Grow your crop steering expertise Crop steering can optimize crop production and production costs, but to crop steer successfully, you need to do it right. You have to understand how to obtain the right soil water contents and soil electrical conductivities to either stress the crop or avoid stressing the crop in a controlled way. To do this, you’ll need to perform crop steering calculations. Steer your way to higher quality, productivity, and profit In part 3 of our greenhouse webinar series, Dr. Gaylon Campbell, internationally recognized soil physics and environmental measurement expert, teaches how to perform crop steering calculations that give you the information you need to stress or de-stress your crop at the right time and in the right way to achieve your goals. In this 30-minute webinar you’ll learn: The water balance equation - How to calculate the irrigation amount - How to calculate the transpiration variables that affect recharge drainage, and changes in stored water - How to determine the field capacity of the substrate - Environmental factors that influence the water balance - How to determine the leaching fraction - How to manage substrate electrical conductivity crop steering, environment, field capacity, gaylon campbell, indoor cultivation, irrigation, leaching fraction, substrate electrical conductivity, transpiration, water balance, webinar

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

3. SUBSTRATES – MAIN POINTS
3
• We “hire” a substrate to provide water, nutrients and oxygen to plants
• Understanding how substrates work requires knowledge of both 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
• Water is typically readily available to plants over the whole useful water
content range
• 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

4. NUTRIENTS – MAIN POINTS
4
• Electrical conductivity directly measures the concentration of nutrients in
the irrigation water
• Sensors measure bulk EC, which, in substrates, must be converted to pore
water EC to be useful
• Water and nutrients vary over time. Both need to be monitored
• Osmotic potential varies directly with EC.
• We can manipulate osmotic potential to stress plants for crop steering
• Careful monitoring and control of water and nutrients is essential for
optimum production

5. 5
DS = I - D - E
• DS - Change in stored water
• I - Irrigation
• D - Drainage
• E - Evaporation
WATER BUDGET OF A PLANT

6. IRRIGATION AMOUNT (I)
I = N x Q x T [ml/plant/day]
• N - number of drippers per plant
• Q - flow per dripper (ml/min)
• T - irrigation minutes per day

7. EXAMPLE
• 2 emitters, 1.2 L/hr, 30 minutes per day irrigation (6
shots, 5 min/shot). Assume substrate volume is 6 liters
• Q = 1200 ml/hr x 1 hr/60 min = 20 ml/min
• I = N x Q x T = 2 x 20 ml/min x 30 min/day = 1200
ml/plant/day
• Shot volume = N x Q x t = 2 x 20 ml/min x 5 min = 200 ml
• One shot is (200 ml/6000 ml) x 100 = 3.3% of the
substrate volume

8. VERIFYING ACCURACY OF
EMITTERS
To properly manage water in controlled environments you must
know how much water each plant receives, and that the amount of
water each plant in a zone receives in a day is the same.
Don’t assume your emitters are accurate – measure them
1. Stakes in a cup
2. Run irrigation for 5 minutes
3. Measure the water; calculate Q
4. Do this for several locations in the zone

9. EVAPORATION (E)
Evaporative loss includes both transpiration from
plants and evaporation of water from substrate –
mainly loss from plants
Depends on
- Leaf area
- Stomatal conductance
- Radiation load (lights)
- Wind
- Temperature and vapor deficit of the air

10. CALCULATING EVAPORATION
𝐸 = 𝑓$ 𝑔&
'( )* +',
𝝆 .,
[ml/plant/day]
fi - fraction of incident light intercepted by the canopy
r - plant density [plants/m2]
gv - vapor conductance (stomatal and boundary) [ml/m2/day]
es(Tc) - saturation vapor pressure at canopy temperature [kPa]
ea , pa - vapor pressure of air and atmospheric pressure [kPa]

11. CALCULATING CANOPY
TEMPERATURE
𝑇0 = 𝑇1 +
3∗
563∗
7,8(+9(:;<
=>?0@
−
B
.,3∗
[C]
Ta - air temperature [C]
Rabs - absorbed radiation [W/m2]
D - vapor deficit of the air [kPa]

12. CALCULATING EVAPORATION
• The theory is complete and sound
• The equations are big and many, but are easy for a
computer
• All of the inputs to do the calculations are readily
measured
• To calculate E , we need to know:
air temperature, leaf area, stomatal conductance,
radiation, wind, and vapor deficit

13. MEASUREMENTS
Fractional interception Radiation load (lights)
Wind
Air temp & vapor deficit
Stomatal conductance

14. WATER BALANCE EXPERIMENT
• Plants on load cells monitor
I , E , DS , and field capacity
• TEROS 12 sensors monitor
substrate water content
• Sensors monitor light,
temperature, humidity, wind
for E calculation

15. LOAD CELL RESULTS

16. LOAD CELL VS. TEROS 12

17. RECHARGE AND DRAINAGE

18. SOME POINTS TO REMEMBER
• “Field capacity” is the water content to which the substrate
drains after recharge
• Shots given after the substrate reaches field capacity are lost
to drainage
• Shots given before drainage starts replace yesterday’s
transpiration

19. ADDITIONAL POINTS
• Change in storage is directly measured by the load cell;
it is also equal to the water content change multiplied by
the substrate capacity
• Slope of the load cell trace is transpiration rate; slope of
water content multiplied by substrate capacity is
transpiration rate

20. LEACHING FRACTION
• Definition: LF = D/I = ECirrigation / ECdrainage
• At steady state, and with no uptake of salts:
ECdrainage = ECirrigation / LF
• Example: assume LF = 0.5, ECirrigation = 3 dS/m,
then ECdrainage = 6 dS/m

21. CONCLUSIONS
21
• Irrigation minus evaporation minus drainage must equal the change in water storage
• The use of good emitters, carefully checked and calibrated, along with known
irrigation durations provide the basis for good irrigation practice, and allow us to
quantify irrigation amounts
• Appropriate theory plus measurements of air temperature and vapor deficit,
radiation, wind, stomatal conductance, and fractional interception allow direct
computation of transpiration rate
• Field capacity of a substrate sets an upper limit for the amount of water it can hold
• Shots given after a substrate reaches field capacity go to drainage
• Leaching fraction gives insight into the relationship of drainage EC to irrigation EC

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