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