Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published in Agron J 100:658-664 (2008)
DOI: 10.2134/agronj2006.0154
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Agricola
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Related Collections
Right arrow Nitrogen
Right arrow Field-Scale Studies
Right arrow Nutrients
Right arrow Maize
Right arrow Nutrient Management
Right arrow Plant Nutrition
Right arrow Maize Management

CORN

Luxury Production of Leaf Chlorophyll and Mid-Season Recovery from Nitrogen Deficiencies in Corn

Jun Zhanga,*, Alfred M. Blackmerb, Jason W. Ellsworthc, Peter M. Kyverygad and Tracy M. Blackmerd

a Statistical Consulting Center, Wright State Univ., 130 MM Bldg., 3640 Colonel Glenn Hwy., Dayton, OH 45435
b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
c Wilbur Ellis Company, 150 Burlington St., Pasco, WA 99301
d Iowa Soybean Association, 4554 114th St., Urbandale, IA 50322

* Corresponding author (jun.zhang{at}wright.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Deficiencies of N during the growth of corn (Zea mays L.) are often diagnosed by using chlorophyll meters that measure chlorophyll content in leaves. The diagnoses are based on the assumption that above-optimal supplies of N do not significantly influence chlorophyll meter readings (CMRs). The objective of this research was to assess the possibility that above-optimal supplies of N impacted chlorophyll concentration and the effects imposed a limitation on the minimum N deficiencies that can be detected by chlorophyll meters. Our approach was to monitor temporal patterns in CMRs of nonirrigated corn that received various rates of N at various times. The results showed that the time at which N deficiency symptoms first become detectable was closely related to the amounts by which N rates fell short for maximizing grain yield. The measured symptoms of N deficiency changed with time. Temporal patterns in CMRs were affected by N treatments while yields were not greatly affected. In-season N applications made to plants that started to show N deficiencies caused CMRs to converge with those taken on plants that always had adequate N. These observations suggest that above-optimal supplies of N may induce a luxury production of chlorophyll that is analogous to luxury uptake of nutrients. These problems severely limit the value of using chlorophyll meters to guide in-season fertilization in fields having near-optimal supplies of N. The underlying problem is the uncertainty caused by difficulties associated with distinguishing luxury production of chlorophyll from symptoms of N deficiencies.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication May 17, 2006.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CHLOROPHYLL METERS have been widely used to monitor seasonal patterns of N status in corn (Feil et al., 1997; Zhang and Blackmer, 1999; Zhang et al., 2008a), to estimate the need for the in-season application of N for corn (Wood et al., 1993; Blackmer and Schepers, 1995; Varvel et al., 1997a; Shapiro, 1999; Scharf et al., 2002; Blackmer and Zhang, 2005; Varvel et al., 2007), or to refine estimates of fertilizer need for future years (Piekielek and Fox, 1992; Varvel et al., 1997b; Scharf et al., 2006; Zhang et al., 2008b). The chlorophyll meter is useful because symptoms of N deficiencies are expressed in leaves well before harvest, and because it is easier to take CMRs on a few plants than to measure yields in response trials. Each application of the chlorophyll meters relies on the use of established relationships between two symptoms of N deficiencies (i.e., chlorophyll content in leaves and production of dry matter fall below-optimal level for maximum growth or yield). When the chlorophyll meters are used to guide in-season applications of N, these relationships must originate from data collected in previous years with identical conditions.

Because CMRs can also be influenced by factors other than sufficiency of N for plant growth, deficiencies of N are usually diagnosed by expressing readings as a percentage of those observed under conditions that are identical except for having enough N applied to ensure that N does not limit growth (Peterson et al., 1993; Zhang et al., 2000). This practice is based on the assumption that CMRs are the same whether supplies of N are optimal or substantially above optimal (Wood et al., 1993; Peterson et al., 1993). This assumption would greatly simplify diagnoses if any significant difference in CMRs could be considered to indicate a deficiency of N where less N is applied. Diagnoses would be much more complicated, however, if above-optimal supplies of N induced a luxury production of chlorophyll that is analogous to luxury uptake of N in tissue testing (Dwyer et al., 1995). Luxury uptake of N refers to fertilizer-induced increases in N concentration in tissues that are not accompanied by an increase in dry matter (Macy, 1936).

