Chemistry Reference
In-Depth Information
addition, soil pH is important for predicting nutrient availability and the
mobility and bioavailability of metals in soils. It is currently estimated that
58% of terrestrial semi-natural habitats across Great Britain receive acidic
deposition in excess of their buffering capacity, thus potentially causing long-
term damage according to the critical load methodology.
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Soil pH data from the CS between 1978 and 2007 has given a unique
national assessment of soil pH change over this time period. It was found that
pH increased over the time period in the majority of Broad Habitats, with soils
in general becoming less acidic. This agrees with recent independent work
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and is consistent with the expected benefit of continued reductions in sulfur
emissions. Data analysis continues to explore whether the smaller sulfur
deposition reductions experienced in the north (Scotland) and west (Wales) of
GB or other drivers of change, such as N deposition and land management, are
responsible for a lack of significant pH increase between 1998 and 2007 in
organic-rich soils that occur most commonly in this part of Britain. Land use
change was presumably not responsible for this pH change, as detailed analysis
separating plots where Aggregate Vegetation Class (AVC) had remained
unchanged from those where change had been recorded showed similar pH
trends.
One of the impacts of an increase in the mean soil pH across GB is that there
has been a concurrent increase in the number of soils with pH above 8.3.
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The
theoretical pH of a solution exposed to the atmosphere (p
CO2
5 10
23.5
atm) in
the presence of calcite is 8.3. The partial pressure of CO
2
in soils can be more
than 100 times higher than in the atmosphere so that pH values from 7.5 to 8.5
may indicate calcite saturation. When pH is controlled by the calcite system
only, soils do not in general exhibit structural problems since the abundance of
Ca provides good aggregate stability. However, when monovalent cations like
sodium accumulate in the soil, the pH can rise above 8.3 causing dispersion of
soil colloids, soil sealing, reduced infiltration rates and enhanced soil erosion.
Countryside Survey data collected from 1978, 1998 and 2007 showed that the
mean soil pH for locations with pH .8.3 and sampled in all three surveys had
experienced an increase of ca. 1 pH unit from 1978 to 2007. The majority of
these samples were taken from calcareous soil types or over calcareous parent
material, and predominantly in the Arable Broad Habitat type; farming tillage
practices may therefore be bringing calcareous minerals to the surface, which
when affected by sodium can cause a further increase in the pH. Sources of
sodium may include mineral weathering, salting of roads or atmospheric
deposition. Moreover, the region where soil pH is increasing is coincident with
the greatest decrease in acid atmospheric deposition over the last 20 years,
enhancing the stability of alkali minerals.
The implications of these findings are that current emission control policies,
combined with policies to protect soil through sustainable land management
practices, have had some major benefits in reducing acidity. However, there
may be a trade-off as alkaline soils may increase in abundance and become
more susceptible to structural problems if sodium levels in the environment
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