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Understanding soil air/water dynamics
No-till soil Tilled soil
Porosity
the soil’s respiratory and circulatory system
(Young and Ritz, 2000)
White zones are pores > 1mm
http://www.mtm.kuleuven.ac.be/Research/NDT/IDO_SHerman_final.ppt
Soil pores come in many
sizes and shapes
Intensive tillage Long term no-till
AgriCanada
plow pan
well connected network of biopores
Effect of previous 20
years of rotations on
SOM and corn growth on Beltsville silt loam in Maryland
Effect of previous 20
years of rotations on
SOM and corn growth on Beltsville silt loam in Maryland
Continuous corn with tillage
Continuous bluegrass sod
25 yrs of CT corn25 yrs of CT corn
20 yrs of bluegrass, then 5 yrs CT corn
20 yrs of bluegrass, then 5 yrs CT corn
After adding waterAfter adding water
25 yrs of 25 yrs of conventiconventional onal corncorn
25 yrs of 25 yrs of conventiconventional onal corncorn
20 yrs of bluegrass, then 5 yrs conventional corn
20 yrs of bluegrass, then 5 yrs conventional corn
After adding water
Least Limiting Water Range
dry Soil water content, cm3/cm3 *100 wet
20 25 30 35 40 45 50 55
Ro
ot
gro
wth
rat
e LLWR for loose well-aggregated soil
LLWR for compacted soilNot enough O2 for root respiration
Soil too hard for roots to penetrate
From Weil, 2003Ray Weil
June July August
So
il M
ois
ture
(%
of s
atur
atio
n)
100
75
50
25 8”16”
How does compaction affect a soil’s least limiting water range?
Uncompacted soilCompacted soil
LL
WR
Compacting effects of wheel traffic
Chapter 7 in Ross (1989)
Cone resistance
Bulk density
Number of wheel passes
AgriCanada
Without restricted traffic, most field surfaces receive traffic each year
(Watts and Dexter, 1997)
Compaction
Soils with more OM are weaker when dry and stronger when wet !
Tillage and traffic damages wet soils !!!
Why are soils more compactible at field capacity than at saturation?
Soil will flow before water filled pores collapse
http://www.bettersoils.com.au/module6/6_3.htm
< 1 MPa1 - 3 MPa> 3 MPa
1 MPa = 145 psi
Prenetrometer pressureMoisture strongly affects penetration resistance!!The same soil can be hard
when dry and weak when wet.
Understanding bulk density
Soil structure must be intact and soil must be oven dry
It is often said that bulk densities > 1.6 g / cm3
are root restrictive… Is this true ??
Compactive force
So how does compaction impact soil water relationships ?
Loss of drainage pores
Gain in small pores
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
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)
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 is rated using a number (1 to 4) and letter (A or B) system. 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
A
Poorly drained
The water table is at or near the surface during the wetter seasons of the year
Very poorly drained
The water table remains near, at, or above the surface much of the time
BSomewhat poorly drained
The water table is near the surface only during the very wettest periods
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 ?
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