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 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 EC e – Best measure of soil salinity and crop response Soil bulk EC b - Measured by in situ sensors Soil water EC w - Sensed by the plant At saturation EC e = EC w

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? WaterSaturated SoilField Capacity 1.Cross section for flow is smaller in soil 2.Flow path is longer in soil ECb = ECw ECb = ECw/3ECb = ECw/10

Getting EC e from EC b

Getting EC w from EC b ECe ECw

Bulk EC (EC b ) Decreases with water content Measured by probes in soil Depends on soil water content, soil salt content and temperature

Saturation Extract EC (EC e ) 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 (EC w ) What the plant sees Equal to EC e at saturation Predictions from EC b 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

Pore water EC Rainfed winter wheat

Maintaining Soil Productivity: Leaching fraction Defined as the ratio of drainage water to applied water: LF = D drain /D irrig Can use it to compute drainage required for a particular irrigation water quality: LF = EC irrig /EC drain If EC i were 0.3 dS/m and EC d were 3 dS/m, then LF would be 0.1; 1/10 th 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 = D drain /D irrig = EC irrig /EC drain New way: D drain = D appl EC appl /Ec drain Measure D appl, EC appl and EC drain to know D drain

Monitor D rain with a rain gauge Monitor D irrig with a flow meter Monitor EC irrig with an EC sensor or rain gauge Monitor EC drain with a deep moisture/EC/T Making the measurements

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