There is abundant evidence that CMRs are not greatly influenced by supplies of N that exceed optimal, but we rarely found reports that evaluate the possibility that errors in diagnoses may be introduced by effects of above-optimal supplies of N on plant growth. This possibility needs to be assessed because Zhang et al. (2008b) recently found that the value of the chlorophyll meters for guiding in-season applications of N may be limited by the minimum N deficiency that can be detected (i.e., the sensitivity of the diagnoses). The basic problem observed is that useful relationships between CMRs and yield responses could not be established in studies where N deficiencies were small enough and grain yields were within 10% of the highest yield that could be attained by addition of N. Relationships with high r2 values were usually obtained in studies that included much more severe N deficiencies in irrigated corn, but interpretations within the range of interest were largely based on extrapolation and often included substantial errors associated with the extrapolation. Economic analyses showed that N deficiencies resulting in 10% loss of yield were very important to corn producers even though the deficiencies are often too small to be detectable in many experiments (Kyveryga et al., 2007).

The objective of this paper is to explore the possibility that above-optimal supplies of N influence CMRs and that these effects impose a basic limitation on the minimum deficiencies that can be detected by chlorophyll meters. During the course of this study it became apparent that commonly used terms were not defined precisely enough to adequately describe what was observed. For this reason it is necessary to specify exactly what is denoted by key terms in this paper.

In accordance with the idea (Levitt, 1980) of distinguishing a biological stress (the cause) from a biological strain (the effect), we suggest that it is necessary to distinguish a "deficiency of N" from a "symptom of N deficiency." When two or more plots are compared at the same time, a difference in growth stages, grain yields or CMRs caused by addition of N can be a "symptom" of an N deficiency, but it is not a "deficiency of N" itself (Morris et al., 2006; Zhang et al., 2007, 2008a). The term "deficiency of N" is used throughout this paper to denote the amount of N falling short for grain production at a given site. To avoid problems discussed by Greenwood (1976), it should be noted that a deficiency of N as defined here should not be confused with the amount by which a given rate of N application falls short of the rate that produces maximum growth within a small segment of the growing season (Zhang et al., 2008a). Although we recognize that the explicit definition of deficiency is not appropriate for all conditions, it is appropriate within the context of this paper.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experiments were conducted on corn after soybean (Glycine max L. Merr.) at four sites located in Boone, Hamilton, and Greene counties of Iowa in 1998–1999. Major soil series includes Canisteo (calcareous, mesic Typic Endoaquolls), Clarion (mesic Typic Hapludolls), Webster (mesic Typic Endoaquolls), and Nicollet (mesic Aquic Hapludolls). All fields were more than 25 ha in size. Each was divided into six-row (Sites 1, 2, 4) or eight-row (Site 3) strips going the full lengths of the fields (500–820 m). Corn hybrids were Pioneer 34R06 (medium Bt) at Site 1, Dekalb 595 (top-cross high oil) at Site 2, Merschman 7114 (114-d maturity) at Site 3, and Great Lakes 5050 (104-d maturity) at Site 4. All fields were planted in late April or early May under rotation of corn with soybean. These fields were managed by farmers using their normal practices except for N fertilizer treatments. Sites 1 and 3 have been under no-till management for more than 15 yr. Sites 2 and 4 have been managed by conservation tillage methods.

Urea–ammonia–nitrate (UAN, 28% N) was applied to strips in various combinations of time and rate treatments. No fertilizer N was applied in the fall prior or in the spring before planting corn. The time-of-application treatments were soon after planting and sidedress (19 May and 1 July for Site 1, 27 May and 10 July for Site 2, 13 May and 8 July for Site 3, and 2 June and 9 July for Site 4). Fertilizer N applied soon after planting was injected midway between every other row to a depth of 15 cm in strips at rates of 0, 56, 112, and 224 kg N ha–1. The second fertilization involved uniformly dribbling liquid UAN on the soil surface between every other row, at rates of 56 and 112 kg N ha–1 for Sites 1 and 3, and only at the rate 56 kg N ha–1 for Sites 2 and 4. Each treatment was replicated five times in a stratified block design.

Five blocks were selected from each field. Each block extended 12 m along the rows and was wide enough to include all N treatments within a block. The block was divided into plots that corresponded to the strips having different treatments in increasing or descending order of N rates. Blocks were positioned to have minimal variation in soil characteristics among plots within a block.

Chlorophyll content in corn leaves was measured in June through September (corresponding to growth stages of V4 through R6 as defined by Ritchie et al., 1993) at approximately 1-wk intervals, using a Minolta SPAD-502 meter (Spectrum Technologies, Inc., Plainfield, IL). The youngest fully expanded leaf was used for measurement until tassel emergence; thereafter the ear-leaf was measured. All readings were taken halfway between the stalk and the leaf tip. The mean of the individual CMRs for each treatment was calculated by randomly selecting 30 plants from the center four rows of each plot. Relative CMRs for each treatment were obtained by expressing the CMRs in a lower N rate plot as a percentage of the CMR in the highest N rate plot (224 kg N ha–1) within the same block. Overall relative CMRs for each treatment were averaged across five blocks.

