understanding soil water

Understanding Soil Water

What is the term for the moisture status of a soil

when its pores are 100% full of water?

Saturation

Why do large soil pores (aka macropores)

normally drain within a few days?

GRAVITY

Large soil pores are drained by

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

Capillarity and surface attraction combine to pull more

strongly than gravity on: 1) water in “micropores” and

2) water close to the “soil skin”.

Field capacity Wilting point

When plants have extracted

as much water as they can When water is no longer

drained by gravity

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

Field capacity Wilting point

Some water is held

too tightly to be

pulled away by roots

When plants have extracted

as much water as they can When water is no longer

drained by gravity

Pull of the soil matrix on H2O

+

surface attraction + capillarity

So

il Sk

in H

H

O

Hydrogen bonding

H

H

O

H

H

O

?

cohesion

+

adhesion

pull H2O

into small

pores

Pull of the soil matrix on H2O

+

surface attraction + capillarity

So

il Sk

in H

H

O

Hydrogen bonding

H

H

O

H

H

O

?

cohesion

+

adhesion

pull H2O

into small

pores Water is pulled into

the micropores and

toward the soil skin

by matric forces

What do I mean by “soil skin”?

humus clay minerals

http://www.ccma.csic.es/dpts/suelos/ Brady and Weil (2002)

Plant available water

10-30 μm

Gravitational water in

drainage pores

Unavailable water

Adapted from Buol (2000)

Most available

Soil circulatory system

~0.2

μm

les

s

availab

le

Field Capacity

Wilting point

Saturation model soil pore

Plant available water

Unavailable water


Most available


~0.2

μm

les

s

availab

le

Field Capacity

Wilting point

model soil pore

Unavailable water



~0.2

μm

Wilting point

model soil pore

So

il skin

Thickness of water film

Unavailable

water

high energy H2O Low energy H2O

high energy H2O = molecules bouncing around

low energy H2O = molecules moving slowly

So

il skin

Thickness of water film

high energy H2O Low energy H2O

high energy H2O = molecules bouncing around

low energy H2O = molecules moving slowly

There is still some water in air dry soils!

Mars Lander probe finds no water in Martian soils

A conductivity probe on the Mars Lander sensed rising and falling humidity levels in the

Martian atmosphere, but when stuck into the ground, the probe found “Martian soil” to

be completely and perplexingly dry.

On Earth, “if you have water vapor in the air, every surface exposed to that air will have

water molecules adhering to it that are somewhat mobile, even at temperatures well

below freezing," said Aaron Zent , lead scientist for the Lander’s conductivity probe.

http://www.innovations-report.com/html/reports/physics_astronomy/esa_039_s_gravity_satellite_goce_117553.html

~ 1

m

15

0 m

10

,00

0 m

Wilting point Air-dry

There are many

other methods

of expressing

soil water

tension

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 created by a hanging column of water

-1500 kPa

-15 bars

-100,000 kPa

-1000 bars

Field capacity

~ 1

m

15

0 m

10

,00

0 m

Wilting point Air-dry

There are many

other methods

of expressing

soil water

tension

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 created by a hanging column of water

You should be

familiar with

these units

-1500 kPa

-15 bars

-100,000 kPa

-1000 bars

Field capacity

Saturation

Are all of the water

molecules in this pore

under the same tension ?

Field

Capacity

Wilting

point

-1500 kPa

Air-dry

-100,000 kPa

Ψtotal + Ψmatric + Ψosmotic = Ψgravitational

Understanding soil water tension

Pull

of

gravity

Pull by

micropores

and soil skin ?

Understanding osmotic tension

Salt added

?

Understanding osmotic tension

Salt added

What causes fertilizer burn?

Osmotic tension

The same phenomena that causes “dishwashing hands”

Osmotic tension

How do

water

molecules

get from the

soil to the

top of a

plant?

Continuous

chains of water

molecules

move upward

through the

xylem

The chain

moves upward

if there is a

negative energy

gradient

H20

H20 H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20 H20

H20

H20

Continuous

chains of water

molecules

move upward

through the

xylem

Solar energy

drives

transpiration

Plants provide

the conduit

H20

H20 H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20

H20 H20

H20

H20

Transpiration = air conditioning for plants

~ 4000 gallons H2O

per acre on a hot

sunny day

~ 30 gallons H2O

per corn plant per

season

The tallest living tree is a coast redwood that stands 112 meters

(367 feet, 6 in.), or ~ five stories higher than the Statue of Liberty.

