Illinois
Fertilizer Conference Proceedings
January 25-27, 1993
| Home | 1993 Index | Search |
| Home | 1993 Index | Search |
D. Anderson, D. Bullock, G. Johnson, C. Taets1
Nitrogen is necessary for corn production and even moderate deficiencies will substantially reduce yield and profit, but excessive N can pollute both surface and ground water. Thus, farmers are walking a thin line. Too much N and they threaten the environment and add an unnecessary cost; too little N and they reduce their profit by reducing yield.
Virtually everyone is willing to decrease N fertilizer use if that reduction will improve N efficiency, increase profit and protect the environment. But current technology does not allow for rapid, accurate, on-farm decision making regarding N fertility. Thus, we can not answer the questions of if, when, where, and by how much N fertilizer rates can be changed without running the very real risk of affecting U.S. farm profit, efficiency, and competitiveness in the world market. For any given field the consequences of making the wrong decision are not trivial. Failure to reduce N fertilizer rates in many situations will result in farmers using more N than is necessary and in a continued decline in water quality. On the other hand, if we blindly accept large across-the-board reductions in N fertilizer rates we will, without a doubt, hurt many farmers and the U.S. position in world trade. To answer the questions of when, where and how much, we require a monitoring technique to evaluate the N status of corn. Ideally the technique will be fast and inexpensive and allow for on-the-spot decision making.
One tool has been suggested by Blackmer at Iowa State University. In their procedure the nitrate concentration in the top foot of soil is quantified when the plants are six inches tall. They have shown that most Iowa soils containing at least 21 ppm nitrate do not require additional N fertilizer. This research was the basis for the N test kit developed and offered by Iowa State University and the Hach Chemical Company during the 1990 growing season. The process is faster than sending soil samples to a lab, but it is still labor intensive and slow. It requires several hours to get a single result. The test kit has met with mixed, but overall positive, reviews by most growers.
Another possible tool is the Minolta SPAD-520 chlorophyll meter. The meter determines the relative amount of chlorophyll present in plant leaves. Chlorophyll amount in plant leaves is closely related to the N content of the leaves so if the relative chlorophyll level is known then the relative N content is known. Correlations of R=0.91 between the measured SPAD value and leaf N concentration of rice have been reported in Minolta literature. The SPAD-520 meter is very fast. A value can be obtained in less than one minute. The SPAD-520 meter is used currently in Asia for N management of rice.
The SPAD-520 chlorophyll meter is not used commercially in the United States. Preliminary work at University of Illinois and the University of Nebraska suggests that it may be applicable to corn. At the University of Illinois we used the SPAD-520 during the 1990 season in a N fertility rate experiment in which we have two hybrids (B73XLH51 and LHE136XLH82) and nine N rates (0, 25, 50, 75, 100, 125, 150, 175, 200 lb N ac''). In that study we used the SPAD-520 at the mid-greensilk stage only. We found that the SPAD-520 could distinguish between the 25 lb incremental rates and the correlation between the SPAD-520 meter reading at the mid-greensilk stage and final grain yield was an impressive R=0.90.
It's interesting to note that Kjeldahl analysis is considered very accurate for determining the N content of tissue, but the correlation between final grain yield and the Kjeldahl N analysis was only R=0.77. Thus, the SPAD-520 gave an estimate in less than a minute that was a better predictor of final yield than was the standard Kjeldahl method.
While the SPAD-520 may be a valuable tool for on-farm decision making, several critical questions remain however. First, we do not know if the SPAD-520 can be used earlier in the season, which is essential if it is to be used for side dress decisions. Second, we do not know how the meter will respond to different environments and in particular different soil types. Third, we do not know if the meter needs to be calibrated for individual hybrids. And fourth we do not how the meter compares to standard Kjeldahl procedures and the new Blackmer kit.
The objective of this research was to evaluate the Minolta SPAD-520 chlorophyll
meter as a rapid on-farm decision making aid for N management in corn in Illinois.
This project was originally proposed and funded as a three year field study. However, after funding was received we decided additional greenhouse work was necessary in order to understand how leaf number, meter position, and leaf age interact with N fertilizer in affecting the SPAD 520 chlorophyll reading. Therefor, a greenhouse component was added to the study and will be discussed here.
Greenhouse
The greenhouse portion of this study consisted of two identical experiments ran in the winter and spring of 1992. The experimental design was a randomized complete block with five replications. Nitrogen fertilizer was applied at six levels (0, 50, 100, 150, 200, 250 lb N/acre equivalent) to the soil after corn emergence. In each pot six seeds of Pioneer brand hybrid 3790 were planted. The corn was thinned to a single plant per pot at the second leaf stage. For each plant weekly SPAD readings were taken for each leaf at three positions; bottom, middle, and tip. Those positions were approximately 20, 50, and 80% of the distance from the collar to the tip of the leaf. Soil samples, were pulled for total soil N analysis every two weeks for a total of eight soil samples.
Field
The field component of this project was conducted on a cooperating farm in
northwestern Illinois, near Geneseo, and at the University of Illinois experimental
farms at Urbana and Brownstown. The statistical design was a randomized complete
block with three replications. Individual plots were six rows wide by 50 feet
long. At each location ten different open pedigree hybrids (B73 X LH51, LH74
X LH123, LH119 X LH51, LH74 X LH59, LH119 X LH123, LHE136 X LH82, LH132 X LH82,
LH132 X LH59, LH132 X LH51, LH119 X LH82) were over planted and then after emergence
thinned to a final population of 26,000 plants per acre. Nitrogen rates were
0, 80, 160, and 2401b N/acre and applied as 28 % solution injected into the
soil at the first leaf stage. All sampling was conducted on the middle two rows.