Grain yields were measured by hand-harvesting 15 plants from the center four rows of each plot. Relative yields for each treatment were obtained in accordance with relative CMRs. Temporal CMR data were analyzed for means and standard deviations at each individual date by using Statistical Analysis System (version 9.1, SAS Institute, Cary, NC).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Single Application for Severe Nitrogen Deficiency
Data presented in Fig. 1 illustrate relationships usually observed when chlorophyll meters are used to monitor symptoms of N deficiency in an experiment where fertilizer is applied early in the season at rates that produce substantial differences ( >10%) in grain yield. These observations clearly confirm that the chlorophyll meters have the capacity to detect severe symptoms of N deficiency in corn. For the respective sites included in this study, corn plants without N fertilization during the entire growing season yielded 42, 39, 12, and 11% less than the referencing plants that received 224 kg N ha–1 soon after planting (the early application). At Sites 1 and 2, the early application of 56 kg N ha–1 alleviated yield reduction by 15 and 16%, respectively, less than the reference.


Figure 1
View larger version (17K):
[in this window]
[in a new window]

  		Fig. 1. Temporal patterns in relative chlorophyll meter readings (CMRs) for early fertilization (E) resulting in a >10% reduction in yields. RY labeled as the mean relative yield for each treatment. Error bars indicate ± 1 standard deviation of the mean relative CMR with five replicates for each date and each treatment.

 
Noteworthy trends illustrated in Fig. 1 are (i) the magnitude of N deficiency symptoms tends to increase with time, and (ii) the N deficiency symptoms become detectable later in the season as rates of N fertilization are increased. It is clearly indicated that until late June, it is hard to see a significant difference in CMRs between the N rates 0 and 224 kg ha–1 applied soon after planting. This trend should be expected because a severe symptom of N deficiency would occur as the effects of continued shortages of N on plant growth accumulate over time. The second trend, as shown at Sites 1 and 2, that the N deficiency was delayed by a small amount of N fertilizer (56 vs. 0 kg ha–1) should also be expected because concentrations of NH4+ and NO3– in soils are poorly buffered (Liu et al., 2005). Therefore, the length of time required for plant uptake to reduce concentrations of this N to growth-limiting levels should be expected to increase with amounts of fertilizer added.

Split Application for Mild Nitrogen Deficiency
Relationships illustrated in Fig. 2 are usually observed when the chlorophyll meters are used to monitor symptoms of N deficiency in an experiment where fertilizer is applied under the same conditions as illustrated in Fig. 1, but additional plots with a second application of N are included to observe the effects of in-season fertilization. The added plots and treatments are important to assess the value of in-season diagnoses of N deficiency. These additional treatments enable evaluation of two hypotheses simultaneously: (i) the chlorophyll meter is really detecting deficiencies of N, and (ii) the crop will respond to fertilizer N applied after the CMRs are taken.


Figure 2
View larger version (27K):
[in this window]
[in a new window]

  		Fig. 2. Temporal patterns in relative chlorophyll meter readings (CMRs) for treatments with early (E) and in-season fertilization (L) resulting in a >5% reduction in yields at the four sites. RY labeled as the mean relative yield for each treatment. Error bars indicate ± 1 standard deviation of the mean relative CMR with five replicates for each date and each treatment.

 
Data in Fig. 2 clearly indicate that the crops responded to fertilizer applied under each of the field conditions studied. The observed responses in CMRs and yields confirm that deficiencies of N were present and were detectable by the chlorophyll meter. They also show that the crop was able to benefit from fertilizer applied after the severe or mild deficiencies of N were diagnosed.

Situations with only Minor Nitrogen Deficiencies
Figure 3 shows temporal patterns in CMRs for treatments that produced yield levels at least 95% of the highest treatment mean within the same site. The situation illustrated in Fig. 3 seems more appropriate for agricultural systems in the Corn Belt where fertilizer needs are already fairly well established and (or) there is concern that farmers are applying more fertilizer than is needed (Zhang et al., 2008b).


Figure 3
View larger version (31K):
[in this window]
[in a new window]

  		Fig. 3. Temporal patterns in relative chlorophyll meter readings (CMRs) for treatments with early (E) and in-season fertilization (L) resulting in a <5% reduction in yields at the four sites. RY labeled as the mean relative yield for each treatment. Error bars indicate ± 1 standard deviation of the mean relative CMR with five replicates for each date and each treatment.

