Geoscience Reference
In-Depth Information
Wilting
point
Evapotranspiration
Deficit of
soil water
Precipitation
Surplus of
soil water
Surplus of
soil water
Figure 11.5 Hypothetical soil-water bud-
get for a location in the midlatitudes of the
Northern Hemisphere.
Although precipitation
is relatively low from October to May, it is greater
than the amount of soil water lost through
evapotranspiration, causing a soil-water sur-
plus. During the summer months, however, the
wilting point is sometimes reached because
more soil water is lost by evapotranspiration,
even though more precipitation actually occurs
during that time of year.
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F
M
A
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J
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Time of year (months)
drops of water onto the surface of an upright bottle. Notice that
each drop is enclosed within a film that pulls water inward into a
rounded shape that withstands the force of gravity. Within soils,
surface tension works in the same way, holding water onto soil
particles until they are lost through evapotranspiration.
After the excess water has drained, the soil is left at
ield
capacity
, which refers to the maximum amount of water that
a soil can hold after free gravitational drainage ceases. If
additional precipitation occurs shortly after field capacity is
reached, then the soil will again become saturated and the cycle
begins anew. On the other hand, if drought conditions begin
and little rain falls, soil water will not be replenished. Even so,
soil water will continue to be lost through evapotranspiration,
resulting in progressively less water held by surface tension. If
dry conditions persist, the soil reaches the
wilting point
, which
represents the time when no soil water is available for plant use
and plants literally begin to wilt.
The term
soil-water budget
reflects the balance between
soil-water gains and losses. To understand this concept,
Figure 11.5 presents a simple example that reflects changes
over the course of the year in a typical continental region in
the midlatitudes in the Northern Hemisphere. During most of
the year, the region has a surplus of soil water, which occurs
from October through April. Interestingly, this surplus happens
to correlate with the time of year when the least precipitation
falls. Why does it happen at this time of year? First, very little
evaporation occurs because the Sun angle is low, temperatures
are cool to cold, and it is frequently cloudy. Second, plants are
not active during this time of year, so little soil water is lost due
to transpiration. During the summer months, in contrast, defi-
cits of soil water frequently develop, even though precipitation
is greater at that time of year, because the amount of evapo-
transpiration exceeds rainfall.
4.
Air
The air in soil is mostly carbon dioxide, which
plants give off during respiration and then take in during
photosynthesis. Soils contain so much carbon dioxide that
the soil layer is sometimes considered to be part of the
atmosphere and thus a transition to the solid Earth below.
The amount of air in the soil fluctuates depending on how
wet the soils are. As you can see in Figure 11.4, when
soils are saturated, the pore spaces are full of water and
thus contain little room for air. In contrast, when soils dry
out, the pore spaces contain less water and more air.
Soil-Forming (Pedogenic) Processes
A key concept to understand at this point is that soils form and
evolve through a complex sequence of interrelated
pedogenic
processes
. It is useful to think of these processes as consisting
of
additions
,
depletions
,
translocations
, and
transformations
that occur within the soil (Figure 11.6). The following discus-
sion illustrates how these processes operate individually.
Field capacity
The amount of water remaining in the soil
after the soil is completely drained of gravitational water.
Soil-water budget
The balance of soil water that involves the
amount of precipitation, evapotranspiration, and water storage
and loss.
Wilting point
The threshold amount of soil water below which
plants can no longer transpire water.
Pedogenic processes
The natural processes of soil formation
that involve additions, translocations, transformations, and losses.