Cohesion

theory

Why don’t trees grow

any taller ?

Hydrogen bonding is only

strong enough to hold

together ~ 400’

of water

molecules

Soil water is a switch that activates and

deactivates soil biology

Water is biologically available, when soil

organisms are able to win the

“tug of war” with the soil

What is meant by the term water content?

Up to this point, we have been discussing

water tension

Determining gravimetric soil moisture content

Collect sample. Weigh moist. Weigh after oven drying.

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

Water content can also be

expressed volumetrically

v.m.c. = volume of water in soil / total soil volume

volume

of H2O

volume

of dry

soil

mass of H2O

mass of dry soil

mass of dry soil

volume of dry

soil

Converting from gravimetric to volumetric MC

volume of H2O

mass of H2O * * =

inappropriate

for expansive soils

Bulk density

Gravimetric

moisture

content Density

of H2O

Volumetric

moisture

content

Why would you want to do this conversion?

Gravimetric MC is easier to measure

but volumetric MC is more useful

for managing irrigation

0

Translating between

water tension (aka potential)

and water content using

a “characteristic curve”

A characteristic

curve (aka

water release

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 ceramic plate.

Why are

all those

bolts

needed?

Different soils have different characteristic curves

Field capacity

Wilting point

Brady and Weil, 2002


Field capacity

Wilting point

0.09 – 0.02 = 7%



Field capacity

Wilting point

34

% - 8

% =

26

%



Field capacity

Wilting point 54

% - 2

4%

= 3

0%


Use the diagram to interpret how much water

is held in the clay @ saturation, FC and WP.

Calculate how many inches of water are

needed to bring a 3’ rooting zone of this soil

from 50% of FC to FC.

0.54 * 36” = 19.4” of water @ FC

The volumetric water content @ FC = 0.54

50% of 19.4” = 9.7”

Real soils rarely hold more than 2.5” of

plant available water per foot… based on

this fact, do you think the characteristic

curve for the clay soil is realistic?

The volumetric water content @ FC = 0.54

The volumetric water content @ WP = 0.24

PAW = 0.30

0.3* 12” = 3.6” >> 2.5”

So how does compaction impact soil water relationships ?

So how does compaction impact soil water relationships ?

Loss of drainage

pores

Gain in

small

pores

Field capacity line

Which soil texture can hold the

most plant available water?


Plant

Available

water

~ 2.5” of plant

available water

(PAW) per foot

Wilting point line

How does SOM affect PAW?

Adapted from Brady and Weil

Prairie soil Farm field

Impressive example of the impact of

soil organic matter on

water holding capacity

So when should you irrigate a clay soil?

Wimpy crops

Tough crops

So when should you irrigate a clay soil?

Wimpy crops

Tough crops

So when should you irrigate a loam soil?

So how does one measure soil water tension in the field?


A tensiometer is a

water filled tube

with a porous

ceramic tip on one

end and a vacuum

gauge on the other.

Water tension in the

tube equilibrates

with the water

tension outside the

porous tip.

Tensiometers are

useful for montioring

tensions between

0 and -85 kPa (-0.85 bars)

a range that includes

about half the water in

most soils.

When soils are too dry

(> -85 KPa), air is drawn

in through the porous tip

and the vacuum fails.

Measuring soil

moisture as a

function of

electrical

resistance

Gypsum block


Measuring soil

moisture as a

function of

electrical

resistance

Calibration is

critical !!

Gypsum block


Resistance

drops as

gypsum

starts to

dissolve

What is this gizmo?

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

Measuring infiltration rate

http://soilquality.org/images/infiltration_photo1.jpg

Why do the wetting fronts have different shapes?

http://www.ext.colostate.edu/mg/gardennotes/images/213-7.jpg

Capillarity pulls the water farther in finer textured soils

http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf

Capillary rise in a sandy soil


Capillary rise in a silt loam

What happens when capillary rise lifts water to the soil surface?


