Why doesn’t gravity pull all the water out of soil ?

When water is no longer drained by gravity

When plants have extracted as much water as they can

Capillarity and surface interactions combine to pull more strongly than gravity on the water in micropores and the water close to

the surfaces of soil particles.

Field capacity Wilting point

Matric forces

+

Hydrogen bonding

cohesion +

adhesion pull H2O

into small pores

surface interactions + capillarity

H

H

O= H 2

O

Water is pulled into the micropores and toward the soil skin

by matric forces

So

il Skin

Plant available water

10-30 μm

Drainage pores

Unavailable water

Adapted from Buol (2000)

Most available

Soil circulatory system

~0.2

μm

less

av

aila

ble

Field Capacity

Wilting point

Saturation

Soi

l ski

n

Distance from soil skin

Unavailable water

high energy H2O Low energy H2O

http://piru.alexandria.ucsb.edu/~geog3/concept_illus/279_sc.jpg

Adhesivewater

Plant available water

Air dry

Gravitationalwater

Low energy H2O high energy H2O

Soil feels wet

00

Relationship between water film thickness and moisture tension

1-3.3 m 150 m 10,000 m

Field capacity Wilting point Air-dry

There are many other methods of expressing

soil water potential

All of the following are equivalent:

-1 m of H2O-100 cm of water

-75 mm of mercury-10 kPa

-0.01 MPa-0.1 bars

-0.0987 atmospheres-1.45 PSI

Soil water tension (aka potential) can be visualized as the suction from a hanging column of water

You should be familiar with these units

-1500 kPa-15 bars

-10 to -33 kPa-0.1 to -0.33 bars

Saturation

Do all of the water molecules in this pore have the same energy

status ?

Field Capacity

-10 K

Pa

-20 K

Pa

Wiltingpoint

-1500 kPa

Air-dry

-100,000 kPa

Ψtotal + Ψmatric + Ψosmotic= Ψgravitational

Understanding soil water potential

Osmotic potential

Salt added

H20

H20

H20H20

A continuous chain of water molecules is

pulled up through the

plant

Solar energy drives the process

Plants provide the conduit

Soil water is a switch that activates or deactivates soil biology

Water is considered biologically available, when soil organisms are able to win the

“tug of war” with the soil

The soil matrix presents its inhabitants with many challenges

Structural rigidity

Tortuous, loosely connected and highly constricted porosity

Low quality nutritional resources

Moisture fluctuations

0

Translating betweenwater potential (aka tension) and water content using a “characteristic curve”

A characteristic curve describes the relationship between water

tension and water content for a specific

soil.

A pressure plate system can be used to bring soil to specific

water tensions

A known positive pressure is applied inside the chamber. Soil water is pushed

out through a porous plate.

After intact cores of soil are

brought to specific

tensions, the moisture

content at those tensions can be

determined.

Why are all those

bolts needed?

Different soils have different characteristic curves

Field capacity

Wilting point

0.09 – 0.02 = 0.07

34% - 8%

= 26%

54% - 24%

= 30%

Brady and Weil, 2002

So how does compaction impact soil water relationships ?

Loss of drainage pores

Gain in small pores

Impact of texture on soil water

Available water

Brady and Weil, 2002

35 - 14

21%

21% of 12”

~ 2.5”

SOM increases plant available H20

Adapted from Brady and Weil (2002)

Determining gravimetric soil moisture content

Collect sample. Weigh moist. Weigh after oven drying.

g.m.c. = (moist – dry soil mass) / dry soil mass

volumeof H2O volume of dry soil

mass of H2Omass of dry soil

mass of dry soilvolume of dry

soil

Converting from gravimetric to volumetric

volume of H2Omass of H2O* * =

inappropriate for expansive soils

So when should you irrigate ?

Wimpy crops

Tough crops

TensiometerBrady and Weil, 2002

Measuring soil moisture as a function of electrical resistance

Calibration is critical !!

Gypsum block

Brady and Weil, 2002

“An affordable and practical substitute for tensiometric measurement in most agricultural and landscape irrigation

environments.”

The technique involves determination of the propagation velocity of an electromagnetic pulse sent down a fork-like probe installed in the soil. The velocity is determined by measuring the time taken for the pulse to travel down the probe and be reflected back from its end. The propagation velocity depends on the dielectric constant of the material in contact with the probe (i.e. the soil). Water has a much higher dielectric constant than soil.

Time Domain Reflectometry

A neutron probe contains a source of fast neutrons and slow neutron detector.  Neutrons are emitted from a radioactive source (e.g. Radium or Americium-beryllium) at a very high speed.  When these fast neutrons collide with a small atom such as the hydrogen contained in soil water, their direction of movement is changed and they lose part of their energy.  These “slowed” neutrons are measured by a detector tube and are calibrated to indicate the amount of water present in the soil.

Neutron probe

Measuring infiltration rate

12”

6”

50% porosity

satu

rati

onMacropores

Plantavailable

H2O50% plant

available H2O

2.5”1.25”3.5”

Total water at field capacity

50% solids

6”

Visualizing water in a 1 foot layer of soil

50% plantavailable H2O

1.25”

How much water is need to bring the soil to field capacity ?

What will happen if more than 1.25” of water infiltrates into this soil ?

Water will percolate deeper than 1’

How fast does water move through soil ?

Flow rate = Area*Ksat *pressure head/lengthBrady and Weil, 2002

Darcy’s Law

Hyd

raul

ic c

ondu

ctiv

ity

Permeability = Hydraulic conductivity

Flow rate ~ pore radius4

Coarse textured layer

Fine textured layer

How does the presence of a coarse textured layer under a fine textured layer affect

percolation ?

http://www.personal.psu.edu/asm4/water/drain.html

Coarse textured layer

Water will not enter the coarse

textured layer until the upper layer is near saturation

After water enters the coarse textured layer, it

will percolate more quickly.

Does a thin layer of coarse

material improve

drainage ?

NO !

Thin layer with coarser texture

Slit filled with coarse material

Soil capped slit

Systems for rapidly draining surface water should be open to the surface

Slit trenching equipment

Outlets are needed !!

The current guide reflects recent developments in drainage science and technology. Most of

these are related to new equipment and materials, widespread use of computers, and

water quality considerations. It includes information not in the previous edition on

pipeline crossings, water and sediment control basins, drain fields for septic systems, design of drainage water management systems, and

design charts for smooth-walled pipes.

In Illinois soil drainage groups are assigned a number (1 to 4) and a capital letter (A or B). The number

indicates the degree of soil permeability. The letter indicates the natural drainage.

1

Rapidly permeableMore than 6 inches per hour

Moderately rapidly permeable

2 to 6 inches per hour

2Moderately permeable

0.6 to 2 inches per hour

3Moderately slowly permeable

0.2 to 0.6 inch per hour

4

Slowly permeable0.06 to 0.2 inch per hour

Very slowly permeable

less than 0.06 inch per hour

IL Permeability classes

Fields at the Allison Farm

Open field ditch

Bioreactor vs. standard tile outlet

One calorie is the amount of thermal energy required to raise the temperature of one gram of water by one Celsius degree.

3000 calories of thermalenergy enters each cup.The temperature of thewater on the left rises by30 Celsius degrees.

By how much does thetemperature of thewater in the cup on theright rise ??

Why does soil heat up faster than water ?

The heat capacity of water is ~ 5 times higher than the heat capacity dry soil.

As a result, moist soils heat up and cool down more slowly than dry soils.

Water has a high thermal conductivity

Air has a low thermal conductivity

What can be done to maximize geothermal

heat transfer ?

compacted vs. loose ?

moist vs. dry ?