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” 15 – 20 molecules thick
Remove by air drying
Most is available to plants
some unavailable to plants (especially in clay or high OM soils)

7. WILTING POINT Divides available and unavailable water
“the amount of water in the soil when plants have removed all that they can”

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

10. 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

12. FIELD CAPACITY Divides capillary water from gravitational water
“amount of water after gravity has removed all freely drained water”

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

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

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

17. 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)

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

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

21. 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

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

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

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

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

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

28. 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)
clay
silt
sand
Pan of water

29. at
Insert Fig 9.6

30. Energy status of soil water
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

31. Forces on soil water: Adhesion Ions in solution Gravity
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”

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

33. Water is held at various tensions/attractions

34. potentials
Water is removed by various potentials

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

36. Potentials
Matric
Gravitational
Hydrostatic
Osmotic
Total

37. 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

38. 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

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

40. 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.

41. 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

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

43. 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

44. 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

45. How do plants get water?
Root hairs are in contact with soil water

46. Salts (nutrients) are in a more dilute solution in the soil than in roots (plant “sap”)
By osmosis, water (with dissolved salts) moves into root hairs.
From the roots hairs, water diffuses into other plant cells, eventually to xylem, which is the water conduit of the plant.

47. This “root pressure” pushes water up into the plant.
This mechanism is called “active absorption”
(requires energy)

48. “passive absorption” : Plants transpire during the day.
Transpiration is loss of water vapor to the atmosphere from Stomata (pores) on leaves.
In transpiration water is pulled from the plant, beginning at the leaf tip.

49. This pull from the leaf tip is transmitted all the way down to the roots.
“Domino effect” of water in a continuous film being drawn up column from soil through plant cells, as water is lost by transpiration.
Cohesion holds water together in a continuous film.
No energy required; roots are passive.