Do In and Post-Season Plant-Based Measurements Predict Corn Performance and/ or Residual Soil...
1
Do In and Post-Season Plant-Based Measurements Predict Corn Performance and/ or Residual Soil Nitrate? Patrick J. Forrestal, R. Kratochvil, J.J Meisinger Abstract Plants demonstrate visual and physiological signals that are reflective of their growing conditions. These signals are being used to guide development of early season variable-rate fertilizer nitrogen (FN) applications to corn (Zea mays, L.) using optical sensors. This project examined whether monitoring these signals in-season can quantify residual soil nitrate following corn harvest. Corn was monitored in-season at growth stages R2, R3-R4 and R5 using a portable chlorophyll meter, an optical sensor and by green leaf counts. Data from all three measurement techniques was significantly correlated with corn relative yield and with each other. However, the relationship with residual soil nitrate for all three techniques was weak, reflecting the inability of these tools to accurately characterize soil nitrogen (N) fertility following corn. Nevertheless, it was observed that monitoring these signals late in the growing season could provide a simple and useful on-farm tool to assess corn nitrogen management. For example, a producer could easily evaluate new management practices using on-farm field strips that include a new practice, a standard practice and a high N fertility strip. Use of a simple green leaf count could assess whether the new N management practice was adequate. Corn will luxury accumulate nitrate in the corn stalk; the basis for the corn stalk nitrate test (CSNT), a post- maturity test. The CSNT can characterize corn N fertility status in the above optimum range. This project is also examining ability of the CSNT to measure residual soil nitrate and corn performance. Although much work has been done that relates CSNT results to corn relative yield, little information has been published relating CSNT results to residual soil nitrate. It was found that increased CSNT levels tended to indicate increasing soil nitrate concentrations. However, this relationship was not considered consistent enough to accurately quantify residual nitrate. Materials and Methods Locations and Soil Type: The three in-season plant based measurements were collected from six site- years (2006 and 2007) on Sassafras Sandy Loam, Mattapeake and Mattapex Silt Loam soils located at University of Maryland Research and Education Centers. CSNT data was also collected at these six sites during 2006 and 2007. In addition, CSNT data was collected from sites on Sassafras Sandy Loam, Mattapeake, Mattapex and Delanco Silt Loam soils in 2008 resulting in ten site-years of CSNT data. Experimental Design: A randomized complete block (RCB) design with four replications was used. Four corn FN rates of 0 (67 in 2006), 135, 202 and 270kg ha -1 were applied as 34 kg N ha -1 starter and the balance as side-dress except for the 0 kg N ha -1 treatment which received no FN. Procedures: One corn hybrid (Pioneer 33B51) was planted at the sites in May. Green leaf counts, SPAD chlorophyll meter and optical sensor readings were collected at corn growth stages R2, R3-R4 and R5. Green leaf counts were collected by counting the number of green or partially green leaves below and including the ear leaf. SPAD meter readings were collected from the corn ear leaf. Optical sensor readings (normalized difference vegetative index [NDVI] was used) were collected from within the corn canopy using a Crop Circle TM optical sensor. This was accomplished by carrying the sensor head at approximately 6 feet height and directing its sensing beam toward the ear leaf area. The Pearson correlation matrices presented illustrate the linear correlation between variables. Relative reading scores for the three techniques and relative grain yields were calculated by expressing individual plot plant readings or plot yield measurement as a percentage of the mean response from the highest FN treatments (i.e. the high FN reference where N availability was not limiting corn performance) in a similar manner to the technique used by Fox et al., (2001). Corn stalk samples for the CSNT were collected following the procedure of Binford et al., (1990). Corn grain was harvested at maturity and yield recorded. Following corn grain harvest 8 soil cores were collected from each plot to a depth of 60 cm (30 cm in 2006). Each core was divided into sections by depth (0-15, 15-30 and 