Making Electrical Conductivity Meaningful Gaylon Campbell Decagon Devices, Inc. Pullman, WA
Feb 07, 2016
Making Electrical Conductivity Meaningful
Gaylon CampbellDecagon 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 Saturated Soil Field Capacity
1.Cross section for flow is smaller in soil
2.Flow path is longer in soil
ECb = ECw ECb = ECw/3 ECb = ECw/10
Getting ECe from ECb
514.094.0 sb
e
ECEC
Getting ECw from ECb
ECeECw
514.194.0 b
w
ECEC
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
Soil Bulk EC under rainfed winter wheat
Saturation extract EC rainfed winter wheat
514.094.0 sb
e
ECEC
Pore water EC Rainfed winter wheat
514.194.0 b
w
ECEC
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
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
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
Making the measurements
Conclusions
Managing salinity is a BIG issue in irrigated agriculture
Salts are added with waterSalts prevent germination and
reduce yieldA 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