How fast does water move through soil ?

Flow rate = Area*Ksat *pressure/length Brady and Weil, 2002

Darcy’s

Law

Hyd

raulic

co

nd

uctivity

Hydraulic conductivity = permeability

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 layer

of sandy soil

improve

drainage ?

NO !

Layer with sandy texture

Layer with sandy texture

Soil suitability for septic drainfields

http://organicearthsolutions.wordpress.com/2012/02/16/

What happens if a septic drainfield

does not drain adequately?

Can a drainfield drain too well?

Interstream

divide

SOIL

DRAINAGE

CLASSES

Poorly

drained

Somewhat

poorly

drained

Moderately

well drained

Poorly

drained

Well

drained

Valley floor

Backslope

Shoulder

LANDSCAPE

POSITIONS

N.C. Agric. Res. Bull. 467

Common in IL

Impact of topography on drainage

Illinois’

natural drainage

classes

http://www.il.nrcs.usda.gov/technical/soils/Suite_Maps.html

What is a

hydric soil?

A hydric soil is a soil that

formed under conditions of

saturation, flooding or

ponding long enough during

the warm season to develop

anaerobic conditions in the

upper horizons.

Soils in which the hydrology

has been artificially modified

are still considered hydric if

the soil, in an unaltered

state, was hydric.

htt

p:/

/ww

w.il.n

rcs.u

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/Suite_M

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Hydric soils are dominated by low chroma colors

http://www.wtamu.edu/~crobinson/soils/clayskn05s.jpg

Mottles are

indicative of a

fluctuating

water table.

Some hydric soils in McDonough Cty

http://www.ars.usda.gov/SP2UserFiles/Place/36251500/TheExtentofFarmDrainageintheUnitedStates.pdf

Artificial drainage in the United States

% of land drained

IL has experienced some very wet

springs in recent years

?

Yield maps

have made

drainage

problems

more

obvious

Increasing yield by installing drainage

By Mindy Ward, Missouri Farmer Today

BOONVILLE --- For more than 100 years, the

Hoff family has fought to farm wet areas of their

fields.

For Eddie Hoff, the fourth generation to farm the

creek bottom ground in Cooper County, the loss

of yield and added expense of working the

ground was ultimately affecting his bottom line.

“We were losing 60 to 70 bushels per acre in

some spots,” he says.

We were working the ground over and over. I

just wanted to no-till and save some cost.”

So, he decided to drain the soils with pattern tile.

Could this story be about your farm?

http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm

Pattern Tiling in Ontario

http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm

http://www.fastline.com/flimages/internet/032/169/3959312_4.jpg

Installing corrugated plastic tile with a tile plow

Ontario Ministry of Ag and Food

Why do crops on tiled-drained land tend to

be more drought resistant ?

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.

?

http://wrc.umn.edu/prod/groups/cfans/@pub/@cfans/@wrc/documents/asset/cfans_asset_212844.jpg

Bioreactor

filled with

woodchips

Controlled

drainage system

Conservation Drainage

Maximum conveyance Crop productivity Environmental quality

Artificial drainage has greatly increased the number of days when

soils in the Upper Midwest are suitable for field operations and

deep root growth

but has also

contributed

to some

environmental

problems

Pollution of

water resources Loss of SOM

Which is worse??

Saturated soil is

less compressible

than wet soil

Compaction

probably extends

several feet deep

What is the

optimum soil

moisture for

compacting

soil?

Soils are most compactible

near field capacity because

the particles are well

lubricated and the large

pores are empty and most

collapsible

Soil resistance to penetration is very

related to soil moisture content.

Healthy crops tend to use more

water which can result in higher

penetrometer readings.

heat capacity of water =

1 calorie / gram / degree C

Understanding Heat Capacity

How much will the temperature of the water increase in cup A if

300 calories of thermal energy are added? How about cup B?

A

B

Why does soil heat up faster than water ?

The heat capacity of

water is ~ 5 times

higher than the heat

capacity of 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 ?

understanding soil water

Education

understanding soil water

unitssoil water

water vapor

mpasoil water

water moleculesget

soil water tension total

hanging column of water

aroundlow energy h2o