Water content can leave you in the dark Everybody measures soil water content because it’s easy. But if you’re only measuring water content, you may be blind to what your plants are really experiencing. Soil moisture is more complex than estimating how much water is used by vegetation and how much needs to be replaced. If you’re thinking about it that way, you’re only seeing half the picture. You’re assuming you know what the right level of water should be—and that’s extremely difficult using only a water content sensor. Get it right every time Water content is only one side of a critical two-sided coin. To understand when to water or plant water stress, you need to measure both water content and water potential. In this 30-minute webinar, METER soil physicist, Dr. Colin Campbell, discusses how and why scientists combine both types of sensors for more accurate insights. Discover: - Why the “right water level” is different for every soil type - Why soil surveys aren’t sufficient to type your soil for full and refill points - Why you can’t know what a water content “percentage” means to growing plants - How assumptions made when only measuring water content can reduce crop yield and quality - Water potential fundamentals - How water potential sensors measure “plant comfort” like a thermometer - Why water potential is the only accurate way to measure drought stress - Why visual cues happen too late to prevent plant-water problems - Case studies that show why both water content and water potential are necessary to understand the condition of soil water in your experiment or crop

2. WHY WATER CONTENT CAN’T
TELLYOU EVERYTHING YOU
NEED TO KNOW
Colin Campbell, Ph.D.
METER Group, Inc. USA

7. Without more information,
we cannot tell how many logs will keep
us adequately in our comfort range.

9. ?

10. DO YOU NEED TO UNDERSTAND
TEMPERATURE TO USE IT?
Typically we only connect these
values with comfort
Easy to remember ranges if you
are familiar with the units
˚F
˚C
K

11. = water potential

12. WATER POTENTIAL IS THE
SAME WAY
-30 to -50 kPa
Sterling Taylor, 1972,Physical Edaphology:
The physics of irrigated and non-irrigated
soils

13. WHY ISN’T WATER POTENTIAL
MORE COMMONLY USED?
1. Water potential is difficult to understand
2. Water potential is hard to measure
3. Even using water potential, we still don’t know HOW MUCH water to add
We’ll now focus on each of these challenges

14. WATER POTENTIAL
UNIT OVERLOAD
kPa cbars
Bars
mm water
atm
MPa
What do they all mean?!!
J/kg
pF

15. “WATER POTENTIAL IS HARD TO
UNDERSTAND”
Temperature
Defined by the third law of thermodynamics
with respect to molecular movement at
absolute zero…
Simply learn relevant temperature unit and
scale and accept values for optimal ranges
Water potential
Energy required, per quantity of water, to
transport, an infinitesimal quantity of water
from the sample to a reference pool of pure,
free water
Simply learn relevant water potential scale
and units and accept literature values for
optimal ranges

16. “WATER POTENTIAL IS HARD TO
MEASURE”
True –
• Many field sensors are inaccurate
• Some are expensive
• Inherent sensor-to-sensor variability
• The most accurate sensors have limited range (tensiometers)
A poorly calibrated water potential sensor is just a water content sensor
• It will give a relative response but will not tell you if it’s in the right range

17. “WATER POTENTIAL IS HARD TO
MEASURE”
METER Group’s TEROS 21
• Individually calibrated
• Cover broad water potential
range
• Low sensor-to-sensor variability
• Sensor consistency evident in field
data

18. “WATER POTENTIAL LACKS
CONNECTION TO HOW MUCH
WATER TO APPLY”
Let’s consider some scenarios
Good:simply use water potential to know when plants need water
Great:Use water potential and water content together to get complete soil
moisture picture

19. STRESS AND YIELD
POTATOES
Site Days in stress Yield (Mg/ha)
9 42 31.47
12 53 32.77
10 44 37.44
11 0 39.67
6 0 40.08
7 16 40.33

20. FIELD MOISTURE STATUS

21. IRRIGATION STEERING WITH
WATER POTENTIAL
Vine kill
Southern Idaho, USA - Irrigated seed potatoes
Data courtesy of BKR Farms from ZENTRA Cloud

22. RESULTS FROM 2020
Potatoes
• Improved yields
• Higher quality
• Reduced variability
• Lower disease
Inputs
• Reduced water and fertilizer
applications
• Lower pumping costs
• Less frequent field walks
Challenges
• Requires knowledge of water
requirements

23. GREAT:UNITE WATER CONTENT
AND WATER POTENTIAL
Water potential
Water content
15 cm
6 cm
30 cm
15 cm
6 cm
Soil Type Crop Type
Loamy sand Turf grass

24. OVER-IRRIGATION
15 cm
15 cm
6cm
6cm
25cm

25. OPTIMAL IRRIGATION
15 cm
15 cm
6cm
6cm
25cm

26. UNDER IRRIGATION
-500 kPa
15 cm
15 cm
6cm
6cm
25cm

27. OPTIMAL MOISTURE RANGE
15 cm deep layer: Maximum of 12 mm of irrigation to fill up profile
- - - - -

28. WATER AVAILABILITY IN POTATOES
Rough guess: 4% VWC range w/ 50 cm root depth  20 mm water refill

29. We don’t know if we’ll be
comfortable just by knowing how
many logs we’ve added to the fire.
We won’t know if the soil is optimal
for plant growth just by knowing
water content.

30. QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

31. QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

32. QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

33. SUMMARY
Water potential should be understood like temperature
• Rarely consider its complex definition
• Memorize critical range for comfort and apply strategies to keep within boundaries
Water potential is difficult to measure accurately, but not impossible
• Some sensors have large variability
• Individual calibration of TEROS 21 sensors shows excellent consistency
Combining water potential and water content is powerful
• Straightforward to irrigate with only water potential, but miss how much water to add
• Together, they give a complete moisture picture
• Comfort range generates the metaphorical logs to add

34. QUESTIONS