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Transcript monitoring_soil_salinity_
Making Electrical
Conductivity Meaningful
Gaylon Campbell
Decagon Devices, Inc.
Pullman, WA
Richard Stirzaker’s
Goldilocks Principle
Soil water measurements: useful but
too detailed for the big picture
Groundwater and river monitoring:
too slow for management decisions
Monitoring salinity in the soil profile:
“just right”
Virtual Seminar
at www.decagon.com
Solute Signatures: Monitoring and
Interpreting Salt and Nitrate Levels in
the Root-Zone
July 8, 2010
Dr. Richard Stirzaker
Principal Research Scientist
CSIRO Australia
Three Measures of
Electrical Conductivity
Saturation extract ECe – Best
measure of soil salinity and crop
response
Soil bulk ECb - Measured by in situ
sensors
Soil water ECw - Sensed by the plant
At saturation ECe = ECw
1 gram of salt, 1 kg of water
Measuring 1g/kg EC using
GS3 sensor and ProCheck
Add the 1.8 dS/m water to
soil
Saturated soil bulk EC
1.8 dS/m water
“Field capacity” soil bulk EC
1.8 dS/m water
Why is soil EC lower than
water EC?
Water
ECb = ECw
Saturated Soil
Field Capacity
ECb = ECw/3
ECb = ECw/10
1.Cross section for flow is smaller in soil
2.Flow path is longer in soil
Getting ECe from ECb
10
9
ECb
ECe
0.94 s 0.514
8
ECe/ECb
7
6
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
Water Content (m3/m3)
0.5
Getting ECw from ECb
10
9
8
ECe
7
EC/ECb
ECw
EC b
EC w
0.94 1.514
6
5
4
3
ECe
2
1
0
0
0.1
0.2
0.3
Water Content (m3/m3)
0.4
0.5
Bulk EC (ECb)
Decreases with water content
Measured by probes in soil
Depends on soil water content, soil
salt content and temperature
Saturation Extract EC (ECe)
A measure of the amount of
salt in the soil
Tells us what crops will grow in
that soil
Is typically 3 to 10 times the
bulk EC of the soil
Pore water EC (ECw)
What the plant sees
Equal to ECe at saturation
Predictions from ECb are uncertain
when soil water content is low
Water Content under
rainfed winter wheat
Water Content (m3/m3)
0.6000
0.5000
0.4000
30 cm
0.3000
60 cm
90 cm
0.2000
120 cm
150 cm
0.1000
0.0000
0
100
200
300
Day of Year
400
500
Soil Bulk EC under rainfed
winter wheat
1.40
1.20
ECb (dS/m)
1.00
0.80
30 cm
60 cm
0.60
90 cm
120 cm
0.40
120 cm
0.20
0.00
0
100
200
300
Day of Year
400
500
Saturation extract EC
rainfed winter wheat
4.00
ECb
ECe
0.94 s 0.514
3.50
ECe (dS/m)
3.00
2.50
30 cm
2.00
60 cm
1.50
90 cm
1.00
150 cm
120 cm
0.50
0.00
0
100
200
300
Day of Year
400
500
Pore water EC Rainfed
winter wheat
EC b
EC w
0.94 1.514
5.00
4.50
ECw (dS/m)
4.00
3.50
3.00
2.50
30 cm
2.00
90 cm
60 cm
120 cm
1.50
150 cm
1.00
0.50
0.00
0
100
200
300
Day of Year
400
500
Maintaining Soil Productivity:
Leaching fraction
Defined as the ratio of drainage water to
applied water: LF = Ddrain/Dirrig
Can use it to compute drainage required
for a particular irrigation water quality: LF
= ECirrig/ECdrain
If ECi were 0.3 dS/m and ECd were
3 dS/m, then LF would be 0.1; 1/10th of the
water would need to drain to keep the
drainage water at this EC
EC of water from rain and
irrigation
Drain ECrain + Dirrig ECirrig
Dirrig
ECa =
» ECirrig
Drain + Dirrig
Drain + Dirrig
Rain is almost salt free so it dilutes
the soil solution
EC of applied water is approximately
EC of irrigation times the fraction of
the total water depth from irrigation
A new way to think about
leaching fraction
Old way:
LF = Ddrain/Dirrig = ECirrig/ECdrain
New way: Ddrain = Dappl ECappl/Ecdrain
Measure Dappl, ECappl and ECdrain to
know Ddrain
Making the measurements
Monitor Drain with a
rain gauge
Monitor Dirrig with a
flow meter
Monitor ECirrig with an
EC sensor or rain
gauge
Monitor ECdrain with a
deep moisture/EC/T
Conclusions
Managing salinity is a BIG issue in
irrigated agriculture
Salts are added with water
Salts prevent germination and reduce
yield
A good way to measure the salt
content of soil is to measure its
electrical conductivity
Conclusions
Proper irrigation management requires a
knowledge of the EC of applied water and
drainage water
EC of the saturation extract can be
reliably determined from bulk EC
measurements in soil
Drainage can be measured using EC