pH, and hence they effectively regulate
both acidity and anion flux.
In this article we define acidity in
relation to rain and soil, describe how
soil formation can acidify soil and water
as well as leach nutrients and aluminum,
show how humic acids buffer soils, and
finally discuss the impact of changing
land use and consequent vegetational
additional 10 to 12 kg/ha, and rain strongly
acidified to pH 4 can dissolve an
additional 50 kg/ha or about 1 kiloequivalent
per hectare. For comparison, managed
agricultural soils in the northeastern
United States may require limestone
additions of 50 to 100 keq/ha per annum.
Measurements of acid in soil are somewhat
analogous to measurements of acid
in rain. With clay minerals and organic
macromolecules acting as immobile anion
phases, soils resemble weak acids,
making a thermodynamic interpretation
of soil pH uncertain. However, certain
principles apply: increasing concentrations
of neutral salt decrease the measured
pH, whereas increasing dilution
with water increases pH. This salt effect
is significant in measuring acidification
of runoff.
The exchange acidity of soil is the
analog of the total acidity of rain and is
Summary. Acid rain is widely believed to be responsible for acidifying soil and water
in areas of North America and northern Europe. However, factors commonly
considered to make landscapes susceptible to acidification by acid rain are the same
factors long known to strongly acidify soils through the natural processes of soil
formation. Recovery from extreme and widespread careless land use has also
occurred in regions undergoing acidification. There is evidence that acidification by
acid rain is supenmposed on long-term acidification induced by changes in land use
and consequent vegetative succession. Thus, the interactions of acid rain, acid soil,
and vegetation need to be carefully examined on a watershed basis in assessing
benefits expected from proposed reductions in emissions of oxides of sulfur and
nitrogen.
succession. We wish to show that natural
soil formation is often more important
than acid rain in determining the acidity
of lakes and streams.
Acidity of Rain and Soil
Rain in equilibrium with carbon dioxide
in the air has a pH of about 5.6. Rain
at pH 4.6 is often said to be ten times
more acidic, but this neglects the acidity
of undissociated carbonic acid (H2CO3)
normally present in rain. The strong acid
necessary to reduce the pH of rain from
5.6 to 4.6 actually represents a twofold
rather than a tenfold increase in total
acidity (5).
The acidity of rain has also been expressed
in terms of the amount of limestone,
CaCO3, that can be neutralized or
leached from the soil (6), but the
amounts that can be dissolved by rain at
pH 5.6 are often ignored. An annual
rainfall of 1 meter per hectare at pH 5.6
can dissolve 400 to 500 kilograms of
CaCO3 per hectare, depending on the
values of the thermodynamic constants
used (5). Rain at pH 4.6 can dissolve an
defined in terms of hydrogen ions readily
exchanged by neutral salts. Acid humus
typically has an exchange acidity of
about 1 eq/kg, which is about 10,000
times greater than its acidity expressed
as pH. In mineral soils, much exchange
acidity is produced by hydrolysis of ionic
aluminum. Hence, at pH values that are
typical of strongly acid soils, ion exchange
sites on minerals are occupied
primarily by ionic aluminum and some
additional aluminum is complexed by
organic matter, giving a total exchange
acidity on the order of 1000 keq/ha.
Soil Formation
The more mature a soil is the more it
differs from its parent material, although
the degree of difference is strongly influenced
by geology. In humid temperate
climates soils are more acid than their
parent materials; water percolating
through freshly ground granite is alkaline
(pH 8 to 9), but soils developed from
granite are acid. There is sufficient acidification
that mature soils developed even
on limestone or marble can be acid (7).
SCIENCE, VOL. 221
on February 11, 201