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Soil Moisture: Why Water Content Can’t Tell You Everything You Need to Know

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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

Published in: Environment
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Soil Moisture: Why Water Content Can’t Tell You Everything You Need to Know

  1. 1. WHY WATER CONTENT CAN’T TELLYOU EVERYTHING YOU NEED TO KNOW Colin Campbell, Ph.D. METER Group, Inc. USA
  2. 2. Without more information, we cannot tell how many logs will keep us adequately in our comfort range.
  3. 3. ?
  4. 4. 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
  5. 5. = water potential
  6. 6. WATER POTENTIAL IS THE SAME WAY -30 to -50 kPa Sterling Taylor, 1972,Physical Edaphology: The physics of irrigated and non-irrigated soils
  7. 7. 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
  8. 8. WATER POTENTIAL UNIT OVERLOAD kPa cbars Bars mm water atm MPa What do they all mean?!! J/kg pF
  9. 9. “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
  10. 10. “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
  11. 11. “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
  12. 12. “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
  13. 13. 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
  14. 14. FIELD MOISTURE STATUS
  15. 15. IRRIGATION STEERING WITH WATER POTENTIAL Vine kill Southern Idaho, USA - Irrigated seed potatoes Data courtesy of BKR Farms from ZENTRA Cloud
  16. 16. 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
  17. 17. 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
  18. 18. OVER-IRRIGATION 15 cm 15 cm 6cm 6cm 25cm
  19. 19. OPTIMAL IRRIGATION 15 cm 15 cm 6cm 6cm 25cm
  20. 20. UNDER IRRIGATION -500 kPa 15 cm 15 cm 6cm 6cm 25cm
  21. 21. OPTIMAL MOISTURE RANGE 15 cm deep layer: Maximum of 12 mm of irrigation to fill up profile - - - - -
  22. 22. WATER AVAILABILITY IN POTATOES Rough guess: 4% VWC range w/ 50 cm root depth  20 mm water refill
  23. 23. 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.
  24. 24. QUIZ SHOULD I WATER? Data courtesy of BKR Farms from ZENTRA Cloud
  25. 25. QUIZ SHOULD I WATER? Data courtesy of BKR Farms from ZENTRA Cloud
  26. 26. QUIZ SHOULD I WATER? Data courtesy of BKR Farms from ZENTRA Cloud
  27. 27. 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
  28. 28. QUESTIONS

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