The document discusses various concepts related to soil water, including:
- Saturation refers to when soil pores are 100% full of water
- Large pores drain quickly by gravity while smaller pores retain water due to capillarity and adhesion
- Field capacity is when gravity drainage stops and wilting point is when plants can no longer extract water
- Matric forces like capillarity pull water into small pores and toward the soil surface more strongly than gravity
- Not all water is under the same tension due to differences in pore size
- Characteristic curves relate soil water content to tension for a given soil type
2. What is the term for the moisture status of a soil
when its pores are 100% full of water?
Saturation
3. Why do large soil pores (aka macropores)
normally drain within a few days?
Large soil pores are drained by
GRAVITY
4. Why doesn’t gravity drain all the water out of soil pores ?
Field capacity Wilting point
When water is no longer When plants have extracted
drained by gravity as much water as they can
Capillarity and surface attraction combine to pull more
strongly than gravity on: 1) water in “micropores” and
2) water close to the “soil skin”.
5. Why doesn’t gravity drain all the water out of soil pores ?
Field capacity Wilting point
When water is no longer When plants have extracted
drained by gravity as much water as they can
Some water is held
too tightly to be
pulled away by roots
6. Pull of the soil matrix on H2O
surface attraction + capillarity
cohesion
H
+
Soil Skin
O adhesion
H
H
+ pull H2O
into small ?
O H pores
H
O
H
Hydrogen bonding
7. Pull of the soil matrix on H2O
surface attraction + capillarity
cohesion
H
+
Soil Skin
O adhesion
H
H
+ pull H2O
into small ?
O Water is pulled into
H pores
H
the micropores and
O toward the soil skin
H
by matric forces
Hydrogen bonding
8. What do I mean by “soil skin”?
Brady and Weil (2002) http://www.ccma.csic.es/dpts/suelos/
clay minerals humus
9. Soil circulatory system
Unavailable water
~0.2
μm
Wilting point
available
less
Plant available water
Most available
Field Capacity
10-30 μm
Gravitational water in
drainage pores
model soil pore Saturation
Adapted from Buol (2000)
10. Soil circulatory system
Unavailable water
~0.2
μm
Wilting point
available
less
Plant available water
Most available
Field Capacity
model soil pore
Adapted from Buol (2000)
11. Soil circulatory system
Unavailable water
~0.2
μm
Wilting point
model soil pore
Adapted from Buol (2000)
12. high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
Soil skin
Unavailable
water
Thickness of water film
Low energy H2O high energy H2O
13. high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
Soil skin
Unavailable
water
Thickness of water film
Low energy H2O high energy H2O
14. high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
Soil skin
Unavailable
water
Thickness of water film
Low energy H2O high energy H2O
15. high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
Soil skin
There is still some water in air dry soils!
Thickness of water film
Low energy H2O high energy H2O
16. Mars Lander probe finds no water in Martian soils
A conductivity probe on the Mars Lander sensed rising and falling humidity levels in the
Martian atmosphere, but when stuck into the ground, the probe found “Martian soil” to
be completely and perplexingly dry.
On Earth, “if you have water vapor in the air, every surface exposed to that air will have
water molecules adhering to it that are somewhat mobile, even at temperatures well
below freezing," said Aaron Zent , lead scientist for the Lander’s conductivity probe.
17. Soil water tension (aka potential) can be visualized
as the suction created by a hanging column of water
~1m
Field capacity Wilting point Air-dry
All of the following are equivalent:
150 m
1 m of H2O
There cm of many
are water
10,000 m
100
other methods
75 mm of mercury
-10 kPa
of expressing
-0.01 MPa
soil water
-0.1 bars
-0.0987 atmospheres
tension
-1.45 PSI
-1500 kPa -100,000 kPa
-15 bars -1000 bars
18. Soil water tension (aka potential) can be visualized
as the suction created by a hanging column of water
~1m
Field capacity Wilting point Air-dry
All of the following are equivalent:
150 m
1 m of H2O
There cm of many
are water
10,000 m
100
other methods
75 mm of mercury
-10 kPa You should be
of expressing with
-0.01 MPa familiar
-0.1 bars these units
soil water
-0.0987 atmospheres
tension
-1.45 PSI
-1500 kPa -100,000 kPa
-15 bars -1000 bars
19. Saturation
Are all of the water
molecules in this pore
under the same tension ?
29. How do
water
molecules
get from the
soil to the
top of a
plant?
