1. The document summarizes methods for measuring and estimating nitrogen (N) levels in crop canopies, including direct measurement through plant tissue testing and indirect estimation using chlorophyll meters, crop sensors, and drones. 2. Direct measurement of N involves testing plant tissues like potato petioles in a lab to determine their nitrate levels, while indirect methods estimate N through relationships with chlorophyll or vegetation indices. 3. The presentation evaluates tools like SPAD meters, leaf color charts, optical sensors, and unmanned aerial vehicles (UAVs) that can help farmers estimate crop N levels and determine optimal nitrogen fertilization rates.

1. Measuring and Estimating N
Levels in the Crop Canopy
Olga Walsh, PhD
Cropping Systems Agronomist & Extension Specialist
University of Idaho, Parma Research & Extension Center
FWAA Winter Conference, Boise, ID
January 12, 2017

2. BS Soil Sci: St Petersburg Russia – 1997; MS Soil Sci – 2007 and
PhD Soil Sci- 2009, OSU, Stillwater, OK;
Soil Nutrient Management - Montana State Univ - 2010 – 2014
Cropping Systems Agronomist, Univ Idaho – from 2014

3. Presentation Outline:
1. Nitrogen (N) role in plants
2. Plant N content
4. Measurement vs estimation
5. Measuring N: plant tissue testing
6. Estimating N: chlorophyll meters &
charts
7. Estimating N: crop sensors
8. Using UAVs (Drones)

4. Nitrogen role in plants

5. N function and deficiency
symptomsN is important for high photosynthetic activity,
vegetative growth and protein building

6. N and photosynthetic activity
 Process by which plants use the energy from
sunlight to produce sugar from CO2and H2O.
 Sugar is converted into energy in plant cells,
used for growth and development
 N – major component of Chlorophyll (green)
 Plants sufficient in N
are healthy and
vigorous and have
high photosynthetic
activity

7.  Chlorosis = yellowing of the leaves  Pale green
to yellow lower (older) leaves
 Why lower leaves? N is MOBILE in plant
 New leaves develop, take nutrients from the old
leaves and use them to grow. The old leaves are
left without enough nutrients  display the
symptoms.
N and chlorosis
Adequate
N
Limited
N
okstate.edu, 2005

8. N and vegetative growth
 Low N  smaller leaves, poor stand
establishment, stunted plants, slow growth
CYMMIT, 2006
Low
N
Adequate
N

9. N and vegetative growth
 N - essential element of all amino acids in plant
structures (building blocks of plant proteins)
 Important in the growth and development of vital
plant tissues and cells
 Enables plants to grow and reproduce
(component of DNA)

10. N and root growth
 Plants adapt to nutrient availability by
changing their root system architecture to
efficiently explore soil zones containing the
limited nutrient.
 Root growth affects
overall plant growth
 Root growth is
affected by N
Giehl & von Wirén, 2014

11. Plant N content

12. Plant N content
 Healthy plants often contain 3 to 4 % of N in
above-ground tissues.
 This is a much higher concentration compared
to other nutrients.
 N content is an important characteristic often
used for evaluation of crop health and nutrient
status.

13. N demand by crops
High
demand (> 120
lb N/a removed in 1
season
Moderate (50-
120 lb N/a removed
in 1 season
Low demand
(<50lb N/a removed
in 1 season

14. Sufficiency
 Deficient – nutrient is at low concentration,
severely limits yield, produces distinct deficiency
symptoms; Extreme deficiency plant death
 Insufficient - nutrient is below amount required for
optimum yields; Symptoms seldom evident
 Sufficient - nutrient present in adequate amounts
for optimum crop growth
 Excessive - nutrient is sufficiently high to result in
a corresponding shortage of another nutrient
 Toxic – element concentration is sufficiently high
to significantly reduce plant growth; Severe toxicity
 plant death

15. Measuring vs estimation

16. Measurement vs Estimation
 Several methods are available for direct
measurement and indirect estimation of N
content in plant biomass.
 Direct = measuring exactly the thing that
you're looking to measure
 Indirect = measuring something by
measuring something else

17. N measurement & estimation

18. Measuring N:
Plant tissue analysis
(potato petioles)

19. Plant tissue analysis
 Directly measures nutrient levels in the plant
 The supply of available nutrients is reflected in plant
nutrient content
 Use of plant tissue analysis allows producers to
evaluate the effectiveness of fertilizer
management
 N taken up by potatoes can be detected in petiole
nitrate analysis within 3-4 days after fertilizer
application
 Indicates the N status of the plant in the past 2-3
days, very good tool for in- season N decisions

20. Response to NResponsetoNitrogen
Deficiency
Sufficiency
Toxicity
Relative foliar N content

21. N content & fertilizer rates
 N content can be used to guide N fertilizer
applications
Increasing N fertilizer rate
Increasingcropyield Physiological optimum
Top yield
Bare
economic
optimum
Can cause
10-15% yield loss
BUT symptoms are
rare

22. N uptake in potatoesTotalNuptake,
kg/ha
200
100
50
0
N
uptake
Effective N management matches N

23. Petiole sampling Agvise Lab, 2017

24. Effect of petiole position on
nutrient concentration
Agvise Lab, 2017

25. Petiole N – lab test
• Petiole samples tested each week help
determine the current nitrogen status of the
potato crop
• N in the form of nitrate (NO3-N) is extracted
from potato leaf petioles with 2% acetic acid
(at room temperature) and analyzed by flow
injection