30-60cm). Soil samples were analyzed for NO 3 - -N and NH 4 + -N using a Lachat auto-analyzer. Analysis: Results The correlations between relative corn leaf based measurements and relative grain yield were highly significant for all three methods and growth stages examined (Tables 1-3). However, the strength of this relationship, as shown by the correlation coefficient (r), varied between the methods and time of observations. All methods produced the strongest correlation with relative grain yield at corn growth stage R3-R4 (r = 0.84, 0.72, 0.83 respectively Tables 1- 3). Relative green leaf counts and relative optical sensor readings were more strongly correlated with relative yield (r 2 = 0.70 and 0.69 respectively, Fig. 1 & 5) than relative SPAD meter readings (r 2 = 0.51, Fig. 3). The correlations between relative corn leaf based measurements and post-harvest soil nitrate concentration in the surface 30 cm was significant for the three methods examined at the R3-R4 observation time (P<0.05) , however the linear relationship for each of the methods was weak (Fig. 2,4, and 6). In treatments where relative leaf measurements at R3-R4 were < 90% (< 80% for green leaf counts) of the high FN reference measurements, residual soil nitrate concentration tended to be relatively low (< 8 mg/l). However, when relative Conclusions Green leaf counts and optical sensor generated NDVI readings collected at corn growth stage R3-R4 are equally effective for assessing the impact of FN management practice on corn grain yield (r = 0.84 and 0.83 respectively, Tables 1&3). Collection of green leaf counts and optical sensor readings at growth stage R3-R4 maximized yield prediction accuracy. The relationship between SPAD meter readings and relative grain yield was less affected by time of observation (Table 2) but less accurate for assessing the impact of FN management on yield than either leaf counts or optical sensor readings. None of the three in-season measurements examined were effective methods of quantifying soil residual nitrate because readings did not increase in response to increased FN availability once N sufficiency for the corn was reached. However, if readings were <80-90% of the high N reference, soil nitrate concentration (0-30cm) was consistently low (< 8mg/l). Increased CSNT levels tended to reflect increasing soil nitrate concentrations. The CSNT can provide an indication of residual soil nitrate level, however, the relationship is considered not consistent enough to accurately quantify residual Literature Cited Binford G.D., A.M. Blackmer and N.M. El-Hunt. 1990. Tissue Test for Excess Nitrogen during Corn Production. Agron. J. 82:124-129. Fox R.H., W.P. Piekielek., K.E. Macneal. 2001. Comparison of Late-Season Diagnostic Tests for predicting nitrogen status of corn. Agron. J. Corn Yield Residual Nitrate 0-30 cm Corn FN rate Relativ e leaf count R1 Relative leaf count R2 Relative leaf count R3-R4 Relative leaf count R5 Corn Yield 1.00 0.30 <0.01 0.65 <0.000 1 0.57 <0.000 1 0.56 <0.0001 0.84 <0.0001 0.31 <0.01 Residual Nitrate 0-30cm 1.00 0.32 <0.002 -0.07 0.567 0.22 <0.03 0.33 <0.001 0.10 0.312 Corn FN rate 1.00 0.49 <0.000 1 0.61 <0.0001 0.72 <0.0001 0.23 0.02 Relative leaf count R1 1.00 0.50 <0.0001 0.60 <0.0001 0.09 0.48 Relative leaf count R2 1.00 0.73 <0.0001 0.20 0.049 Relative leaf count R3- R4 1.00 0.35 <0.001 Relative leaf count R5 1.00 Table 1: Pearson Correlation Matrix relationship between relative corn Green Leaf Counts at 4 observation times, relative corn grain yield, soil residual nitrate concentration and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha -1 . Corn Yield Residual Nitrate 0-30 cm Corn FN rate Relative SPAD R1 Relative SPAD R2 Relative SPAD R3-R4 Relative SPAD R5 Corn Yield 1.00 0.30 <0.003 0.65 <0.000 1 0.63 <0.0001 0.70 <0.001 0.72 <0.001 0.59 <0.0001 Residual Nitrate 0-30cm 1.00 0.32 <0.002 0.08 0.558 0.22 <0.04 0.21 <0.05 0.24 <0.02 Corn FN rate 1.00 0.52 <0.0001 0.61 <0.0001 0.65 <0.0001 0.49 <0.0001 Relative SPAD R1 1.00 0.58 <0.0001 0.60 <0.0001 0.29 <0.03 Relative SPAD R2 1.00 0.78 <0.0001 0.44 <0.0001 Relative SPAD R3-R4 1.00 0.53 <0.0001 Relative SPAD R5 1.00 Corn Yield Residual Nitrate 0-30 cm Corn FN rate Relative NDVI R2 Relative NDVI R3-R4 Relative NDVI R5 Corn Yield 1.00 0.30 <0.003 0.65 <0.0001 0.66 <0.0001 0.83 <0.0001 0.73 <0.0001 Residual Nitrate 0-30cm 1.00 0.32 <0.002 0.23 <0.03 0.28 <0.005 0.37 <0.001 Corn FN rate 1.00 0.57 <0.0001 0.67 <0.0001 0.71 <0.0001 Relative NDVI (R2) 1.00 0.71 <0.0001 0.64 <0.0001 Relative NDVI R3-R4 1.00 0.81 <0.0001 Relative NDVI R5 1.00 R 2 = 0.7043 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 Relative G reen Leafcountatcorn grow th stage R3-R4 RelativeCornGrainYield R 2 = 0.1079 0 10 20 30 40 50 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 Relative G reen Leafcountatcorn grow th stage R3-R4 Post harvest residual nitrate(mg/l) R 2 = 0.5111 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 R elative SPAD m eter readings atR3-R4 RelativeCornGrainYield R 2 = 0.0427 0 10 20 30 40 50 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 Relative S PA D m eterreadings atcorn grow th stage R3-R 4 Post-harvest residual nitrate(mg/l) R 2 = 0.6886 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 R elative O pticalS ensor readings atcorn grow th stage R 3-R 4 RelativeCorn Grain Yield R 2 = 0.1454 0 10 20 30 40 50 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 Relative O pticalSensor readings atcorn grow th stage R3-R4 Post-harvest residual nitrate(mg/l) R 2 = 0.4231 0 10 20 30 40 50 60 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 C orn stalk nitrate concentration (m g/l) Post-harvest soil nitrate0-30cm (mg/l) Table 2: Pearson Correlation Matrix relationship between relative SPAD Meter Readings at 4 observation times, relative corn grain yield, soil residual nitrate concentration and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha -1 . Table 3: Pearson Correlation Matrix relationship between relative Optical Sensor Readings (NDVI) at 3 observation times, relative corn grain yield, soil residual nitrate and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha -1 . Figure1: Relationship between relative green leaf counts at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 2: Relationship between relative green leaf counts and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 3: Relationship between relative SPAD meter readings at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 4: Relationship between relative SPAD meter readings at R3-R4 and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 6: Relationship between relative NDVI at R3-R4 and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 5: Relationship between relative NDVI at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha -1 Figure 7: Relationship between corn stalk nitrate concentration and residual soil nitrate concentration in the surface 30cm (10 site- years ) that received FN rates ranging 0 to 270 kg ha -1 . Discussion The in-season plant-based measurements examined in this study were significantly correlated with relative grain yield. Timing of the measurements influenced the strength of this relationship and was especially important for leaf counts and optical sensor readings. These measurements had stronger correlations with relative grain yield at the R3-R4 growth stage than at the other observation timings examined (Tables 1 & 2). SPAD meter readings had weaker correlations with relative grain yield but this relationship was more stable across observation times (Table 2). The difference observed between the relative strength and stability of this relationship between readings over time may be attributable to the contrasting size of the sampled area for each of the techniques. The SPAD meter collects information from a small area of ear leaf tissue. In contrast, green leaf number conveys more information about total plant green leaf area. Similarly, the optical sensor assimilates information from a relatively large sensed area of the plant to produce an NDVI reading. Nitrogen stress is manifested as senescence of the leaves starting with the older (lower) leaves and advancing upwards. SPAD meter readings taken from the ear leaf do not capture well the stress exhibited by lower canopy senescence. Use of in-season green leaf counts and optical sensor readings provided the most accurate snapshot of corn yield potential when taken at R3-R4 growth stage. A producer could include a high FN reference strip in his field along with a standard practice FN rate and a reduced rate. The use of either green leaf counts or an optical sensor could provide an easy way to assess the impact of new FN management practices on corn yield potential. Collection of readings with the SPAD meter at R3-R4 growth stage did not improve yield prediction accuracy as it did when using the other two techniques. This indicates that observation timing is less important when using SPAD meters. However, SPAD meter readings were less