30. H20
H20
H20
H20
Continuous H20
chains of water H20 The chain
molecules H20 moves upward
move upward if there is a
H20
through the negative energy
H20
xylem gradient
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
31. H20
H20
H20
H20
Continuous H20
chains of water Solar energy
H20
molecules drives
H20
move upward transpiration
H20
through the
xylem
H20 Plants provide
H20 the conduit
H20
H20
H20
H20
H20
H20
H20
H20
H20
32. Transpiration = air conditioning for plants
~ 4000 gallons H2O
per acre on a hot
sunny day
~ 30 gallons H2O
per corn plant per
season
33. The tallest living tree is a coast redwood that stands 112 meters
(367 feet, 6 in.), or ~ five stories higher than the Statue of Liberty.
Why don’t trees grow
any taller ?
Hydrogen bonding is only
strong enough to hold
together ~ 400’
of water
molecules
Cohesion
theory
34. Soil water is a switch that activates and
deactivates soil biology
Water is biologically available, when soil
organisms are able to win the
“tug of war” with the soil
35. Up to this point, we have been discussing
water tension
What is meant by the term water content?
37. Water content can also be
expressed volumetrically
v.m.c. = volume of water in soil / total soil volume
38. Why would you want to do this conversion?
Converting from gravimetric to volumetric MC
Volumetric
Gravimetric moisture
moisture Density content
content Bulk density of H2O
volume
mass of H2O mass of dry soil volume of H2O = of H2O
mass of dry soil * volume of dry * mass of H2O volume
soil of dry
soil
Gravimetric MC is easier to measure
but volumetric MC is more useful inappropriate
for expansive soils
for managing irrigation
39. Translating between
water tension (aka potential)
and water content using
a “characteristic curve”
A characteristic
curve (aka
water release
curve) describes
the relationship
between water
tension and
water content
for a specific
soil.
0
40. A pressure plate system can be
used to bring soil to specific
water tensions
Why are
all those
bolts
needed?
A known positive pressure is applied
inside the chamber. Soil water is pushed
out through a porous ceramic plate.
41. Different soils have different characteristic curves
Field capacity
Wilting point
Brady and Weil, 2002
42. Different soils have different characteristic curves
Field capacity
Wilting point
0.09 – 0.02 = 7%
Brady and Weil, 2002
43. Different soils have different characteristic curves
Field capacity
Wilting point
34% - 8% = 26%
Brady and Weil, 2002
44. Different soils have different characteristic curves
Field capacity
Wilting point
54% - 24% = 30%
Brady and Weil, 2002
45. Use the diagram to interpret how much water
is held in the clay @ saturation, FC and WP.
Calculate how many inches of water are
needed to bring a 3’ rooting zone of this soil
from 50% of FC to FC.
The volumetric water content @ FC = 0.54
0.54 * 36” = 19.4” of water @ FC
50% of 19.4” = 9.7”
46. Real soils rarely hold more than 2.5” of
plant available water per foot… based on
this fact, do you think the characteristic
curve for the clay soil is realistic?
The volumetric water content @ FC = 0.54
The volumetric water content @ WP = 0.24
PAW = 0.30
0.3* 12” = 3.6” >> 2.5”
47. So how does compaction impact soil water relationships ?
48. So how does compaction impact soil water relationships ?
Loss of drainage
pores
Gain in
small
pores
49. Which soil texture can hold the
most plant available water?
Field capacity line
~ 2.5” of plant
Plant
available water
Available
(PAW) per foot
water
Wilting point line
Brady and Weil, 2002
50. How does SOM affect PAW?
Adapted from Brady and Weil
51. How does SOM affect PAW?
Adapted from Brady and Weil
52. Prairie soil Farm field
Impressive example of the impact of
soil organic matter on
water holding capacity
53. So when should you irrigate a clay soil?
Wimpy crops
Tough crops
54. So when should you irrigate a clay soil?
loam soil?
Wimpy crops
Tough crops
55. Brady and Weil, 2002
So how does one measure soil water tension in the field?
A tensiometer is a Tensiometers are
water filled tube useful for montioring
with a porous tensions between
ceramic tip on one 0 and -85 kPa (-0.85 bars)
end and a vacuum a range that includes
gauge on the other. about half the water in
most soils.
Water tension in the
tube equilibrates When soils are too dry
with the water (> -85 KPa), air is drawn
tension outside the in through the porous tip
porous tip. and the vacuum fails.
57. Gypsum block
Measuring soil
moisture as a
function of
electrical
resistance Resistance
drops as
gypsum
starts to
Calibration is dissolve
critical !!
Brady and Weil, 2002
60. Time Domain Reflectometry
The technique involves
determination of the propagation
velocity of an electromagnetic
pulse sent down a fork-like
probe installed in the soil. The
velocity is determined by
measuring the time taken for the
pulse to travel down the probe
and be reflected back from its
end. The propagation velocity
depends on the dielectric
constant of the material in
contact with the probe (i.e. the
soil). Water has a much higher
dielectric constant than soil.
