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Soil Water Potential

Soil Water Potential. Peter Cull ICT International. Water Content or Potential?. Water Content: Total amount of water in the soil Does not indicate how much is available to a plant. Water Potential: The amount of energy needed to extract water from soil

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Soil Water Potential

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  1. Soil Water Potential Peter Cull ICT International

  2. Water Content or Potential? • Water Content: • Total amount of water in the soil • Does not indicate how much is available to a plant • Water Potential: • The amount of energy needed to extract water from soil • Direct measure of how much water is available to a plant

  3. Water Potential Units • Water Potential Units of Measurement: High Potential Low Potential

  4. Soil-Plant-Atmosphere Continuum

  5. Gypsum Blocks • Standard matrix equilibrates with soil • Electrical resistance proportional to water content of matrix • Inexpensive, but poor stability, accuracy and response • Sensitive to salts in soil

  6. MPS-2 Capacitance Sensor • Standard matrix equilibrates with soil • Water content of matrix is measured by capacitance • Stable (not subject to salts and dissolution) • Good accuracy from -0.01 to -0.5 MPa

  7. TM229 Thermal Matric Sensor • Robust (ceramic with embedded heater and temperature sensor) • Large measurement range (wet and dry end) • Stable (not subject to salts and dissolution • Requires complex temperature correction and calibration

  8. Tensiometers • Equilibrates water under tension with soil water through a porous cup • Measures tension of water • Highest accuracy of any sensor in wet range • Limited to potentials from 0 to -0.09 MPa • Significant maintenance requirements

  9. Limitations of Field Techniques • TM229 Thermal Matric Sensor: • Calibration essential • Relatively expensive • Gypsum Blocks: • Inaccurate • Requires individual calibration • Limited range: 0 to -500kPa • Tensiometers: • High maintenance • Limited range: 0 to -80kPa • MPS-2 Capacitance Sensor: • Inaccurate below -100kPa • Requires individual calibration • Limited range: 0 to -500kPa

  10. Lab Based Measurements Pressure Extractor WP4C DewpointPotentiameter

  11. Pressure Extractors • Apply known amount of pressure into a • sealed chamber and monitor outgoing • moisture • Long established, accepted technique • Excellent accuracy between • -0.03 and -1.5 MPa

  12. WP4C Dewpoint Potentiameter • Based on psychometric principle • Dew forms on small mirror inside measurement chamber • WP4C measures humidity and temperature when dew forms • Poor results at wet end, excellent results at dry end • Fast, accurate, easy maintenance

  13. Limitations of Lab Techniques • Pressure Extractors: • Slow and expensive • Limited range: -0.03 to -1.5 MPa • All Methods: • Destructive • Destroys soil structure • Artificial result • One-off, spot, measurement • No continuous, in-situ data • WP4C: • Small sample • Limited range: -0.1 to -300 MPa

  14. Lab & Field Technique • Moisture Release Curves: • Quantifying soil moisture content Against soil water potential for Varying Soil Textures • Standard practice to measure Between Field Capacity and Plant Wilting Point: -0.03 to -1.5 MPa

  15. SMM Soil Moisture Meter • Stand-alone data logging solution • Up to 10 sensor capacity • Wireless communication • Low power requirement • Real and virtual sensors

  16. SMM Soil Moisture Meter • Real and Virtual Channels: • 5 channels to measure moisture content • 5 virtual channels to measure soil water potential

  17. SMM Soil Moisture Meter • Calibration Feature: • Individual channel calibration • Precision measurement of soil moisture parameters in any soil type

  18. SMM – How it Works • Step 1: • Collect soils from field and air dry for 2 weeks • Step 2: • Sieve soil through 2mm sieve • Place in separate containers

  19. SMM – How it Works • Step 3: • 1st container, keep air dry • 2nd container, 1 unit of water • 3rd container, 2 units of water • 4th container, 3 units of water • and so on…

  20. SMM – How it Works • Step 4: • Measure soil with MP306 sensor in raw, millivolt format in all of the containers • Step 5: • Measure water potential of soil in each container with a WP4C or pressure extractor

  21. SMM – How it Works • Step 6: • Once water potential is known, determine volumetric water content with a balance and drying oven

  22. SMM – How it Works • Step 7: • Find equation from your moisture release curve Curve A Sandy Soil: Soil Water Potential = 5343 x Soil Water Content-1.852 Curve B Clay Soil: Soil Water Potential = 6x107 x Soil Water Content-4.228

  23. SMM – How it Works • Step 8: • Enter equation into the SMM software as a script

  24. SMM – How it Works • Step 9: • Connect sensors to SMM and install in the field. Start taking measurements!

  25. Simultaneous Content & Potential % Volumetric Water Content Soil Water Potential (kPa) Day 1 Day 2 Day 3 Day 4 Day 5 Day 6

  26. Virtual Channels = Flexibilty • SMM Soil Moisture Meter can handle other equations: • Hydraulic Conductivity; Degree of Saturation; Oxygen Content Water Potential and Water Content Hydraulic Conductivity and Water Content

  27. ICT International Pty Ltd Solutions for soil, plant and environmental research www.ictinternational.com sales@international.com.au Phone: 61 2 6772 6770 Fax: 61 2 6772 7616 PO Box 503, Armidale, NSW, Australia, 2350 INTERNATIONAL

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