SOIL WATER
CHAPTER 7
SOIL WATER
Functions:
plant cells 50-90% water
seed germination
photosynthesis
nutrients available
chemical reactions
keeps turgor
transpiration
moves products
lowers soil strength
microbial activity
Water Stress
Initially, decreased photosynthesis . . .
Continued . . .
temporary wilting point
further . . .
permanent wilting point
Forces on Soil Water
Gravitational – pull of gravity downward
Adhesion – attraction of water to soil
Cohesion – attraction of water to water
adhesion and cohesion result from shape of water
molecule and sharing of electrons in oxygen-hydrogen
covalent bonds
http://www.biology.arizona.edu/biochemistry/tutorials/chem
istry/page3.html
Polarity of Water
Polarity of Water
Effects of Water Molecule Polarity:
Hydrogen of one molecule attracted to oxygen of
another molecule in a hydrogen bond accounts
for cohesion
Hydrogen bond between hydrogen of water and
oxygen of silica (SiO ) accounts for adhesion
Adhesion water is very tightly held!!!
Cohesion water can move and is available for use
2
Capillarity
Additive force of adhesion and cohesion
- can move against force of gravity
- small pores conduct capillary water
Soil Water Potential
Work water can do
Potential energy
Tendency of water to flow/move freely in soil
http://www.fhsu.edu/biology/ranpers/ert/wp_tut.htm
Water will always try to move from a state of high energy
to a low-energy state
The lower the soil water potential the more tightly water is
adsorbed to soil particles
Water POTENTIAL
Refers to the ability of water to
move in soil
More water in soil = More water potential
At saturation, potential is near 0 (zero)
As soil dries, values become more negative
Water is held more tightly by soil!!
WATER FILM – WATER POTENTIAL
Three Forces of Water Potential
Gravitational – potential energy due to gravity
positive
Matric – most common force; effect of soil on water
negative
Osmotic – special case of salty soils
negative
Total water potential is sum of three forces
Units of Potential
Official unit is the Pascal (Pa), kilopascal (kPa), or
Megapascal (MPa)
- common usage of older unit bar
- equivalent to 0.1 MPa or 100 kPa
Soil water potential is usually negative because of
negative matric potential
TYPES OF SOIL WATER
Gravitational – at saturation, will drain from larger
pores within 24 to 48 hours in well-drained soils
Available – can be absorbed by plants; held
between gravitational water and wilting point
Cohesion – held between gravitational and
adhesion (hygroscopic) water
Hygroscopic – held tightly by soil particles; air dry
REFRERENCE POINTS RELATED TO
SOIL WATER
FOUR CATEGORIES OF
SOIL MOISTURE
Chemically combined . . . unavailable
Hygroscopic . . . unavailable
Gravitational . . . moves downward by gravity
Capillary . . . taken up by plants
WATER RETENTION
Total water-holding capacity and available waterholding capacity are based on soil texture
WATER RETENTION
Medium-textured soils have the highest available
water-holding capacity e.g. Silt Loam
Organic matter influences water-holding capacity
Increases amount of available water
WATER MOVEMENT
Gravitational flow – moves by gravity
• occurs only under saturated conditions
• rapid in course soils – large pores
• usually percolation through soil profile
SATURATED SOILS
Sandy soil:
gravitational water moves rapidly downward
Clay loam:
gravitational water retained 2-3 days afterward
Once soils lose gravitational water (drain)
movement is by . . .
Capillarity – movement due to attraction between
water molecules and soil particles
Rapid in sandy soils but limited in distance
Slow in clay soils but may move great distances
WATER MOVEMENT
Unsaturated flow – lateral movement; capillary flow
• depends on unbroken films of water spreading
through connected capillary pores
• moves from moist to dry soil
• can move in any direction
WETTING FRONT
A distinct “line” where water is moving in soil –
Wet behind, Dry ahead
• Soils must be nearly saturated in order for the
front to advance; Why?
• Dry soil cannot “pull” the water deeper
• All the soil must be wet in order for the front to
advance
CAPILLARY RISE
Upward movement of water from higher to lower
potentials
Explains evaporation of water from soil to atmosphere
• Continuation of capillary rise when entire soil column dries
• Boundary in soil serves to protect from further losses
• Unsaturated flow only moves over short distances
• Saturated soil near the surface encourages capillary rise
Responsible for accumulation of salts at surface of soils
in dry climates and in potted plants
•
Effect of Soil Horizons
water flows differently in different textures . . .
stratified layers will slow percolation
Vapor Flow
occurs when water vapor moves from moist to
drier soil . . .
- condenses on cooler soil particles
- very slow
- minimal water moved
Preferential Flow
Saturated soil conditions . . .
water enters biopores or other soil channels
Increases infiltration and percolation
May also move pollutants!!!
How Roots Gather Water
Governed by Soil Water Potential
Root hairs draw from higher potential regions
Capillary flow moves water
Soil – Plant – Atmosphere continuum
Plants create “unbroken” column of
water
Driven by plant transpiration
Patterns of Water Removal
Plants will use water near the surface first
Oxygen is highest . . . Respiration drives uptake
As surface dries, plant roots grow deeper . . .
absorption shifts downward
If surface is rewetted, absorption shifts upward
Measuring Soil Water
Four methods:
- gravimetric measurements
- potentiometers
- resistance blocks
- neutron probes (mainly research)
Gravimetric
-
measures soil water content by weight
water content = moist wt – dry wt
dry wt
Example: soil sample at field capacity 162 grams
dry sample 135 grams
water content = 162g – 135g = .20
135g
Volume Basis
More useful – utilizes gravimetric water content
volumetric water content =
gravimetric water content x soil bulk density
water density
From previous gravimetric example . . .
If bulk density of soil is 1.4 grams per cubic cm,
and we know density of water is 1.0 g/cc
Volumetric water content =
.20 x 1.4g/cc = .28
1.0 g/cc
Soil Depth Basis
Measures “inches of water” per foot of soil
- Uses volumetric water content
- Simple calculation . . .
Inches water per foot =
12 inches x volumetric water content
Continue from previous example . . .
Inches water per foot soil =
12 inches x .28 = 3.36
Or simply stated . . . Each foot of soil depth
contains 3.36 inches of water assuming constant
soil conditions
Practical Measuring Devices
Gravimetric method not very practical management
More useful and practical are . . .
Potentiometers (tensiometers)
Resistance Blocks (gypsum blocks)
Potentiometers
-
-
Measure soil moisture potential at given levels
Water exiting tube creates vacuum
Measured by gauge/instrument
Function best at higher potentials
Resistance Blocks
-
Measure resistance of electrical flow between
two electrodes embedded in block buried in soil
- moist soil with ions of salts in solution carry
electrical flow
- resistance blocks designed to buffer salt
effects (gypsum accomplishes this)
- works well between field capacity and WP