accurate than either green leaf counts or optical sensor readings at the R3-R4 timing (Figs. 1,3&5). The linear relationship between the readings and post-harvest residual soil nitrate was weak but an interesting relationship was observed. When adequate or excessive levels of FN were applied to maximize corn performance, relative readings tended to be close to 1.00 but residual nitrate concentration in the surface 30cm varied greatly (1-49 mg/l) (Fig. 2,4, and 6). Use of the in-season plant-based measurements did not detect a plant response resulting from excessive FN applications. These plant based measurements are unable to quantify excessive levels of residual soil nitrate because the in-season measurements plateau in response to additional levels of FN beyond sufficiency, an outcome that is comparable to the response observed for corn yield with excessive FN applications. This study also examined the CSNT as a potential indicator of residual soil nitrate. Corn stalks will “luxury accumulate” nitrate which suggests that this test holds potential as an indicator of residual soil nitrate. The CSNT was found to have a significant relationship with soil nitrate concentration in the surface 30cm (r 2 =0.42 , Fig. 7). This test could act as an indicator of residual nitrate, however, the relationship was not consistent and thus is considered not accurate enough to quantify residual soil nitrate concentration.
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Transcript of Do In and Post-Season Plant-Based Measurements Predict Corn Performance and/ or Residual Soil...
Do In and Post-Season Plant-Based Measurements Predict Corn Performance and/ or Residual Soil Nitrate?
Patrick J. Forrestal, R. Kratochvil, J.J Meisinger Abstract
Plants demonstrate visual and physiological signals that are reflective of their growing conditions. These signals are
being used to guide development of early season variable-rate fertilizer nitrogen (FN) applications to corn (Zea
mays, L.) using optical sensors. This project examined whether monitoring these signals in-season can quantify
residual soil nitrate following corn harvest. Corn was monitored in-season at growth stages R2, R3-R4 and R5 using
a portable chlorophyll meter, an optical sensor and by green leaf counts. Data from all three measurement techniques
was significantly correlated with corn relative yield and with each other. However, the relationship with residual
soil nitrate for all three techniques was weak, reflecting the inability of these tools to accurately characterize soil
nitrogen (N) fertility following corn. Nevertheless, it was observed that monitoring these signals late in the growing
season could provide a simple and useful on-farm tool to assess corn nitrogen management. For example, a
producer could easily evaluate new management practices using on-farm field strips that include a new practice, a
standard practice and a high N fertility strip. Use of a simple green leaf count could assess whether the new N
management practice was adequate.
Corn will luxury accumulate nitrate in the corn stalk; the basis for the corn stalk nitrate test (CSNT), a post- maturity
test. The CSNT can characterize corn N fertility status in the above optimum range. This project is also examining
ability of the CSNT to measure residual soil nitrate and corn performance. Although much work has been done that
relates CSNT results to corn relative yield, little information has been published relating CSNT results to residual
soil nitrate. It was found that increased CSNT levels tended to indicate increasing soil nitrate concentrations. However, this
relationship was not considered consistent enough to accurately quantify residual nitrate.
Materials and Methods
Locations and Soil Type:
The three in-season plant based measurements were collected from six site-years (2006 and 2007) on Sassafras
Sandy Loam, Mattapeake and Mattapex Silt Loam soils located at University of Maryland Research and Education
Centers. CSNT data was also collected at these six sites during 2006 and 2007. In addition, CSNT data was collected
from sites on Sassafras Sandy Loam, Mattapeake, Mattapex and Delanco Silt Loam soils in 2008 resulting in ten
site-years of CSNT data.
Experimental Design:
A randomized complete block (RCB) design with four replications was used. Four corn FN rates of 0 (67 in 2006),
135, 202 and 270kg ha-1 were applied as 34 kg N ha-1 starter and the balance as side-dress except for the 0 kg N ha-1
treatment which received no FN.
