Soil Water Potential Peter Cull ICT International

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

Water Potential Units Water Potential Units of Measurement: J/kg MPa Bars RH -1 -0.001 -0.01 0.999993 -10 -0.1 0.999926 Field Capacity -33 -0.033 -0.33 0.999756 -100 0.999261 -1000 0.992638 Wilting Point -1500 -1.5 -15 0.988977 -10000 0.928772 Air Dry -100000 0.477632 Oven Dry -1000000 0.000618 High Potential Low Potential

Soil-Plant-Atmosphere Continuum

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

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

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

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

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

Lab Based Measurements Pressure Extractor WP4C Dewpoint Potentiameter

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

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

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: -0.03 to -1.5 MPa WP4C: Small sample Limited range: -0.1 to -300 MPa

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

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

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

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

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

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…

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

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

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

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

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

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

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

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