This document provides an introduction to soil moisture release curves and water potential. It discusses how soil moisture release curves can be used to understand plant water availability and make irrigation decisions. Specifically, it defines water potential and its components. It explains that soil moisture release curves relate extensive water content properties to intensive water potential properties. The document also discusses field capacity and factors that can affect it, such as soil texture. It describes how soil moisture release curves provide additional information about soil properties beyond water retention.

2. SOIL MOISTURE 201
MOISTURE RELEASE CURVES-REVEALED
Leo Rivera
METER Group, Inc. USA

3. INTRODUCTION
ABOUT ME
Background in Soil Physics & Pedology
12 years of experience measuring and
interpreting soil hydraulic properties and soil
moisture release curves

4. INTRODUCTION
WHAT DOES SOIL MOISTURE EVEN MEAN?
Are you interested in the water stored in the soil?
Do you care more about water available for
primary productivity?
Are you studying water and solute movement
in soils?
Do you aim to optimize water use of crops?
Are you modeling soil hydrology?
4

5. GOALS
• You will have a better understanding of the
importance of water potential
• How you can utilize soil moisture release curves for
irrigation planning
• What additional information lives within the soil
moisture release curve
• What factors affect field capacity

6. EXTENSIVE VS. INTENSIVE
PROPERTIES
Two variables necessary to describe state of matter or energy in environment
Extensive variable – describes the extent or amount of matter or energy
Intensive variable – describes the intensity or quality of matter or energy
Extensive Intensive
Volume Density
Water Content Water Potential
Heat Content Temperature

7. EXTENSIVE VS. INTENSIVE
Heat Content
Temperature
Energy flow?

8. WHAT IS WATER POTENTIAL?

9. WATER POTENTIAL
DEFINITION
Energy required, per quantity of water, to transport, an
infinitesimal quantity of water from the sample to a reference
pool of pure, free water

10. TOTAL WATER POTENTIAL
SUM OF COMPONENTS
yT = ym + yg + yo + yp
yT – Total water potential
ym – matric potential - adsorption to surfaces
yg – gravitational potential - position
yo – osmotic potential - solutes
yp – pressure potential - hydrostatic or pneumatic

11. SOIL FINGERPRINT
SOIL WATER RETENTION CURVES
EXTENSIVE
INTENSIVE

12. MAKING DECISIONS
IRRIGATION
How can we use Soil Moisture Release Curves to make
irrigation decisions?
We need to understand both water potential and water content
VWC – How much irrigation to apply
Water potential – availability of water to crops

13. 6/11/19
IRRIGATION REQUIREMENTS
SILT LOAM
-100 kPa = 0.24 m3 m-3
-33 kPa = 0.32 m3 m-3
Irrigation zone depth = 15 cm (5.9 in)
Required irrigation = (0.32-0.24) x 15 = 1.2 cm (0.47 in)

14. 6/11/19
IRRIGATION REQUIREMENTS
FINE SANDY LOAM
-100 kPa = 0.10 m3 m-3
-33 kPa = 0.16 m3 m-3
Irrigation zone depth = 15 cm (5.9 in)
Required irrigation = (0.16-0.10) x 15 = 0.9 cm (0.35 in)

15. WHAT FACTORS LIMIT PLANT
WATER AVAILABILITY?
Field Capacity
Permanent Wilting Point
What other information is in a
soil moisture release curve?
Uniformly-Graded
Well-Graded
Gap-Graded

16. WHAT FACTORS LIMIT PLANT
WATER AVAILABILITY?
0
10
20
30
40
50
60
70
80
0.01 0.1 1 10 100 1000 10000 100000 1000000
WATERCONTENT,%
WATER POTENTIAL, -KPA
Uniformly-Graded
Well-Graded
Gap-Graded

17. FIELD CAPACITY
DEFINITION
Field water capacity is “the content of water on a mass or volume basis,
remaining in a soil 2 or 3 days after having been wetted with water and after
free drainage is negligible.”
Glossary of Soil Science Terms. Soil Science Society of America, 1997
About -33 kPa
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18. TRUE FIELD CAPACITY
WHAT DO WE MEASURE IN THE FIELD
What does field capacity look like in the real
world.
Where does the water potential stabilize
Does this represent my soil?

19. TRUE FIELD CAPACITY?
CAN VARY SITE TO SITE
Typical soil Connection with water table Capillary Barrier Effect
Silt Loam
Water Table
Silt Loam
Water Table
Sand

20. TOOLS FOR MEASURING
WATER POTENTIAL

21. HOW FAR WE HAVE COME
TOOLS FOR MEASURING WATER POTENTIAL
HYPROP WP4C

22. SOIL MOISTURE RELEASE CURVE
WHAT ELSE CAN IT TELL ME?
• Shrink swell capacity
• Cation exchange capacity
• Soil specific surface area

23. HYDRAULIC CONDUCTIVITY
CAN IT AFFECT PLANT WATER AVAILABILITY?
Limits water redistribution
Water potential near root-soil interface
can be much lower.
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1 10 100 1000
K,cm/s
y, kPa
FSL LFS SiL Potting Soil McCorkle

24. RECAP
WHAT’S NEXT?
Soil Moisture 101 – basics of water content and water potential
Soil Moisture 201 – A deeper look at water potential
Focused on states
What are we missing? What is the end goal?
Flux data & rate data.

25. ECOSYSTEM FLUXES
THE BIG PICTURE

26. SOIL MOISTURE 201
QUESTIONS?