Procedures:
One corn hybrid (Pioneer 33B51) was planted at the sites in May. Green leaf counts, SPAD chlorophyll meter and
optical sensor readings were collected at corn growth stages R2, R3-R4 and R5. Green leaf counts were collected by
counting the number of green or partially green leaves below and including the ear leaf. SPAD meter readings were
collected from the corn ear leaf. Optical sensor readings (normalized difference vegetative index [NDVI] was used)
were collected from within the corn canopy using a Crop CircleTM optical sensor. This was accomplished by carrying
the sensor head at approximately 6 feet height and directing its sensing beam toward the ear leaf area. The Pearson
correlation matrices presented illustrate the linear correlation between variables. Relative reading scores for the three
techniques and relative grain yields were calculated by expressing individual plot plant readings or plot yield
measurement as a percentage of the mean response from the highest FN treatments (i.e. the high FN reference where
N availability was not limiting corn performance) in a similar manner to the technique used by Fox et al., (2001).
Corn stalk samples for the CSNT were collected following the procedure of Binford et al., (1990). Corn grain was
harvested at maturity and yield recorded. Following corn grain harvest 8 soil cores were collected from each plot to a
depth of 60 cm (30 cm in 2006). Each core was divided into sections by depth (0-15, 15-30 and 30-60cm). Soil
samples were analyzed for NO3- -N and NH4
+ -N using a Lachat auto-analyzer.
Analysis:
Statistical analysis was conducted using the SAS PROC CORR and PROC REG procedures (version 9.1).
Results
The correlations between relative corn leaf based measurements and relative grain yield were highly significant for all
three methods and growth stages examined (Tables 1-3). However, the strength of this relationship, as shown by the
correlation coefficient (r), varied between the methods and time of observations. All methods produced the strongest
correlation with relative grain yield at corn growth stage R3-R4 (r = 0.84, 0.72, 0.83 respectively Tables 1-3). Relative
green leaf counts and relative optical sensor readings were more strongly correlated with relative yield (r 2 = 0.70 and
0.69 respectively, Fig. 1 & 5) than relative SPAD meter readings (r2 = 0.51, Fig. 3).
The correlations between relative corn leaf based measurements and post-harvest soil nitrate concentration in the
surface 30 cm was significant for the three methods examined at the R3-R4 observation time (P<0.05) , however the
linear relationship for each of the methods was weak (Fig. 2,4, and 6). In treatments where relative leaf measurements
at R3-R4 were < 90% (< 80% for green leaf counts) of the high FN reference measurements, residual soil nitrate
concentration tended to be relatively low (< 8 mg/l). However, when relative yield exceeded 90% (80% for leaf
counts), post-harvest soil nitrate concentrations varied greatly (1-49 mg/l).
As corn stalk nitrate concentration increased, residual soil nitrate concentrations in the surface 30cm tended to increase
(Fig. 7).
ConclusionsGreen leaf counts and optical sensor generated NDVI readings collected at corn growth stage R3-R4 are equally
effective for assessing the impact of FN management practice on corn grain yield (r = 0.84 and 0.83 respectively, Tables
1&3).
Collection of green leaf counts and optical sensor readings at growth stage R3-R4 maximized yield prediction accuracy.
The relationship between SPAD meter readings and relative grain yield was less affected by time of observation (Table
2) but less accurate for assessing the impact of FN management on yield than either leaf counts or optical sensor readings.
None of the three in-season measurements examined were effective methods of quantifying soil residual nitrate because
readings did not increase in response to increased FN availability once N sufficiency for the corn was reached. However, if
readings were <80-90% of the high N reference, soil nitrate concentration (0-30cm) was consistently low (< 8mg/l).
Increased CSNT levels tended to reflect increasing soil nitrate concentrations. The CSNT can provide an indication of
residual soil nitrate level, however, the relationship is considered not consistent enough to accurately quantify residual
nitrate. Literature Cited
Binford G.D., A.M. Blackmer and N.M. El-Hunt. 1990. Tissue Test for Excess Nitrogen during Corn
Production. Agron. J. 82:124-129.
Fox R.H., W.P. Piekielek., K.E. Macneal. 2001. Comparison of Late-Season Diagnostic Tests for predicting
nitrogen status of corn. Agron. J. 93:590-597.
