1. Soil Water

2. Section 1
The types of soil water
and measuring soil moisture content

3. The importance of soil water：
It is the major constituent of plant protoplasm.
It is essential for photosynthesis and conversion of starches to sugar .
It is the solvent in which nutrients move into and through plant parts.

4. What are the main components of soil?
Mineral Matter
Air
Water
Organic Matter

5. Classification of soil fractions
Classification System
USDA
International (ISSS)
DIN, BSI, MIT
Sand mm mm mm
Silt mm mm mm
Clay
< mm
USDA U.S. Department of Agriculture
ISSS International Soil Science Society
DIN German Standards
BSI British Standards Institute
MIT Massachusetts Institute of Technology

7. Spaces for Gas and Water
Partical size effects spaces for gases and water.
Water movement is dependent on the spaces

8. Adhesion and cohesive forces
This is called capillary water

9. 1、Classification of soil water
一、The types and available
of soil moisture
1、Classification of soil water
Adsorbed water
Capillary water
Gravitational water

10. （1） Soil adsorbed water ：
held by strong electrical forces - low energy
little movement- held tight by soil
exists as a film
unavailable to plants
removed from soil by drying in an oven

11. Soil Water Adhesion Water- water attracted to solid surfaces
held by strong electrical forces - low energy
little movement- held tight by soil
exists as a film
unavailable to plants
removed from soil by drying in an oven

12. Sketch map of adsorbed water

13. （3） Soil capillary water ：
Capillary water-The water held in the “capillary” or small pores of a soil,

14. Water is drawn up into the capillary tube
Capillarity：
0.1-1mm
Capillarity obvious mm
Capillarity strong mm
Capillarity very strong
〈0.001mm
Capillarity disappears
Water is drawn up into the capillary tube

15. Capillary water sketch map
Soil particle

16. （4） Gravitational water
Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.

17. Water adheres to soil particles Water held in large pores
Hydroscopic
Water
Capillary
Water
Gravitational
Water
Water adheres to soil particles
Water held in large pores
Available for crop use
Water drains through soil profile
Wilting
Point
15 bars
Field
Capacity
1/3 bar

18. What is Field Capacity?
when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity

19. Section 4、Control of soil water
一、Availability of soil water
Available soil water-The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of -1.5 MPa).

20. Estimating water contents
Gravimetric method: The soil sample is dried in an oven at 105°C and the mass of dry soil recorded.
Water potential
Neutron scattering method

21. Calculating Soil Moisture
Gravimetric
Pw = (weight of wet soil – weight of oven dry soil) X 100
weight of wet soil

22. Soil Water Potential Description
Measure of the energy status of the soil water
Important because it reflects how hard plants must work to extract water
Units of measure are normally bars or atmospheres
Soil water potentials are negative pressures (tension or suction)
Water flows from a higher (less negative) potential to a lower (more negative) potential

23. Components of Water Potential
Pressure potential: pushing (positive pressure, like the hose) or sucking (negative pressure, like a straw) Major factor moving water through plants
Osmotic, or Solute potential: reduction in water potential due to the presence of dissolved solutes
salty water has lower water potential (lower concentration) than pure water
Matric potential: reduction in water potential due to the presence of matric forces (tendency for water to adhere to surfaces)
pressure potential and Matric potential dominates soil water

24. Soil water potential
Total soil water potential = Matric potential + gravitational potential + Osmotic (salts)
As the soil dries the water potential decreases or a larger negative number
sat. wet dry > very dry

25. Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Vacuum Gauge
Porous Ceramic Tip

26. Units of soil water potential:
bars
MPa
-0.01
-0.001
-0.1
-0.33
-0.033 -1
-10
-15
-1.5
~FC
Note vapor pressure in atmosphere is ~2 kPa
~PWP

27. Soil Water Classification- a way to quantitatively describe the water in the soil.
= Field Capacity
-15 bar = wilt point
Between -0.3 & -15 is plant available water (AWC)
AWC
0 bar
Saturated Field Cap Wilt point

28. Water Moves through soil by bulk flow
The rate of water flow depends on:
Size of the pressure gradient
Soil hydraulic conductivity (SHC)
Measure of the ease in which water moves through soil
SHC varies with water content and type of soil
Sandy soil high SHC
Large spaces between particles
Clay soil low SHC
Very small spaces between particles

29. Water moves from areas of high potential
(wet soil : -2 or -4) to areas of low
potential (dry soil -8)
-.4 -3 -7 -8 -2
Root
Soil
Soil

30. The End

31. Water secretion

32. Water equilibration method

33. Pressure Chamber

37. Root and root hairs

38. Root hair

44. (b) A close-up of the stele of the buttercup root.
Cortex cells
filled with
amyloplasts
Endodermis
cell
Pericycle cell
Phloem cell
Xylem vessel
elements
Figure 6.4: Structure of an herbaceous eudicot root.
Intercellular
space
(b) A close-up of the stele of the buttercup root.
Note the solid core of vascular tissues.
Fig. 6-4b, p. 116

45. Casparian strip

53. Root hairs increase surface area and make intimate contact with components of the soil.
Soil Structure

56. Root Absorption of Water and Solutes
\figures\ch04\pp04030.jpg

57. Symplast and Apoplast

58. Apoplast vs Symplastic transport

59. Apoplast vs. Symplast

60. Ranunculus root tip: undeveloped

61. Ranunculus root tip: functional

62. Casparian Strip

66. Root Hairs –
increase absorption
develop in region of maturation
extensions of epidermal cells