Illinois Fertilizer
Conference Proceedings
January 24-26, 1994
| Home | 1994 Index | Search |
|
|
| Home | 1994 Index | Search |
D. Bullock1
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 possible tool is the Minolta SPAD-502 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-502 meter is very fast. A value can be obtained in less than one minute. The SPAD-502 meter is used currently in Asia for N management of rice.
The SPAD-502 chlorophyll meter is not used commercially in the United States. Preliminary work at University of Illinois suggests it may be a valuable tool. This report summarizes the second year of a three year study to evaluate the SPAD-502 in Illinois.
The objective of this research is to evaluate the Minolta SPAD-502 chlorophyll
meter as a rapid on-farm decision making aid for N management in corn in Illinois.
This project was conducted on University of Illinois experimental farms at
Brownstown, Dekalb, and Urbana., 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 LH 123, LH 119 X LH51, LH74 X LH59, LH 119 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 240 lb 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.
At the time of this writing soil and leaf analysis have not been completed so discussion will be limited to grain yield and SPAD-502 readings. Means are presented in Tables 1, 2, and 3 for Brownstown, Dekalb, and Urbana respectively.
Increasing N fertilizer rate increased grain yield and SPAD readings at each location. The increases were quadratic in most cases and N efficiency was highest at lower ranges. Nitrogen fertilizer rate was correlated with yield at all locations (Table 4), but SPAD readings were more closely correlated by V1 1 at Brownstown and Dekalb and by Rl (mid-greensilk) at Urbana. The correlation (0.81) between yield and N fertilizer rate at Urbana is unusually large and may reflect the wet summer experienced at Urbana, but all locations received far more than the normal rainfall in 1993. SPAD readings at all locations increased from V5 up to V16-R1 and then declined. Environment did affect the values. For example, the VII SPAD reading at Dekalb (Table 2) appear smaller than expected and was probably due to the wet and cloudy conditions that preceded those measures. We have noticed that SPAD readings decline under cloudy weather. That is reasonable for a measure of relative leaf color.
Grain yield and SPAD readings were also affected by hybrid. In this experiment hybrid is considered a random effect and desired inference space is over all hybrids. The results indicate a substantial difference between hybrids. As noted earlier SPAD readings were correlated with yield (Table 4), but the relationship is not perfect. For example, LH192 X LH82 was one of the largest yielding hybrids examined in this study, but SPAD measures were consistently lower than most other hybrids (Table 1).
The results from this experiment (1992 and 1993) indicate the SPAD meter is
an effective predictor of yield and has the ability to distinguish N fertilizer
rates, but absolute values fluctuate widely depending upon environment, hybrid,
and plant age. Thus, it appears that it will be difficult, if not impossible,
to produce a given critical level for a SPAD reading which would suffice for
all situations. It would be possible to make comparisons within a situation.
For example, a given hybrid growing in a ponded part of a field could be compared
to the same hybrid growing in a non-ponded area of the same or similar field.
The results in Tables 1, 2,
and 3 would suggest that reading differences
between the two areas would indicate a N deficient situation.
The results of this experiment in 1993 are similar to those of 1992. The SPAD
meter readings were significantly correlated to yield; particularly during the
grain-fill period. However, absolute values were substantially affected by hybrids,
environment, and plant age. Thus it does not seem reasonable to expect this
technology to allow use of a given critical level to indicate N status in corn
plants under all conditions. It does seem reasonable to suggest that the SPAD
meter can be used to compare a given hybrid in different parts of a given field
or similar fields.
Table 4: Correlation analysis for yield with N rate and SPAD readings
1Assistant Professor of Agronomy, Univ. of IL
