A system for spectrally monitoring the stages of crop development for corn and soybeans based upon red and photographic infrared spectral radiances is proposed. The red and photographic infrared spectral radiance, highly correlated with the green leaf area index or green leaf biomass, enable nondestructive monitoring of the crop canopy throughout the growing season. Five distinct periods are apparent which are related to crop development for corn and soybeans

Elgin, J. H., Jr.

Mcmurtrey, J. E., III

Tucker, C. J.

English

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Technical Memorandum 79607
I
Monitoring Corn and Soybean
Crop Development by Remote
Sensing Techniques
Compton J. Tudcer,
James H. Elgin, Jr, and
James E. McMurtrey,111
(NASA-TH-79607) MOSITCOING CCRN INC SCYEEAN
CPOP DEVELOPMENT EY BEMOTT SENSING
TECHNIQUES (NASA) 16 F HC AO2/MF A01
CSCI 02C
AUGUST 1978
National Aeronautics and
Space Administration
Goddwd Slows FDght NmW
Greenbelt, Maryland 20771
N78-306?9
Onclas
G3/43 29348
r^_i,:
TM 79607
MONITORING CORN AND SOYBEAN CROP DEVELOPMENT
BY REMOTE SENSING TECHNIQUES
Compton J. Tucker
Earth Resources Branch, Code 923
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771 USA
and
James H. Elgin, Jr.
James E. McMurtrey III
BARC, Field Crops Laboratory
Plant Genetics and Germplasm Institute
USDA/SEA
Beltsville, Maryland 20705 USA
August 1978
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
-	 -,_
ABSTRACT
A system for spectrally monitoring the stages of crop development for corn
and soybeans based upon red and photographic infrared spectral radiances is
proposed. The red and photographic infrared spectral radiances, highly corre-
lated with the grean leaf area index or grean leaf biomass, enable nondestructive
monitoring of the crop canopy throughout the growing season. Five distinct peri-
ods are apparent which are related to crop development for corn and soybeans.
iii
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CONTENTS
INTRODUCTION .......................................
MATERIALS AND METHODS ..............................
RESULTS AND DISCUSSION ..............................
SUMMARY ................................
ACKNOWLEDGMENTS ........................
REFERENCES ..............................
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MONITORING CORN AND SOYBEAN CROP DEVELOPMENT
BY REMOTE SENSING TECHNIQUES
INTRODUCTION
The application of remotely sensed multispe:tral imagery for monitoring
the condition of agricultural crops and predicting yields has advanced rapidly
1
in recent years. By necessity, much of this effort is coupled with ground-based
data on crop development and climatic conditions throughout the growing season. 	 1
Several numerical schemes for recording stages of crop development have been i
developed (Hanway, 1963; Hanway and Thompson, 1971). One of the problems, 	 -
1
however, in trying to monitor crop condition and predict crop yields has been 	 +`
is
the inability to collect ground-based crop development data :,vei the large areas
generally surveyed in remote sensing projects. Put simply, it is not feasible
to collect crop development data to supplement satellite imagery for any large-
scale agricultural monitoring project. In extra-country situations, on-site
	 J
access may be impossible and, in locations like North America or Australia,
manpower requirements may be too costly. A possible solution would be to
relate remotely sensed data to crop development data, thereby generating a
"spectral" crop development value whose inputs would come entirely frOW re-
mote sensing sources. This paper describes such a method using remotely
sensed data collected by a two-channel hand-held radiometer.
1
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MATERIALS AND METHODS
A field of Elinsboro sandy loam soil located on the USDA Beltsville Agri-
cultural Research Center was selected for this study. Four 6 x 6 m plots each
of corn and soybeans were planted on April 28 (Julian date 118) and May 20
(Julian date 140) 1977, respectively. Row spacing was 91 cm (23 em within the
row) for the c, rn and 76 cm (5 cm within the row) for the soybeans. Agronomic
data pertaining to crop development, percentage crop cover, and plant height
were taken weekly throughout the growing season. Crop development was re-
corded using numerical scales devised by Hanway (1963) for corn and by Hanway
and Thompson (1971) for soybeans. Percentage crop cover (percent of ground
