Lecture 6: Soil Physical Properties;
Behavior of Water in Soil Pores
Porosity
Percentage of soil volume that is
taken up by holes
Soil Porosity
• Soils contain about 50% porosity unless compacted
• Pores occur in a range of sizes, each having different
effects on soil properties
• Macropores: >0.08 mm in size, allow for ready
movement of air and water in soil
– Accommodate plant roots and small soil animals
– Primary control on water drainage
• Micropores: <0.08 mm is size
– Air does not move through these pores easily
– Almost always water filled, only some is available to plants
Distribution of Pores
Example medium-texture soil with good structure
Types of Soil Macropores
Packing Pores
• Between aggregates or
sand grains
• Common in A horizons
Interped Pores
• Between soil peds
• Common in B horizons
• Size varies with soil
water content
Biopores
• Pores produced by roots
and soil organisms
• Can extend deep into soil
profiles but more
common near surface
Soil Micropore Classes
Note: Don’t memorize these! Know that micropores of different sizes have different effects.
• Micropores vary in their ability to provide water to
plants and habitats for microorganisms
• Mesopores (0.03-0.08 mm): Retain water after soil
drains; accessible to fungi and root hairs
• Micropores (0.005-0.03 mm): Found within peds;
retain plant-available water; microbial habitat
• Ultramicropores (0.0001-0.005 mm): Retain water
but not available to plants; exclude most microbes
• Cryptopores (<0.0001 mm): Exclude all microbes and
large molecules of OM; associated with clays
Estimating Porosity from Density
• Bulk density is easy to measure, and particle density
can usually be assumed to be 2.65 g/cm3 for a
mineral soil
• Using these values, one can estimate the percent
porosity of a soil:
%porosity = 100 – (DB/Dp x 100)
• Percent porosity is useful, in combination with bulk
density, for evaluating soil compaction, maximum
water holding capacity, and suitability for agriculture
See Box 4.6 for more information on calculating pore space
Key Concepts in Soil Physical
Properties: Part 2
• Soils show structure that vary with depth and soil type
and are determined by stresses on soil
– Soil aggregation is promoted by clays and organic matter
and improves water holding ability and aeration
• Soil bulk density is controlled by the soil texture and
porosity
– High bulk density prevents root penetration, inhibits water
drainage, and reflects low soil porosity
• Soil pores cover a wide range of sizes and are
important for air and water movement through a soil
Glass Capillary Experiment
• Glass is hydrophilic, like most soil solids, and will
attract water molecules due to adhesion
– Cohesion will bring additional water molecules along for
the ride, causing capillary action
• Explore how capillary action works and how it depends
on pore size (glass capillary diameter):
– Pick up a cup of colored water and one of each size glass
capillary: 1 (0.5 mm), 2 (1.0 mm), 3 (2.0 mm), and 4 (0.5 x
5 mm rectangle)
– Estimate the relative height of capillary rise for each
– Suggest a relationship between capillary rise and the
diameter/radius of the pore
– If time: Insert a narrow capillary into a wider capillary and
see how far the water rises compared to the same narrow
capillary by itself
Properties of Soil Water
Water is a Polar Molecule
Unique Properties of Water
Property
Heat Capacity
Heat of Vaporization
Dissolving Ability
Surface Tension
Heat Conduction
Comparison to Other Substances
Highest of all common liquids (except NH3)
Highest of all common substances
Dissolves more substances and in greater
quantities than any other common liquid:
Universal Solvent
Highest of all common liquids
Highest of all common liquids
Link to 73 Anomalous Properties of Water: http://www.lsbu.ac.uk/water/anmlies.html
Adhesion
and
Cohesion
Adhesion: Water sticking to solid surfaces
Cohesion: Water sticking to itself
Adhesion and Cohesion
Surface Tension (T)
Water Cohesion >> Adhesion to Air
• One measure of adhesive and cohesive forces
– For water: 72.86 mN/m at 20°C
Contact Angle (α)
Adhesion > Cohesion
Adhesion < Cohesion
• The contact angle (α) is a measure of the relative
strength of adhesion and cohesion
• Hydrophilic (water-loving) surfaces have small
contact angles: High adhesion
• Hydrophobic (water-hating) surfaces have large
contact angles: Low adhesion
Vertical Capillary Action: Competition
between Adhesion, Cohesion, and Gravity
• Capillary Action: Rise of water in small pores above level
of free water when α < 90° (hydrophilic soil)
– Adhesion pulls a layer of water upward along surface, and
cohesion pulls more water with it
• Capillary rise occurs until pull of gravity equals pull of
adhesion and cohesion
• For a tube/pore of radius r:
Capillary rise, h = 2T cos(α)/rρg
– ρ = water density, g = acceleration due to gravity
– Simplified version for soils: h (in cm) = 0.15/r (in cm)
Capillary Action versus Pore Size
Derivation of Vertical Capillary Action
• The force a solid surface exerts on water through
capillary action relates to:
– Strength of adhesion (from the contact angle)
– Strength of cohesion (from the surface tension)
– The length of the solid-water contact (tube circumference)
cos(α) x T x 2πr
• This is balanced by gravity pulling downward on the
volume of water in the capillary:
πr2 x h x ρ x g
• If you set these two terms equal and simplify:
h = 2T cos(α)/rρg
See Box 5.1 for derivation
Capillary Experiment Revisited
Capillary
0.15/r
α=0°
α=30°
α=45°
α=60°
α=75°
α=90°
0.5 mm
60
59.3
51.4
41.97
29.7
15.4
0
1 mm
30
29.7
25.7
21
14.8
7.7
0
2 mm
15
14.8
12.8
10.5
7.4
3.8
0
32.6
28.3
23.1
16.3
8.4
0
0.5 x 5 mm
• Glass capillaries (and real soils) may deviate from simple
predictions because of differences in contact angles or
other properties
Consequences of Hydrophilicity and
Hydrophobicity
• If soil particles are
hydrophilic:
–
–
–
–
α < 90°
cos(α) > 0
h>0
Capillary rise
–
–
–
–
α > 90°
cos(α) < 0
h<0
Capillary depression
• If soil particles are
hydrophobic
Capillary Rise Varies with Pore Size
Smaller pores lead to greater capillary
action but slower water flow
Note: Simple formula above assumes contact angle is 0, i.e., very hydrophilic soil
Capillary Action at Soil Surfaces
Horizontal Motion
Vertical Motion
• Capillary action occurs in all directions
– Horizontal motion controlled by flow rate and
available water rather than gravity
Types of Water in Soil
Unsaturated Soil
Saturated Soil
• Water occurs in soil in 3 distinct states in soil
– Adsorbed water: Molecules bonded to soil particle surfaces;
source of adhesion; unique physical and chemical properties
– Capillary water: Water held by cohesion to adsorbed water
– Pore/gravitational water: Water in large pores not held in
place by capillary action; readily drains from soil
Figures from: Introduction to Environmental Soil Physics by Hillel
Key Properties of Soil Water
• Water is attracted to itself (cohesion) and soil
particles (adhesion)
• Adhesion will pull water into small soil pores
through capillary action
• This may occur in both the horizontal and
vertical directions
– Horizontal movement is not limited by gravity but
is constrained by other factors (see next section)
• Water exists in three distinct forms in soil