Making Electrical Conductivity Meaningful

1. Making Electrical Conductivity Meaningful Gaylon Campbell Decagon Devices, Inc. Pullman, WA

2. 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”

3. 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

4. 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

5. 1 gram of salt, 1 kg of water

6. Measuring 1g/kg EC using GS3 sensor and ProCheck

7. Add the 1.8 dS/m water to soil

8. Saturated soil bulk EC 1.8 dS/m water

9. “Field capacity” soil bulk EC 1.8 dS/m water

10. Why is soil EC lower than water EC? Water Saturated Soil Field Capacity ECb = ECw/3 ECb = ECw/10 ECb = ECw • Cross section for flow is smaller in soil • Flow path is longer in soil

11. Getting ECe from ECb

12. Getting ECw from ECb ECw ECe

13. Bulk EC (ECb) • Decreases with water content • Measured by probes in soil • Depends on soil water content, soil salt content and temperature

14. 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

15. Pore water EC (ECw) • What the plant sees • Equal to ECe at saturation • Predictions from ECb are uncertain when soil water content is low

16. Water Content under rainfed winter wheat

17. Soil Bulk EC under rainfed winter wheat

18. Saturation extract EC rainfed winter wheat

19. Pore water EC Rainfed winter wheat

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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