Part 1: Substrates and Water Stop guessing. Start measuring. When you irrigate in a greenhouse or growth chamber, you need to get the most out of your substrate so you can maximize the yield and quality of your product. But if you’re lifting a pot to gauge how much water is in the substrate, it’s going to be difficult—if not impossible—to achieve your goals. To complicate matters, soil substrates and potting mixes are some of the most challenging media in which to get the water exactly right. Without accurate measurements or the right measurements, you’ll be blind to what your plants are really experiencing. And that’s a problem, because irrigating incorrectly will reduce yield, derail the quality of your product, deprive the roots of oxygen, and increase risk of disease. Supercharge yield, quality—and profit At METER, we know how to irrigate substrates. We’ve been measuring soil moisture for over 40 years. Join Dr. Gaylon Campbell, founder, soil physicist, and one of the world’s foremost authorities on soil, plant, and atmospheric measurements, for a series of irrigation webinars designed to help you correctly control your crop environment to achieve maximum results. In this 30-minute webinar, learn: Why substrates hold water differently than normal soil How the properties of different substrates and potting mixes compare Why it’s difficult if not impossible to irrigate correctly without accurately measuring the amount of water in the substrate The fundamentals of measuring soil moisture: specifically water content and electrical conductivity How measuring soil moisture helps you get the most out of the substrate you choose, so you can improve your product Easy tools you can use to measure soil water in a greenhouse or growth chamber to maximize yields and minimize inputs

2. IRRIGATION OF CONTROLLED
ENVIRONMENT CROPS
SUBSTRATES AND WATER
Gaylon S. Campbell, PhD
METER Group, Inc. USA

3. WHAT DOES A SUBSTRATE DO?
• Holds up the plant
• Stores water for and supplies water to plants
• Stores nutrients for and supplies nutrients to plants
• Supplies oxygen to roots

4. WATER FLOW
SOIL-PLANT-ATMOSPHERE-CONTINUUM
(SPAC)
Why do we irrigate?
• Plant biochemistry highly hydrated
• Water evaporates when leaves take up CO2
• Plant water is replenished from the roots
and soil, so we irrigate to restore lost water

5. WATER FLOW
IN THE SPAC
What causes the water to move?
• Water moves from regions of high energy to regions of
low energy
• Water energy in the root zone is higher than in the
atmosphere
• The plant conducts water along this energy gradient
we call the SPAC

6. MID-DAY WATER POTENTIAL
IN THE SPAC

7. WATER POTENTIAL
HOW IT’S DETERMINED
• Matric Forces:Water in the capillaries and interstices of soil and growth
media has lower energy than free water
• Osmotic Forces:Water containing dissolved salts has lower energy than
pure water when a semipermeable membrane is present (plant membranes,
air gap)
• Gravitational Forces: Water energy changes with elevation in a gravitational
field
• The total water potential is the sum of these components

8. WATER CONTENT VS. POTENTIAL
SILT LOAM

9. WATER CONTENT VS. POTENTIAL
ROCK WOOL VS. SILT LOAM

10. MOISTURE RETENTION
RANGE OF SUBSTRATES

11. MOISTURE RETENTION
COCO VS. ROCK WOOL
]

12. FACTS FROM PLANT
PHYSIOLOGY
• Nominal permanent wilting point of plants
is -1500 kPa
– PWP is the soil water potential at which a plant wilts
and will not recover in a humid atmosphere overnight
• Water potential in the soil is higher than at
any other place in the SPAC
• Cell growth and synthesis are reduced
around -100 kPa
• Transpiration and photosynthesis are
reduced around -1000 kPa

13. • Almost all the water in rock wool is
readily available for crop growth
• We can’t control stress in rock wool
by reducing matric potential
• The water content of rock wool
must be monitored to properly
manage irrigation
CONSEQUENCES
LARGE PORES & NARROW PSD
Reduced
Water
Availability

14. CONSEQUENCES
LARGE PORES & NARROW PSD
• Gravity has a big effect on how water is distributed in and how it flows
through substrates like rock wool
• To distribute water, irrigate in small “shots” (3% of volume) with rest periods
between
• Carefully position drippers and drains to force full use of the substrate

15. CONSEQUENCES
LARGE PORES & NARROW PSD
• Gravitational forces are much
stronger than matric forces,
especially in rock wool, so the
water goes to the bottom
• Be careful to choose the right place
to monitor water content
• The measured water content is of
that spot, not of the slab
17

16. SUBSTRATE OXYGEN
TRANSPORT

17. THE TOOLS
AROYA Nose
• Wireless mesh node
• Sensor logger
• Light measurement
• Solar charger
Rock wool slab with TEROS 12 and AROYA Nose
TEROS 12
• Dielectric permittivity
• Bulk EC
• Temperature

18. WATER & NUTRIENTS
DIELECTRIC & EC
• Dielectric sensors measure dielectric
not water content
• Correctly converting to water content may
require a medium-specific calibration
• To get accurate nutrient estimates from
bulk EC requires good WC
Component
Dielectric
Permittivity
Air 1
Mineral & Organic 3-5
Water 80
EC
Dielectric

19. AROYA
THE INFORMATION YOU NEED
TO MANAGE THE ROOT ENVIRONMENT

20. RECORD FROM A SINGLE SENSOR

21. CONCLUSIONS
• We “hire” a substrate to provide water, nutrients, and oxygen to our plants
• Understanding how substrates work requires knowledge of water content
and water potential
• Large pores and narrow PSD impose limits on how controlled environment
rooting media can be successfully used for growing plants
• Monitoring is essential for success with these media
– Careful management with good feedback from a monitoring system will allow the operator
to provide the exact right conditions for each growth phase

22. QUESTIONS?
Gaylon S. Campbell, Ph.D.
Senior Scientist
METER Group, Inc.
2365 NE Hopkins Ct, Pullman, WA 99163
T 509.332.2756 F 509.332.5158
E support.environment@metergroup.com W www.metergroup.com