Corn Yield
Residual Nitrate
0-30 cm
Corn FN
rate
Relative leaf count
R1
Relative leaf count
R2
Relative leaf count
R3-R4
Relative leaf count
R5
Corn Yield 1.00 0.30
<0.01
0.65
<0.0001
0.57
<0.0001
0.56
<0.0001
0.84
<0.0001
0.31
<0.01
Residual Nitrate
0-30cm
1.00 0.32
<0.002
-0.07
0.567
0.22
<0.03
0.33
<0.001
0.10
0.312
Corn FN
rate1.00 0.49
<0.0001
0.61
<0.0001
0.72
<0.0001
0.23
0.02
Relative leaf count R1
1.00 0.50
<0.0001
0.60
<0.0001
0.09
0.48
Relative leaf count R2
1.00 0.73
<0.0001
0.20
0.049
Relative leaf count R3-R4
1.00 0.35
<0.001
Relative leaf count R5
1.00
Table 1: Pearson Correlation Matrix relationship between relative corn Green Leaf Counts at 4 observation times, relative corn grain yield, soil residual nitrate concentration and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha-1.
Corn Yield
Residual Nitrate
0-30 cm
Corn FN
rate
Relative SPAD
R1
Relative SPAD
R2
Relative SPAD
R3-R4
Relative SPAD
R5
Corn
Yield1.00 0.30
<0.003
0.65
<0.0001
0.63
<0.0001
0.70
<0.001
0.72
<0.001
0.59
<0.0001
Residual Nitrate
0-30cm
1.00 0.32
<0.002
0.08
0.558
0.22
<0.04
0.21
<0.05
0.24
<0.02
Corn FN
rate1.00 0.52
<0.0001
0.61
<0.0001
0.65
<0.0001
0.49
<0.0001
Relative SPAD R1
1.00 0.58
<0.0001
0.60
<0.0001
0.29
<0.03
Relative SPAD R2
1.00 0.78
<0.0001
0.44
<0.0001
Relative SPAD
R3-R4
1.00 0.53
<0.0001
Relative SPAD R5
1.00
Corn Yield
Residual Nitrate
0-30 cm
Corn FN
rate
Relative NDVI
R2
Relative NDVI
R3-R4
Relative NDVI
R5
Corn Yield 1.00 0.30
<0.003
0.65
<0.0001
0.66
<0.0001
0.83
<0.0001
0.73
<0.0001
Residual Nitrate
0-30cm
1.00 0.32
<0.002
0.23
<0.03
0.28
<0.005
0.37
<0.001
Corn FN
rate
1.00 0.57
<0.0001
0.67
<0.0001
0.71
<0.0001
Relative NDVI (R2)
1.00 0.71
<0.0001
0.64
<0.0001
Relative NDVI R3-R4
1.00 0.81
<0.0001
Relative NDVI R5
1.00
R2 = 0.7043
0.000.100.200.300.400.500.600.700.800.901.001.101.201.30
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30
Relative Green Leaf count at corn growth stage R3-R4
Relat
ive C
orn
Grain
Yiel
d
R2 = 0.1079
0
10
20
30
40
50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30
Relative Green Leaf count at corn growth stage R3-R4
Post
har
vest
resi
dual
nitr
ate
(mg/
l)
R2 = 0.5111
0.000.100.200.300.400.500.600.700.800.901.001.101.201.30
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
Relative SPAD meter readings at R3-R4
Rela
tive
Corn
Gra
in Y
ield
R2 = 0.0427
0
10
20
30
40
50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
Relative SPAD meter readings at corn growth stage R3-R4
Post
-har
vest
resi
dual
nitr
ate
(mg/
l)
R2 = 0.6886
0.000.100.200.300.400.500.600.700.800.901.001.101.201.30
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
Relative Optical Sensor readings at corn growth stage R3-R4
Rela
tive
Corn
Gra
in Y
ield
R2 = 0.1454
0
10
20
30
40
50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
Relative Optical Sensor readings at corn growth stage R3-R4
Post
-har
vest
resi
dual
nitr
ate
(mg/
l)
R2 = 0.4231
0
10
20
30
40
50
60
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
Corn stalk nitrate concentration (mg/l)
Post
-har
vest
soi
l nitr
ate
0-30
cm (m
g/l)
Table 2: Pearson Correlation Matrix relationship between relative SPAD Meter Readings at 4 observation times, relative corn grain yield, soil residual nitrate concentration and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha -1.