covered by the crop canopy) was visually estimated for each plot and plant height
was measured in centimeters. In addition to the small plots, a large field (-1 ha)
of each crop was planted at approximately the same time. Four 6 x 6 m areas
were designated in each field and monitored as a check on the validity of data from
the small plots. For both the large fields and small plots soil fertility was opti-
mum and chemical weed controls were used.
Weekly radiance /reflectance measurements were taken on each plot with a
two-channel hand-held radiometer (Pearson et al. 1976) beginning May 31 (Julian
date 151) for the corn and June 21 (Julian date 172) for the soybeans. The two
channels (spectral bands) used were .650-.700 y m and .77i>-.825 µ m. These
wave lengths have been shown to be of excellent utility for monitoring green
vegetation or green leaf biomass (Pearson and Miller, 1972; Gausman et al.,
2
0
(	 ) '	 f	 I-
1973; Tucker, 1978). The .650-.700 µ m band, situated in the red region of the
spectrum, is inversely related to the chlorophyll content. The .775-.825 p m
band, situated in the photographic infrared region, is directly related to the green
leaf area index or green leaf biomass. A vegetation index (VI) was calculated
from the radiance readings as follows after Rouse et al. (1973):
VI - (IR - Red)
(IR+Red)
Each week, 16 red and it measurements were made for each soybean plot; 24
were made for each corn plot. Data were collected from a 1.5-m ladder for
corn plots and from the ground for the soybean plots. The hand-held radiometer
was held — 3.5 m and -1.5 m above the ground surface for the corn and soybean
canopies, respectively. Data from each experimental plot were averaged and the
mean values used thereafter in the analysis (Tucker et al., 1978).
Although an attempt was made to sample the experimental plots every 7 days
on sunny days between the hours of 1100 and 1500 EDT, weather conditions often
precluded the 7-day sampling procedure. Actual measurements were usually
made at 6- to 8-day intervals, with the minimum interval being 4 days and the
:,taximum being 12 days.
RESULTS AND DISCUSSION
Agronomic and spectral measurements were similar for the four corn plots
as were measurements for the four soybean plots (Tucker et al., 1978a). Figure 1
illustrates the relationship between the VI, percentage crop cover, and plant height
220	 260
JULIAN DATE
(A)
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Figure 1. The VI I (ir - red)/(ir + red)], plant height estimated crop cover,
and estimated chlorosis are plotted against Julian date for (A) corn and (B)
soybeans. In addition, agronomic data pertaining to stage of growth is also
noted with respect to Julian date. Each plot is the average of two replicates
having the same agronomic treatment(s).
.;
during the growing season for representative plots of each crop. Both percentage
plant cover and plant height were closely associated with VI (Tucker et al., 1978a).
Data from the large fields were similar to those for the small plots.
Five stages of spectral crop development (SCD) were observed for the corn
and soybean plots (Figures 2a and 2b);
Stage 1. For both crops, the VI for the bare soil before crop emergence
and after emergence up to 10-20% cover was negative.
Stage 2. The rapid increase in vegetative crop cover which occurred for
both corn and soybeans prior to bloom was indicated by a rapid increase in VI.
Stage 3. Once full vegetative crop cover was achieved, a plateau in the VI
was detected. SCD Stage 3 continued during crop bloom and until crop chlorosis
was detected.
Stage 4. During the period of crop maturation and dry-down, the VI declined
gradually. Leaves became chlorotic -md were often lost from the plants.
Stage 5. SCD Stage 5 was detected when the soybeans had lost all leaves
i
and the corn had lost all color, i.e., when the crops were mature and ready for
harvest. The VI was once again similar to that measured at crop emergence.
The two principal factors which influence or control the VI are the chloro-
phyll density and the green leaf biomass. Conditions which affect eitht ^ chloro-
phyll density or groan leaf biomass are expressed in the VI values. For , •xample,
stress conditions such as drought would affect the VI by limiting plant growth
and the plateau point would be reached at a later date or a lower plateau would
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164	 188
	
