Gravitational
water
This is the water which drains through the soil under the influence of
gravity. Such drainage occurs through large soil pores. Small soil
pores have the ability to hold the water against the pull of gravity
through the process of capillarity. (Follow these links to learn more
about capillarity
and capillary
action.)
The next series of images will address drainage of the water from the
large pores in the sponge. Brief video clips are available in Quicktime
(QT) or RealPlayer
(RP) formats. If neither is available on your computer, click the name
to download it.
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Draining sponge, held flat -
When the sponge is removed from the water, and held flat, water drains
from it. This occurs rapidly at first (QT, RP),
then slows to a stop. As it slows,
notice the sponge is not equally wet from top to bottom. The top has
more empty pores than the bottom. (QT,
RP)
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Draining sponge,
short vertical
axis - If the sponge is turned from horizontal to vertical, the
drainage begins again. As before, drainage will be rapid for a time,
then slow to a stop.
(QT, RP)
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Draining sponge, long vertical
axis - If the sponge is turned one more time, the drainage will begin
again, then slow to a stop. Careful observation of the side of the
sponge from top to bottom will allow you to see which pores retain
water, and which ones drain. |
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Watch the bottom of the sponge just as it is turned. Pores will visibly
fill with water, then drain. |
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The only thing that changed as the sponge was turned is the
distance over which gravity could work. In the first case (horizontal
sponge), gravity only had about 4 cm over which it could pull water. In
the second case (short vertical axis), gravity could work through about
10 cm. In the last case (long vertical axis), gravity could pull water
for about 20 cm. Thus, each turn of the sponge provided more water to
be moved by gravity. Upon careful observation, you
noticed that water drained from the large pores in the sponge, but was
retained in the smaller pores.
Field Capacity - When
all the water that can has drained from the soil by gravity, the soil
is at field capacity. A field definition is the amount of water
remaining in the
soil 2 to 3 days after a soaking rainfall or irrigation when
evaporation from the surface has been prevented. A laboratory estimate
places a saturated soil sample on a porous ceramic plate and applies 33
kPA (1/3 bar or 5 psi) of pressure to force some water through the soil
pores, into the plate and out of the system.
This water exists in the soil capillaries, those pores that
are small enough to hold the water against the pull of gravity.
Clays hold the most water at field capacity, while sands hold
the least.
However, field capacity is affected strongly by soil
structure. The amount of aggregation in the soil determines the amount
of small pores available to hold water against gravity.
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| Capillary Water - This is the water
held against gravity in the small soil pores, or capillaries. This
water can be extracted by the plants, just as the next series of
activities with the wet sponge indicates. (Follow these links to learn
more about capillarity
and capillary
action.) |
Water extraction, wet sponge -
Beginning with the sponge at field capacity, when all gravitational
water had drained, begin to squeeze the sponge.
At first it takes little energy to extract water.
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As the sponge
becomes
progressively drier, more energy is required to extract water. (It has
to be squeezed harder.)
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When the sponge is nearly dry,
much more energy is required to extract just a little bit of water. |
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Likewise,
not all of the capillary water held in the soil is equally available to
the
plants. Plants are able to extract water easily from soils that are
near field capacity water contents. As the soil dries, the plants must
work progressively harder to extract water, until finally, the soil is
so dry that the plant can no longer expend enough energy to extract any
more water. This is the concept of wilting point. But, wilting point is
not the same for all plants. Some, like sunflowers, have the ability to
extract more water from soil than others, like corn. This first clip
might represent the ability of corn to extract water from the soil (QT, RP), while the second
clip starts where the first left off, and might represent the ability
of sunflowers to extract more water from a relatively "dry" soil (QT, RP).
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| Squeeze-dried
sponge - It is important to note that, although no more
water can be extracted from the sponge by squeezing it, it is not
"dry". If the sponge is allowed to sit on the counter for a week, it
will become drier as water evaporates from it. In the same way, bare
soil in a field, especially after a tillage event, can become drier
than wilting point. |
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Wilting Point, plant
unavailable water - When no more water could be squeezed from
the sponge, was it dry to the touch? The answer is no, it still retains
water. This water is not absorbed in the pores between the sponge
fibers, but
adsorbed (or adhered) to the fibers of the sponge. The same is true in
soil. All soils retain water that plants are unable to extract; clays
hold the most unavailable water while sands hold the least.
See the page on "Dry" Soils for more
information.
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Plant
Available Water - This is the
water that is considered available for plant use. It is calculated as
the difference between the field capacity and wilting point water
contents. As noted above it
varies for different types of plants and for different types of soils.
Though clays hold the most water at both field capacity and wilting
point, silt loams hold the most plant available water.
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