1. Soil Water Potential
Peter Cull
ICT International

2. Water Content or Potential?
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:
J/kg
MPa
Bars RH -1
-0.001
-0.01
-10
-0.1
Field Capacity
-33
-0.033
-0.33
-100
-1000
Wilting Point
-1500
-1.5
-15
-10000
Air Dry
Oven Dry
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 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 MPa
Significant maintenance requirements

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

10. Lab Based Measurements
Pressure Extractor
WP4C Dewpoint Potentiameter

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
All Methods:
Destructive
Destroys soil structure
Artificial result
One-off, spot, measurement
No continuous, in-situ data
Pressure Extractors:
Slow and expensive
Limited range: to -1.5 MPa
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: to -1.5 MPa

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

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

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

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

20. 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…

21. SMM – How it Works Step 4: Step 5:
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

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

23. 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

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

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

26. Simultaneous Content & Potential
% Volumetric Water Content
Soil Water Potential (kPa)
Day Day Day Day Day Day 6

27. 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

28. ICT International Pty Ltd
Solutions for soil, plant and environmental research
Phone:
Fax:
PO Box 503, Armidale, NSW, Australia, 2350