).
Onsite soil acidification reduces net primary productivity and carbon sequestration by accelerating leaching
of nutrients such as manganese, calcium, magnesium and potassium, resulting in nutrient deficiencies for
plants (Haynes and Swift, 1986). On-site soil acidification is also responsible for the development of subsoil
acidity (Tang, 2004), for the breakdown and subsequent loss of clay materials from the soil (Chen, 2007),
and for the erosion which results from decreased groundcover (Slattery and Hollier, 2002). Soil acidification
also leads to off-site effects such as surface water acidification through sediment losses, and groundwater
enrichment of soluble metals. In turn, these processes mobilize heavy metals into water resources and the
food chain (Driscoll et al., 2003; Reuss and Johnson, 1986; Schindler et al., 1980; Slattery and Hollier, 2002;
Voegelin, Barmettler and Kretzschmar, 2003).
6.4.3 | Responses to soil acidification
Soil acidification is an insidious process. It develops slowly and, if not corrected by lime applications
for example, can continue until the soil is irreparably damaged (Edmeades and Ridley, 2003; Liu and Hue,
2001; Slattery and Hollier, 2002). Biological recovery can potentially be improved by an increase in pH and
acid-neutralising capacity (ANC) (Marschner and Noble, 2000). Of main concern is subsoil acidity, which is
particularly difficult to correct with conventional methods (Farina, Channon and Thibaud, 2000; Liu and Hue,
2001; Hue and Licudine, 1999). Actions to mitigate global warming can reduce the emission of pollutants such
as sulphur dioxide (SO 2
) which contribute to soil acidification (NADP, 2014; Smith, Pitcher and Wigley, 2001;
Vestreng et al., 2007). However, soil response to decreases in acid deposition is slow and acid-affected sites
may require many decades to recover (Zhao et al., 2009).
6.4.4 | Global status and trends of soil acidification
Soil acidity is a serious constraint to food production worldwide. Traditionally it has been counteracted by
applying lime to the topsoil but little could be done to increase the pH of the subsoil. Programmes to improve
soil pH have been undertaken largely in developed countries, which are able to implement soil management
plans to preserve soil properties and to bear the cost of lime to buffer soil acidity. However, even in developed
counties, for example Australia, there have been cases where subsoil acidity increased due to failures in
correcting topsoil acidity. In developing countries the situation is more stark as the use of lime is constrained
by poverty. As a result, the farmed area affected by acidification is on the rise (Sumner and Noble, 2003). Soil
acidification affects not only agricultural areas but also forests and grasslands.
According to Sumner and Noble (2003), topsoil acidity (pH