soil water. section 1 the types of soil water and measuring soil moisture content
TRANSCRIPT
Soil Water
Section 1 The types of soil water and measuring soil moisture content
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.
What are the main components of soil?
• Mineral Matter
• Air
• Water
• Organic Matter
Classification of soil fractionsClassification System
USDA International (ISSS)
DIN, BSI, MIT
Sand 2.0-0.05 mm 2.0-0.02 mm 2.0-0.063 mm
Silt 0.05-0.002 mm
0.02-0.002 mm
0.063-0.002 mm
Clay < 0.002 mm < 0.002 mm < 0.002 mm
USDA U.S. Department of Agriculture
ISSS International Soil Science Society
DIN German Standards
BSI British Standards Institute
MIT Massachusetts Institute of Technology
Soil Components (volume basis)
Spaces for Gas and Water• Partical size
effects spaces for gases and water.
• Water movement is dependent on the spaces
• Adhesion and cohesive forces
• This is called capillary water
一、 The types and available of soil moisture 1 、 Classification of soil water
• Adsorbed water• Capillary water• Gravitational water
( 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
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
Sketch map of adsorbed water
( 3 ) Soil capillary water :
Capillary water-The water held in the “capillary” or small pores of a soil,
Capillarity :
0.1-1mm
Capillarity obvious
0.05-0.1mm
Capillarity strong
0.05-0.005mm
Capillarity very strong
〈 0.001mm
Capillarity disappears
Water is drawn up into the capillary tube
Soil particle
Capillary water sketch map
( 4 ) Gravitational water
Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.
Water held in large pores
Available for crop use
CapillaryWater
GravitationalWater
FieldCapacity1/3 bar
Water adheres to soil particles
Water drains through soil profile
HydroscopicWater
WiltingPoint
15 bars
What is Field Capacity?
• when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity
Section 4 、 Control 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).
一、 Availability of soil water
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
Calculating Soil Moisture
• Gravimetric
Pw = (weight of wet soil – weight of oven dry soil) X 100weight of wet soil
Soil Water PotentialSoil Water Potential• DescriptionDescription
– Measure of the energy status of the soil waterMeasure of the energy status of the soil water– Important because it reflects how hard plants must work Important because it reflects how hard plants must work
to extract waterto extract water– Units of measure are normally bars or atmospheresUnits of measure are normally bars or atmospheres– Soil water potentials are negative pressures (tension or Soil water potentials are negative pressures (tension or
suction)suction)– Water flows from a higher (less negative) potential to a Water flows from a higher (less negative) potential to a
lower (more negative) potentiallower (more negative) potential
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
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
• 00 -5 -8 -10 -15 -55 -100
• sat. wet - -------- dry------- > very dry
Tensiometer for Measuring Soil Water Tensiometer for Measuring Soil Water PotentialPotential
Porous Ceramic Tip
Vacuum GaugeVacuum Gauge
Water ReservoirWater Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
Units of soil water potential:
bars MPa
-0.01 -0.001
-0.1 -0.01
-0.33 -0.033
-1 -0.1
-10 -1
-15 -1.5~PWP
~FC
Soil Water Classification- a way to quantitatively describe the water in
the soil.
• -0.3 = Field Capacity • -15 bar = wilt point• Between -0.3 & -15 is plant
available water (AWC)
0 bar -0.33 -15
Saturated Field Cap Wilt point
AWC
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
Water moves from areas of high potential (wet soil : -2 or -4) to areas of lowpotential (dry soil -8)
SoilSoilRoot
-7
-2
-3-.4
-8
•Water secretion
Water equilibration method
Pressure Chamber
Root and root hairs
Root hair
Cortex cellsfilled withamyloplasts
Endodermiscell
Pericycle cell
Xylem vesselelements
Intercellularspace
(b) A close-up of the stele of the buttercup root.Note the solid core of vascular tissues.
Phloem cell
Fig. 6-4b, p. 116
Casparian strip
Root hairs increase surface area and make intimate contact with components of the soil.
Root Absorption of Water and Solutes
Symplast and Apoplast
Root Hairs –• increase absorption• develop in region of
maturation• extensions of
epidermal cells