63. Why do the wetting fronts have different shapes?
Capillarity pulls the water farther in finer textured soils
http://www.ext.colostate.edu/mg/gardennotes/images/213-7.jpg
64. Capillary rise in a sandy soil
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
65. Capillary rise in a silt loam
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
66. What happens when capillary rise lifts water to the soil surface?
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
67.
68. How fast does water move through soil ?
Hydraulic conductivity
Darcy’s
Law
Flow rate = Area*Ksat *pressure/length
Brady and Weil, 2002
72. How does the presence of a coarse textured
layer under a fine textured layer affect
percolation ?
Fine textured layer
Coarse textured layer
73. Water will not
enter the coarse
textured layer
until the upper
layer is near
Coarse textured layer saturation
After water
enters the coarse
textured layer, it
will percolate
more quickly.
http://www.personal.psu.edu/asm4/water/drain.html
74. Does a layer
of sandy soil
improve
Layer with sandy texture drainage ?
NO ! Layer with sandy texture
76. What happens if a septic drainfield
does not drain adequately?
Can a drainfield drain too well?
http://organicearthsolutions.wordpress.com/2012/02/16/
77. Impact of topography on drainage
Poorly
drained Interstream
divide
Somewhat
Moderately
poorly LANDSCAPE
well drained
drained POSITIONS
Well
drained
Poorly
drained
Shoulder
Common in IL
Valley floor
SOIL Backslope
DRAINAGE
CLASSES
N.C. Agric. Res. Bull. 467
79. What is a
hydric soil?
http://www.il.nrcs.usda.gov/technical/soils/Suite_Maps.html
A hydric soil is a soil that
formed under conditions of
saturation, flooding or
ponding long enough during
the warm season to develop
anaerobic conditions in the
upper horizons.
Soils in which the hydrology
has been artificially modified
are still considered hydric if
the soil, in an unaltered
state, was hydric.
84. Artificial drainage in the United States
% of land drained
http://www.ars.usda.gov/SP2UserFiles/Place/36251500/TheExtentofFarmDrainageintheUnitedStates.pdf
87. Could this story be about your farm?
Increasing yield by installing drainage
By Mindy Ward, Missouri Farmer Today
BOONVILLE --- For more than 100 years, the
Hoff family has fought to farm wet areas of their
fields.
For Eddie Hoff, the fourth generation to farm the
creek bottom ground in Cooper County, the loss
of yield and added expense of working the
ground was ultimately affecting his bottom line.
“We were losing 60 to 70 bushels per acre in
some spots,” he says.
We were working the ground over and over. I
just wanted to no-till and save some cost.”
So, he decided to drain the soils with pattern tile.
88. Pattern Tiling in Ontario
http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm
92. Why do crops on tiled-drained land tend to
be more drought resistant ?
Ontario Ministry of Ag and Food
93. The current guide reflects recent developments
in drainage science and technology. Most of
these are related to new equipment and
materials, widespread use of computers, and
? water quality considerations. It includes
information not in the previous edition on
pipeline crossings, water and sediment control
basins, drain fields for septic systems, design
of drainage water management systems, and
design charts for smooth-walled pipes.
94. Conservation Drainage
Maximum conveyance
Crop productivity Environmental quality
Controlled
drainage system
Bioreactor
filled with
woodchips
http://wrc.umn.edu/prod/groups/cfans/@pub/@cfans/@wrc/documents/asset/cfans_asset_212844.jpg
95. Artificial drainage has greatly increased the number of days when
soils in the Upper Midwest are suitable for field operations and
deep root growth
but has also
contributed
Pollution of to some
water resources environmental Loss of SOM
problems
96. Which is worse??
Compaction Saturated soil is
probably extends less compressible
several feet deep than wet soil
97. What is the
optimum soil
moisture for
compacting
soil?
Soils are most compactible
near field capacity because
the particles are well
lubricated and the large
pores are empty and most
collapsible
98. Soil resistance to penetration is very
related to soil moisture content.
Healthy crops tend to use more
water which can result in higher
penetrometer readings.
99. Understanding Heat Capacity
A heat capacity of water =
1 calorie / gram / degree C
B
How much will the temperature of the water increase in cup A if
300 calories of thermal energy are added? How about cup B?
100. Why does soil heat up faster than water ?
The heat capacity of
water is ~ 5 times
higher than the heat
capacity of dry soil.
As a result, moist soils heat up and
cool down more slowly than dry soils.
101. Water has a
high thermal
conductivity
Air has a
low thermal
conductivity
What can be done to
maximize geothermal
heat transfer ?