Page 71 - Soil and water contamination, 2nd edition

58 Soil and Water Contamination

significant role in the ability to exchange and retain substances that are transported in the soil
solution. Both clays and organic matter have this important property and will be discussed
further in Chapter 4.
The soil texture composition can be described and classified in many different ways. The
soil texture classification adopted by the US Department of Agriculture (USDA, 1999), is
one of the most common classifications based on the content of clay (particles < 2 μm), silt
(2-50 μm) and sand (50 μm – 2 mm) (Figure 3.3).
Because most soils are not fully saturated with water (exceptions include peat soils,
submerged paddy soils, and periodically saturated gley soils), the soil gas phase is in direct
contact with the atmosphere. Respiration by microorganisms that are decomposing
organic matter and plant roots reduces the oxygen concentration of the soil air and greatly
increases its carbon dioxide concentration compared with the free atmosphere. The carbon
dioxide dissolves in the soil water to form carbonic acid . Along with acid compounds
derived from atmospheric deposition and organic acids formed as a result of organic matter
decomposition , the principal acid-forming compound in soil is carbonic acid. These

acids would lower the pore water pH , were it not that they promote the weathering and
dissolution of minerals. These buffering processes also act against temporary pH changes.
Whereas rainwater contains very few dissolved ions, the dissolution of soil minerals causes
the soil water to become enriched in ions. Some ions (in particular, nutrients such as
nitrogen compounds, phosphate , and potassium ) are extracted selectively from the water by
plants, via root uptake. On the other hand, evapotranspiration removes essentially pure water
from the soil, so that the ion concentration always exceeds that of fresh precipitation and
tends to build up in periods without rain. The increase in concentration may even result
in chemical precipitation of solid salts, as occurs in many saline soils in arid and semi-arid
regions.
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Besides weathering and dissolution of soil minerals, the exchange of H ions at
the surfaces of minerals and organic matter also buffers the acidity of the soil solution.
The total buffer capacity is determined by the existence of soluble and weatherable soil
minerals. It is the calcium carbonate , salt, and clay content s in particular that control the
buffer capacity of soils. Buffered soils are rich in such weatherable minerals and the pH of
these soils ranges between 6.0 and 8.5. Poorly buffered soils, such as sandy podsolic soils,
organic (peat ) soils, and intensively leached soils in tropical regions, lack these weatherable
minerals. The parent bedrock on which these soils have been formed often consists of inert
materials, such as nearly pure quartz sand, quartzite, or granite. Poorly buffered soils have
a low pH, with values ranging from 3.5 to 6.0. Note that under natural conditions the pH
of the topsoil is often up to two pH units below that of the underlying soil, because the
buffer capacity is first depleted in the topsoil. In contrast, the application of lime (calcium
carbonate) on agricultural soils may have raised the pH of the topsoil above that of the
subsoil.
The decomposition of organic matter further controls the redox potential of soils. The
presence of oxygen gas in soil causes the redox potential to be high. As mentioned above,
the oxygen consumption by microorganisms and plant roots depletes the oxygen in soil air.
The oxygen is replenished by diffusive exchange between soil air and the free atmosphere at
a rate that is greatly determined by the soil moisture content. Diffusive transport of gases

occurs much faster in the gas phase than in the dissolved phase . So, in soils with high soil
moisture content or in water-saturated soils the oxygen becomes rapidly depleted, which
causes a considerable decrease of the redox potential of the soil. As a consequence, the redox
potential is high in well-aerated sandy soils with low organic matter content . Conversely,
the redox potential is low in saturated soils with a large organic matter content, such as peat
soils.

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