Table 3: Pearson Correlation Matrix relationship between relative Optical Sensor Readings (NDVI) at 3 observation times, relative corn grain yield, soil residual nitrate and corn FN rate at 6 site-years which received inorganic FN rates ranging 67 to 270 kg N ha -1.
Figure1: Relationship between relative green leaf counts at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha -1
Figure 2: Relationship between relative green leaf counts and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha-1
Figure 3: Relationship between relative SPAD meter readings at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha -1
Figure 4: Relationship between relative SPAD meter readings at R3-R4 and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha-1
Figure 6: Relationship between relative NDVI at R3-R4 and post-corn harvest residual nitrate concentration 0-30 cm at 6 site-years which received FN rates 67 to 270 kg ha-1
Figure 5: Relationship between relative NDVI at R3-R4 and relative corn grain yield at 6 site-years which received FN rates 67 to 270 kg ha-1
Figure 7: Relationship between corn stalk nitrate concentration and residual soil nitrate concentration in the surface 30cm (10 site-years ) that received FN rates ranging 0 to 270 kg ha -1.
Discussion
The in-season plant-based measurements examined in this study were significantly correlated with relative grain yield. Timing of the measurements influenced the strength of this relationship and was especially important for leaf counts and optical sensor readings. These measurements had stronger correlations with relative grain yield at the R3-R4 growth stage than at the other observation timings examined (Tables 1 & 2). SPAD meter readings had weaker correlations with relative grain yield but this relationship was more stable across observation times (Table 2). The difference observed between the relative strength and stability of this relationship between readings over time may be attributable to the contrasting size of the sampled area for each of the techniques. The SPAD meter collects information from a small area of ear leaf tissue. In contrast, green leaf number conveys more information about total plant green leaf area. Similarly, the optical sensor assimilates information from a relatively large sensed area of the plant to produce an NDVI reading. Nitrogen stress is manifested as senescence of the leaves starting with the older (lower) leaves and advancing upwards. SPAD meter readings taken from the ear leaf do not capture well the stress exhibited by lower canopy senescence.
Use of in-season green leaf counts and optical sensor readings provided the most accurate snapshot of corn yield potential when taken at R3-R4 growth stage. A producer could include a high FN reference strip in his field along with a standard practice FN rate and a reduced rate. The use of either green leaf counts or an optical sensor could provide an easy way to assess the impact of new FN management practices on corn yield potential. Collection of readings with the SPAD meter at R3-R4 growth stage did not improve yield prediction accuracy as it did when using the other two techniques. This indicates that observation timing is less important when using SPAD meters. However, SPAD meter readings were less accurate than either green leaf counts or optical sensor readings at the R3-R4 timing (Figs. 1,3&5).
The linear relationship between the readings and post-harvest residual soil nitrate was weak but an interesting relationship was observed. When adequate or excessive levels of FN were applied to maximize corn performance, relative readings tended to be close to 1.00 but residual nitrate concentration in the surface 30cm varied greatly (1-49 mg/l) (Fig. 2,4, and 6). Use of the in-season plant-based measurements did not detect a plant response resulting from excessive FN applications. These plant based measurements are unable to quantify excessive levels of residual soil nitrate because the in-season measurements plateau in response to additional levels of FN beyond sufficiency, an outcome that is comparable to the response observed for corn yield with excessive FN applications.
This study also examined the CSNT as a potential indicator of residual soil nitrate. Corn stalks will “luxury accumulate” nitrate which suggests that this test holds potential as an indicator of residual soil nitrate. The CSNT was found to have a significant relationship with soil nitrate concentration in the surface 30cm (r2=0.42 , Fig. 7). This test could act as an indicator of residual nitrate, however, the relationship was not consistent and thus is considered not accurate enough to quantify residual soil nitrate concentration.