232	 266	 300
JULIAN DATE
(A)	
CJTl6SFC 78
JULIAN DATE
C,IT/M 71
(B)
Figure 2. Spectral Crop Development stages for (A) corn and (B) soy-
beans. Five stages are apparent: 1. Emergence up to 10-20% vegeta-
tion cover; 2. Rapid foliar growth and development; 3. Full vegetative
cover; 4. Onset of senescence, crop maturation, and dry down; 5. Crop
maturity, ready for harvest. The averaged VI is plotted against Julian
date for two reulicates for each crop.
6
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be attained. Or, if drought conditions occurred after the plateau had been reached,
the plant canopy would respond by wilting and a reduction in the VI would occur.
Chlorosis, a decrease in chlorophyll density which is brought about by un-
favorable environmental factors, would also be expressed in lower VI values.
Soil fertility, particularly nitrogen, would affect the accumulation and density of
green biomass and thus would be expressed in the VI. High fertility would result
in higher values for the VI while low fertility would result in lower values.
Weeds pose a confounding problem for the VI as the VI is sensitive to green
biomass whether it is weeds, crop, or weeds and crop. However, the sensitivity
to weeds by the VI could possibly be exploited to detect the presence of substantial
patches of weeds. If unexpectedly high values for the VI were found earlier in the
growing period, this could indicate the presence of some other type ( s) of green
leaf biomass (Figure 3).
The relationship between the VI an, crop development observed in our study
indicates the utility for an assessment of crop condition through spectral meas-
uriments. Attaining and sustaining full vegetative cover stage is an important.
factor in conditioning the final yield of any crop. The spectral assessment of the
stages of crop development is an important step toward making yield predictions
by remote sensing.
SUMMARY
1. A system for monitoring the stages of crop development based solely upon
red and photographic infrared spectral measurements is proposed for corn and
soybeans.	 7
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JULIAN DATE
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Figure 3. Comparison between the VI's for three different agronomic
treatments for soybeans. Note how the presence of wee ps in the "no
weed control" plot resulted in i ► igher VI va.ues than the other soybean
treatments with weed control earlier in the growing season.
130
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2. Five distinct and spectrally measurable stagers are apparent for these
two crops:
A. Eme: gene up to 10-20% vegetative cover
B. Rapid foliar growth and development
C. Full vegetative cover
D. Onset of ser#.escence, crop maturation, and dry down
E. Cr;,p maturity, ready for harvest.
ACKNOWLEDGMENTS
The adthors wick to thank the following people for assisting them in the field
data collection effort: Jim Fan, Lee Miller, Shelly Summer, Simon Chang, Steve
Fields, Charley Schnetzler, Lou Walter, George Jacobs, Lelo de Gasparus, Frank
Wood, Sigfried Auer, Susan Dankoff, Per Eckerborn, Jerry Christensen, Craig 	 r
Tom, Bill Wigton, Ruth Whitman, John Barker, Ned Fetcher, Emmett Chappelle,
and Conp ie Miller. The loan of the instrument from Freeman Smith is gratefully
acknowledged.
s,
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9
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REFERENCES
Gausman, H. W , W A. Allen, R. Cardenas, and A. J. Richardson. 1973. Re-
flectance discrimination of cotton and corn at four growth stages. Agron.
J. 65: 194-198.
lianway, J. J. 1963. Growth stages of corn. Agron. J. 55: 487-492.
Hanway, J. J. and H. E. Thompson. 1971. How a soybean plant develops.
Special report 53, Iowa St. Univ. Coop. Extension Service, Ames, Iowa,
June 1971.
Pearson, R. L. and L. D. Miller. 1972. Remote mapping of standing crop bio-
mass for estimation of the productivity of the shortgrass prairie. 8th
International Symp, on Remote Sensing of Environment, Univ. of Michigan,
Ann Arbor.
Rouse, J. W., R. H. Haas, J. A. Schell, and D. W. Deering. 1973. Monitoring
vegetation systems in the great plains with ERTS. Third ERTS Symposium,
NASA Sp-351 I: 309-317.
Tucker, C J. 1978. Red and photographic infrared linear combinations for
monitoring vegetation. Remote Sensing of Environ. (in press).
Tucker, C. J., J. H. Elgin, Jr., and J. E. McMurtrey III. 1978a. Relationship of
red and photographic infrared spectral radiances to corn and soybean bio-
mass, percent cover, plant height, chlorosis, and leaf loss. NASA/GSFC
Technical memorandum (submitted to PE&RS).
10
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Tucker, C. J., C. J. Fan, J. H. Elgin, and J. E. McMurtrey. 1978b. Hand-held
radiometer red and photographic infrared spectral measurements of agri-
cultural crops. NASA/GSFC Tech. Ma morandum 78091 (in press, Remote
Sensing of Environ.).
1.1


Monitoring corn and soybean crop development by remote sensing techniques

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