Soil water
Types of water in soil:

1. Adhesion water

“HYGROSCOPIC WATER”
Remove by oven drying
 Not available to plants

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
Difference between wilting point and
hygroscopic coefficient:
at wilting
point
Moist
Can’t squeeze water
Plant can’t get water
at hygroscopic
coefficient
Dry to touch
Air-dried
Can be oven dried to
remove water
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
(Air dry)
Oven
dry
Hydroscopic
coefficient
Wilting point
Field capacity
No water
Adhesion
water
Plantunavailable
water
(capillary)
Plant-available
Water
(capillary)
Gravitational
water
capillarity
Height water
will rise in
cylinder
depends on
diameter of
tube; due to
adhesion of
water and
tube
Plastic
Glass
Critical levels of water in soil:



Field capacity
Wilting point
Hygroscopic coefficient
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)
Wilting Point


Amount of water in
soil when plants
begin to wilt.
Plant available
water is between
field capacity and
wilting point.
Hygroscopic coefficient


Amount of moisture in air dry soil
Difference between air dry and oven
dry amounts
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
Adhesion water
Micropores full;
macropores have air
Wilting point
Field Capacity
All pores full
Gravitational water
Hydraulic pressure of soil water

Pressure = force / area
Hydraulic pressure
“0”
at surface
increases with
depth
Open body of water

Same in saturated soil
“0”
at surface
increases with depth
Capillary pressure

Thin tube in open pan water
-20 g/cm3
-10
0
Pressure in tube
decreases away
from water surface
(Adhesion to walls of tube;
cohesion in center of tube;
therefore thin tube only)
Same in unsaturated soil:

Capillary water is water in small pores
continuously connected to free water
surface (soil water table)
-20
-10
0
+10
Capillary water
(continuous film)
Soil water table
Saturated soil


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
at

Insert Fig 9.6
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
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”


Soil water potential
Amount of work required to move water
 Expressed in bars or Pascals
 Similar to soil water tension

Water is held at various tensions/attractions
potentials
Water is removed by various potentials

Water moves from areas of higher
water potential (wetter) to areas of
lower water potential (drier).
Potentials





Matric
Gravitational
Hydrostatic
Osmotic
Total
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
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
Hydrostatic Potential

Work required to move water below the
water able; applies only to saturated
conditions
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.
Osmotic potential

Water containing salts is less able to do work
than pure water


The more salts, the lower (higher absolute
value) the potential


e.g., cannot boil at standard boiling point
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
Total water potential=
Matric + osmotic + gravitational +
hydrostatic

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
Hydraulic conductivity


Ability of a soil to transmit water
Depends on :

Pore size


Coarse grained soil has higher cond. than finegrained because movement through large pores
is faster
Amount of water in soil

Cond. decreases as water content increases

Water moves through largest pores first
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

Active absorption