Soil Water

1. Soil water

3. Types of water in soil:
 1. Adhesion water
 “HYGROSCOPIC WATER”
 Remove by oven drying
 Not available to plants

5. 2. Cohesion water
 “CAPILLARY WATER”
 Remove by air drying
 Most is available to plants
 some unavailable to plants (especially in clay or
high OM soils)
 15 – 20 molecules thick

7. Difference between wilting point and
hygroscopic coefficient:
Moist Dry to touch
Can’t squeeze water Air-dried
Plant can’t get water Can be oven dried to
remove water
at wilting
point
at hygroscopic
coefficient

8. 3. Gravitational water
 Not available to plants
 Drains through soil under influence of
gravity
 Through large pores
 Small pores can hold water against pull of
gravity through capillarity

10. Gravitational
water
Plant-available
Water
(capillary)
Plant-
unavailable
water
(capillary)
Adhesion
water
Field capacity
Wilting point
Hydroscopic
coefficient
Oven
dry
No water
(Air dry)

11. capillarity
Height water
will rise in
cylinder
depends on
diameter of
tube; due to
adhesion of
water and
tube
Plastic Glass

12. Critical levels of water in soil:
 Field capacity
 Wilting point
 Hygroscopic coefficient

13. Field Capacity
 Amount of water in soil after free
drainage has removed gravitational
water (2 – 3 days)
 Soil is holding maximum amount of water
available to plants
 Optimal aeration (micropores filled with
water; macropores with air)

15. Wilting Point
 Amount of water in
soil when plants
begin to wilt.
 Plant available
water is between
field capacity and
wilting point.

16. Hygroscopic coefficient
 Amount of moisture in air dry soil
 Difference between air dry and oven
dry amounts

17. Not all capillary water is equally available to
plants
 Plants can extract water easily from
soils that are near field capacity
 Sponge example
 Wilting point is not the same for all
plants
 Sunflowers can extract more water from
soil than corn
 Sponge example

19. Wilting point Field Capacity
Adhesion water
Micropores full;
macropores have air
Gravitational water
All pores full

20. Hydraulic pressure of soil water
 Pressure = force / area
Open body of water
“0”
at surface
increases with
depth
Hydraulic pressure

21.  Same in saturated soil
“0”
at surface
increases with depth

22. Capillary pressure
 Thin tube in open pan water
Pressure in tube
decreases away
from water surface
0
-10
-20 g/cm3
(Adhesion to walls of tube;
cohesion in center of tube;
therefore thin tube only)

23. Same in unsaturated soil:
 Capillary water is water in small pores
continuously connected to free water
surface (soil water table)
Capillary water
(continuous film)
Soil water table
Saturated soil
0
-10
-20
+10

24.  the smaller the pore space, the higher
capillary water will rise in profile
 Smaller pore space, tighter water is
held to particle surfaces against gravity
(i.e., higher field capacity)
Pan of water
clay silt sand

25. at
 Insert Fig 9.6

26. Energy status of soil water
 Energy status
 Things move to lower energy states
 It takes work to keep them from doing so
 E.g. keeping something from falling in response to
gravity
 Influences water movement
 E.g. adhesion attracts water to soil particles so
particles close to soil are at lower energy state

27. Forces on soil water:
 Adhesion
 Attracts water to soil particles
 Holds adhesion(hygroscopic) water and cohesion
(capillary) water
 Called “matric force”
 Ions in solution
 Attracts water to ions
 Called “osmotic force”
 Gravity
 Pulls water downward
 “gravitational force”

28.  Soil water potential
 Amount of work required to move water
 Expressed in bars or Pascals
 Similar to soil water tension

29. Water is held at various tensions/attractions

30. potentials
Water is removed by various potentials

31.  Water moves from areas of higher
water potential (wetter) to areas of
lower water potential (drier).

32. Potentials
 Matric
 Gravitational
 Hydrostatic
 Osmotic
 Total

33. Matric potential
 Work required to remove water held by
adhesion to soil surface and cohesion in
capillary pores.
 Hygroscopic and capillary water
 Zero (if saturated) or negative

34. Gravitational potential
 Work required to draw water down in
response to gravity
 Applies to gravitational water only
 Increases with increasing elevation above
soil water table
 Positive

35. Hydrostatic Potential
 Work required to move water below the
water able; applies only to saturated
conditions

36. Osmotic potential
 If there are solutes in the solution,
water will group around them and reduce
the freedom of water movement, i.e.,
lowering the potential.

37. Osmotic potential
 Water containing salts is less able to do work
than pure water
 e.g., cannot boil at standard boiling point
 The more salts, the lower (higher absolute
value) the potential
 negative
 Important for plant uptake
 In salty soil, potential in soil solution may be lower
than inside plant root cells, impeding ability of
water to pass into plant

38. Total water potential=
Matric + osmotic + gravitational +
hydrostatic

39.  Unsaturated flow: water movement in
soils at less than saturation
 Water moves in response to water potential
gradient (high to low)
 Saturated flow: moves according to
gravitational potential only

40. Hydraulic conductivity
 Ability of a soil to transmit water
 Depends on :
 Pore size
 Coarse grained soil has higher cond. than fine-
grained because movement through large pores
is faster
 Amount of water in soil
 Cond. decreases as water content increases
 Water moves through largest pores first

41. Water uptake by plants
 >90% by passive absorption:
 “Domino effect” of water in a continuous
film being drawn up column from soil
through plant cells, as water is lost by
transpiration
 No energy required

42.  Active absorption