Making Electrical Conductivity Meaningful

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