27. Potato petiole levels

28. Potato petiole sampling
 Early in the season - petiole nitrate levels are
expected to be high (20,000 - 30,000 ppm)
 During rapid vegetative growth & beginning of
tuber bulking - petiole levels are expected to
drop
 If temp warms up quickly after cool period –
explosive rapid vine growth  drop in petiole N
levels to 5000 ppm or lower = dilution of the
nitrate in the potato petiole, BUT the canopy
can look lush and green
 Apply more N? - If the total soil N >60 lb/a  No
need to additional N fertilizer

29. Estimating N:
Chlorophyll meters &
Leaf color charts

30. Chlorophyll & Plant N
• Chlorophyll is positively and linearly correlated
with plant leaf N – can use chlorophyll to
estimate NChlorophyll
concentration
Leaf N concentration

31. How Chlorophyll meters work
 The meter measures the ratio of radiation
transmittance from two wavelengths:
 red……..strongly absorbed by chlorophyll,
and
 near infrared……..not absorbed by
chlorophyll
 Meters expose a portion of a leaf to
abundant light and measure how much was
NOT captured by chlorophyll in the
photosynthetic process

32. Chlorophyll Meters
FieldScout,
$2,850
Spectrum Technologies; Apogee Instruments, atLeaf
2017
SPAD,
$2,700

33. SPAD meter accuracy
Xiong et al,
2017
• SPAD values are highly correlated with
chlorophyll content (left) AND with plant N
content (right)

34. Chlorophyll Meters
 SPAD, FieldScout - values proportional to amount
of chlorophyll in the leaf
 MC-100 – values of actual chlorophyll
concentration
Primack, 2015
atLeaf readings
SPADreadings
$2,700 vs $250

35. From Chlorophyll to N rate
• 6 years of field trials
• Relationship between Chlorophyll and
Economic N rate (Figure 1)
• Developed N recommendations (Table
1)

36. Leaf color charts
Board + App =
$150
IRRI; Spectrum Technologies,
2017
As low as $1.00/unit

37. Estimating N:
Crop Sensors

38. Optical
Crop
Sensors
hollandscientific.com; topconpositioning.com; nue.okstate.edu; agleader.com

39. Sensor-Based N Rate
1. We need
a lot
2. We don’t need
much
3. We need
a little1. High Yield
Potential,
plants some-
what deficient
in N
2. Very high
Yield Potential,
almost
adequate N
nutrition
3. Low Yield
Potential,
plants are very
deficient

40. Sensor Basics
• Emits light and measures reflectance from plants
• Red light is used for photosynthesis (absorbed)
• NearInfrared light – not enough energy, not
used (reflected)
• Sensor reading - Similar to a plant physical
examination
www.nue.okstate.edu, 2014

41. Sensor Basics
• Sensor can detect:
 Plant Biomass
 Plant Chlorophyll
 Crop Yield
 Water Stress
 Plant diseases, and
 Insect damage
 Sensors are used by agronomists, breeders, plant
pathologists, weed scientists, crop consultants,
growers
www.nue.okstate.edu, 2014

42. red
redNIR
NIR
30%50%
60% 8%
NDVI = (NIR-red)/(NIR+red)
Sensor detects the amount of light reflected from
the crop and calculates NDVI Tubana, 2007
NDVI = 0.76
NDVI = 0.25

43. What the GreenSeeker sensor “Sees”
The vigor of the leaves
and
the ratio of plant to soil
affect NDVI values
N-Tech Ind., 2009

44. N-Tech Ind., 2009
What the GreenSeeker sensor “Sees”

45. From sensor to N Rate
 Established Reference Strip (at seeding)
 Compared to the rest of the field (sensed at tillering)
 Now we know: yield potential and crop
responsiveness to N
 How do we determine the needed N rate?
 Need a formula (algorithm) to translate
sensor readings into N recommendation

46. NDVI vs crop yield
NDVI sensors
can estimate
crop yield with
90-95% accuracy
Walsh et al,
2013

47. NDVI vs yield
NDVI explained 68%
of variation in wheat
yield.
Combining NDVI and
plant height increased
accuracy of yield
prediction to 73%.
Walsh et al,
2015

48. Using UAVs (Drones)

49. DRONE STUDY - Objectives
To establish a UAV-based methodology for
in-season prediction of:
 wheat N content
 grain yield potential, and
 and prescribing N fertilizer rates

50. Drone Study

51. N rates and crop sensor data
UAV-based NDVI map
showing N rates
applied
Map showing UAV-
based NDVI values
Walsh et al,
2016
Spring wheat, Parma, ID,
2016

52. 3-D view of wheat plots,
Rupert, ID

53. NDVI data collected with UAV

54. Measured vs estimated plant N
Walsh et al,
2017
Drone
images
provide
accurate
estimation
of plant N
content

55. Thank you!
Questions?
Olga Walsh
owalsh@uidaho.edu
University of Idaho, Parma R&E Center
Tel: (208)722-6701
Web: IDCrops
(http://idcrops.blogspot.com/) Twitter:
@IDCrops