SPAD meter readings and soil sampling were conducted throughout the season.
At each sampling time leaf disk punches were taken in order to analyze for total
N in the leaf tissue. Final yield was obtained by harvesting the entire length
of the middle two rows with a plot combine and adjusting to 15.5% moisture.
Statistical analysis indicated that pooling of the winter and spring experiments was appropriate and was, done. Other pooling was done when appropriate and efficient.
The major argument behind conducting the greenhouse experiment was to ascertain the relationship between leaf number, age, morphology and N fertilizer rate on the SPAD readings. We felt this was important since it is obvious that not all leaves are the same shade of green and that even within a leaf there appears to be a gradient of intensity of green color from the base toward the tip. Thus, we hypothesized that a standard leaf number and age and reading technique would have to be developed. Our results support that hypothesis.
The first question we addressed was if leaf number or N fertilizer rate affected the meter reading and if there was some sort of an interaction between the two. Statistical analysis indicated that both of those main effects significantly affected the SPAD meter reading, but that the interaction term was not significant. From Fig. 1 the relationship is quite evident. In this case a multiple regression approach revealed that the SPAD reading increased as N rate increased and decreased with advancing leaf number. In both cases the leaf number and N rate terms were deemed quadratic. Clearly the greenest leaves were found on very young plants with large amounts of N fertilizer. The large effect due to leaf number suggests that a standard leaf number should be chosen, as is the case for mineral analysis, but that almost any leaf is acceptable. Perhaps different standardization curves could be developed for different leaves for different purposes. For example, curves for young leaves would assist us in making decisions regarding N fertilizer applications prior to layby while curves for the earleaf would assist in monitoring during grainfill.
The second major question addressed was if leaf age or position on the leaf affected the meter reading and if their was interaction between the two. Once again statistical analysis indicated that both of the main effects were significant, but the interaction term was not significant. From Fig. 2 it can be seen that the SPAD meter reading increased in quadratic fashion with increasing distance from the collar of the leaf and decreased in a quadratic fashion with leaf age. It appears the most stable part of the response surface is for measures taken the first to second week after collar development from the middle of the leaf. There does not appear to be a major difference between the middle position (50 % ) and the tip position (80%). It was also noted that the variance of the SPAD readings increased substantially for measures taken from leaves more than two weeks old. The variance was not affected by position.
We interpret the greenhouse work to indicate that successful use of the Minolta SPAD 520 will require standard procedures, i.e. a given leaf at a given time. The field component of the study was used to examine if further recognition of pedigree and environment was necessary as well.
Field
An abbreviated ANOVA and main effect mean values are shown in Table 1. Measures for yield, moisture, and height are presented in combination with a subset of SPAD readings from the sixth leaf stage (V6), mid-greensilk (R1), and early dough (R4).
Significant effects of Location (L), Hybrid (H), and LxH were detected for yield, moisture, and height. It is interesting, however, that of these same three measures only yield was affected by nitrogen fertilizer rate (N) and even in that case yield appears to level off by 80 lb N/acre.
As we would have suspected from the greenhouse work, the SPAD readings differed from V6 to R1 to R4, but rather than declining with age, they actually increased. While this is contrary to what we saw in the greenhouse it does support the suggestion that leaf number and leaf age are important. It is interesting to note that final grain yield was better correlated with SPAD readings than was N fertilizer rate (Table 2). However, early in the season, the correlation was significant, but poor. By the dough stage, the correlation was much better. The reason for the lack of a large correlation coefficient is probably best explained by looking again at Table 1. Hybrids differ in the relative intensity of green color in their leaves and location also has a large effect. Clearly not all hybrids are the same color and even a given hybrid is not the same color with a given N fertilizer rate in all locations.
It is also interesting to note, with the exception of the V6 time, the SPAD readings are more closely correlated to final grain yield than is leaf N content. This indicates that the SPAD meter is a better indicator of final yield during grain-fill than is the standard leaf analysis procedure.
Final analysis of the soil N content is not available at the time of this writing.
The samples were analyzed, but the results clearly show an error in either the
sampling or laboratory technique. The soil samples are being reanalyzed.
Greenhouse and field research were conducted in order to examine the potential
for the use of Minolta SPAD 520 as a N management tool in Illinois. Preliminary
data indicate that standard sampling procedure will have to be developed since
leaf number, leaf age, and position on the leaf all effect the readings. Field
work indicate that locations and hybrids also significantly effect the readings
and thus, may need to be considered when developing values to describe the relationship
between the meter reading and N nutrition and yield. As of now it appears that
the SPAD meter is an excellent indicator of N content of corn leaves and is
a better indicator of final yield than is N content of corn leaves, however,
both are far from ideal.
Figure 1. The effect of leaf number and nitrogen fertilizer rate on Minolta SPAD 520 reading
Table 1: ANOVA and main effect means for SPAD 520 at three locations in 1992
1Graduate Research Assistant and Asst. Prof. of Agronomy, Univ. of IL; Farmer and Farm Manager, Osco, IL; and Farmer, Geneseo, IL respectively.
