2. Why doesn’t gravity pull all the water out of soil ?
Field capacity
Wilting point
When water is no longer drained by gravity
When plants have extracted
as much water as they can
Capillarity and surface interactions combine to pull more strongly than gravity on the water in micropores and the water close to the surfaces of soil particles.

3. adhesion pull H2O into small pores
Matric forces
surface interactions + capillarity
cohesion +
adhesion pull H2O into small pores
Water is pulled into the micropores and toward the soil skin by matric forces
= H2O H O
Soil Skin +
Hydrogen bonding

4. Soil circulatory system
Saturation
Field
Capacity
Drainage pores
Most available
10-30 μm
Wilting
point
Plant available water
available
less
~0.2 μm
Unavailable water
Adapted from Buol (2000)

5. Soil skin Distance from soil skin Low energy H2O high energy H2O
Unavailable
water
Distance from soil skin
high energy H2O
Low energy H2O

6. Relationship between water film thickness and moisture tension
Adhesive
water
Air dry
Soil feels wet
Plant available water
Low energy H2O
high energy H2O
Gravitational
water

7. There are many other methods of expressing soil water potential
Soil water tension (aka potential) can be visualized
as the suction from a hanging column of water
1-3.3 m
150 m
10,000 m
Field capacity
Wilting point
Air-dry
All of the following are equivalent:
-1 m of H2O
-100 cm of water
-75 mm of mercury
-10 kPa
-0.01 MPa
-0.1 bars
atmospheres
-1.45 PSI
There are many other methods of expressing soil water potential
You should be familiar with these units
-10 to -33 kPa
-0.1 to bars
-1500 kPa
-15 bars

8. Field Capacity Saturation
Do all of the water molecules in this pore have the same energy status ?
-10 KPa
-20 KPa

9. -1500 kPa
Wilting
point

10. -100,000 kPa
Air-dry

11. Ψtotal Understanding soil water potential = Ψgravitational + Ψmatric
+ Ψosmotic

12. Osmotic potential
Salt added

13. A continuous chain of water molecules is pulled up through the plant
Solar energy drives the process
Plants provide the conduit
H20
H20
H20

14. Soil water is a switch that activates or deactivates soil biology
Water is considered biologically available, when soil organisms are able to win the “tug of war” with the soil

15. The soil matrix presents its inhabitants with many challenges
Tortuous, loosely connected and highly constricted porosity
Structural
rigidity
The soil matrix presents its inhabitants with many challenges
Low quality
nutritional
resources
Moisture fluctuations

16. water potential (aka tension) and water content using
Translating between
water potential (aka tension)
and water content using
a “characteristic curve”
A characteristic curve describes the relationship between water tension and water content for a specific soil.

17. Why are all those bolts needed?
A pressure plate system can be used to bring soil to specific water tensions
After intact cores of soil are brought to specific tensions, the moisture content at those tensions can be determined.
Why are all those bolts needed?
A known positive pressure is applied inside the chamber. Soil water is pushed out through a porous plate.

18. Different soils have different characteristic curves
Field capacity
Wilting point
54% - 24% = 30%
34% - 8% = 26%
0.09 – 0.02 = 0.07
Brady and Weil, 2002

19. So how does compaction impact soil water relationships ?
Loss of drainage
pores
Gain in small pores

20. Impact of texture on soil water
Available water
21%
21% of 12”
~ 2.5”
Brady and Weil, 2002

21. SOM increases plant available H20
Adapted from Brady and Weil (2002)

22. Determining gravimetric soil moisture content
Collect sample. Weigh moist. Weigh after oven drying.
g.m.c. = (moist – dry soil mass) / dry soil mass

23. Converting from gravimetric to volumetric
of H2O
of dry soil
mass of H2O
mass of dry soil
mass of dry soil
volume of dry
soil
volume of H2O
mass of H2O = * *
inappropriate
for expansive soils

24. So when should you irrigate ?
Wimpy crops
Tough crops

25. Brady and Weil, 2002
Tensiometer

26. Measuring soil moisture as a function of electrical resistance
Gypsum block
Measuring soil moisture as a function of electrical resistance
Calibration is critical !!
Brady and Weil, 2002

27. “An affordable and practical substitute for tensiometric measurement in most agricultural and landscape irrigation environments.”

28. 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.

29. Neutron probe
A neutron probe contains a source of fast neutrons and slow neutron detector.  Neutrons are emitted from a radioactive source (e.g. Radium or Americium-beryllium) at a very high speed.  When these fast neutrons collide with a small atom such as the hydrogen contained in soil water, their direction of movement is changed and they lose part of their energy.  These “slowed” neutrons are measured by a detector tube and are calibrated to indicate the amount of water present in the soil.

30. Measuring infiltration rate

31. Visualizing water in a 1 foot layer of soil
Macropores
50% porosity
saturation 6”
Plant
available
H2O
2.5”
Total water at field capacity
3.5”
50% plant
available H2O
1.25”
12”
50% solids 6”

32. How much water is need to bring the soil to field capacity ?
What will happen if more than 1.25” of water infiltrates into this soil ?
How much water is need to bring the soil to field capacity ?
1.25”
50% plant
available H2O
Water will percolate deeper than 1’

33. How fast does water move through soil ?
Darcy’s Law
Hydraulic conductivity
Flow rate = Area*Ksat *pressure head/length
Brady and Weil, 2002

34. Permeability = Hydraulic conductivity
Flow rate ~ pore radius4

35. How does the presence of a coarse textured layer under a fine textured layer affect percolation ?

36. Water will not enter the coarse textured layer until the upper layer is near saturation
After water enters the coarse textured layer, it will percolate more quickly.

37. Does a thin layer of coarse material improve drainage ?
Thin layer with coarser texture
NO !

38. Systems for rapidly draining surface water should be open to the surface
Soil capped slit
Slit filled with coarse material

39. Slit trenching equipment
Outlets are needed !!

40. 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.

41. IL Permeability classes
In Illinois soil drainage groups are assigned a number (1 to 4) and a capital letter (A or B). The number indicates the degree of soil permeability. The letter indicates the natural drainage.
IL Permeability classes 1
Rapidly permeable
More than 6 inches per hour
Moderately rapidly permeable
2 to 6 inches per hour 2
Moderately permeable
0.6 to 2 inches per hour 3
Moderately slowly permeable
0.2 to 0.6 inch per hour 4
Slowly permeable
0.06 to 0.2 inch per hour
Very slowly permeable
less than 0.06 inch per hour

43. Fields at the Allison Farm

44. Open field ditch

45. Bioreactor vs. standard
tile outlet

49. One calorie is the amount of thermal energy required to raise the temperature of one gram of water by one Celsius degree.
3000 calories of thermal energy enters each cup. The temperature of the water on the left rises by 30 Celsius degrees. By how much does the temperature of the water in the cup on the right rise ??

50. Why does soil heat up faster than water ?
The heat capacity of water is ~ 5 times higher than the heat capacity dry soil.
As a result, moist soils heat up and cool down more slowly than dry soils.

51. Water has a high thermal conductivity low thermal conductivity
Air has a
low thermal conductivity
What can be done to maximize geothermal heat transfer ?
compacted vs. loose ?
moist vs. dry ?