 
There are a few treatments as depicted in Fig. 3 (e.g., Site 2 with 56E-56L, Site 3 with 0E-112L) that made a difference in CMRs within the relevant period chosen to monitor N deficiency with the aim to adjust the in-season-N fertilization. The second application of N in these treatments successfully corrected the minor N deficiencies. As compared to the reference treatment (112E-112L or 224E-0L), differences in relative yields amounted to –4 to 1%. The CMRs on plots having lower rates of N application tended to converge with the CMRs on plots having higher rates of N fertilization at most sites. This phenomenon would be analogous to luxury uptake of nutrients, which indicates that increases in concentrations of nutrients in tissues are not associated with increases in yields of grain. When analyzing these relationships in this figure, it is important to recognize that the yields cannot be measured without error and that errors in yield measurements should be expected within this reasonable range.

Evidence of Luxury Production of Chlorophyll
Each of the lines in Fig. 3 presents evidence for luxury production of chlorophyll if it is recognized that yield differences <5% are not significant. The data presented in Fig. 3 clearly indicate that, especially when supplies of N are in the near-optimal range, relative CMRs are often increased by higher rates of N fertilization without an accompanying increase in yields.

Figure 4 more clearly revealed the problem associated with luxury production of chlorophyll, where CMRs are expressed as percentages of the mean CMR for all treatments that produced yields not significantly different from the highest-yielding treatment (224E-0L or 112E-112L). When compared to the new reference pooled from treatments that showed yield difference <5%, all CMRs from corresponding treatments are close to the dotted 100% referencing line through the growing season. The subtle difference in approach used in Fig. 4 reveals that the chlorophyll meter is much less reliable for detecting N deficiencies than suggested in Fig. 3.


Figure 4
View larger version (24K):
[in this window]
[in a new window]

  		Fig. 4. Temporal patterns in chlorophyll meter readings (CMRs) relative to the CMR averaged for treatments with early (E) and in-season (L) fertilization resulting in a <5% reduction in yields compared to the higher N rate treatment. Vertical bars above the X-axis indicate standard deviations of the mean relative CMRs.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Unstable Appearance of Severe Nitrogen Deficiency as Revealed by Chlorophyll Meter Readings
The experiments illustrated in Fig. 1 are relevant if it is considered that a major role of the chlorophyll meter is to identify fields where some unusual event such as excessive rainfall resulted in losses of large percentages of the N already applied (Liu et al., 2005; Scharf et al., 2006).

The perceived importance of problems associated with the minimum deficiency that can be detected by chlorophyll meters varies greatly with expectations, data analyzed, and methods of data analyses (Zhang et al., 2008a, 2008b). As demonstrated in Fig. 1, sensitivity of chlorophyll measurements does not seem to be a problem in situations where chlorophyll meters are expected only to detect deficiencies that result in yield losses >10% and the data analyzed is appropriate for this expectation.

The trends observed in Fig. 1 suggest a potential problem in interpretation of the CMRs. The magnitude of the deficiency symptoms tends to increase with time even though there was no change in the rate of N application and in the final yield of grain for a specific treatment. Therefore, the quantitative interpretation of a given CMR must change with time of observation. This problem seems unimportant within the context of this situation because deficiencies are correctly identified whenever CMRs fall below some critical value. In accordance with the theory proposed by Bray (1954), similar effects observed in Fig. 1 should not be expected for "immobile nutrients" or nutrients that are taken up only in forms that are highly buffered by various processes in soils (Liu et al., 2005).

Yield Responsiveness to In-Season Nitrogen Application
Most agronomists would recognize that it is not appropriate to conclude that corn showing deficiency symptoms will always benefit from fertilizer N applied after the symptoms can be detected. Although applications of N fertilizer after N deficiencies had developed did increase yields in this case, these applications could not alleviate all effects of shortages of N that had already severely restricted vegetative growth for significant periods of time (Binder et al., 2000). This finding should be expected because the shortages of N significantly reduced the capacity of grain production. The trends observed in Fig. 2 clearly illustrate that the addition of fertilizer often resulted in an actual increase in relative CMRs taken from N-deficient plots. Such a trend suggests that the production of chlorophyll was occurring more rapidly in plants where N deficiencies had been corrected than in plants where N deficiencies had never occurred.

Toxic effects of the additional N fertilizer may be partially responsible for nonincreasing in corn yield following an increase of chlorophyll concentration in leaves as shown in Fig. 3. However, under these experimental conditions where N deficiencies are relatively small, there seems to be a need to recognize the possibility that luxury production of chlorophyll may also be partially responsible. The observations in Fig. 3 present evidence that either luxury production of chlorophyll or the toxic effect of added N could be possible under such conditions that the CMRs on a few plots having lower rates of N application tended to diverge from the CMRs on plots having higher rates of N application. Such trends should not be considered evidence against the occurrence of luxury production of chlorophyll because acceptance of this concept does not require rejection of the concept of N deficiencies. Such divergence in CMRs should be expected in situations where growth of plants exhausts the supplies of N needed for growth. In fact, the divergence of the CMRs on plants having lower rates of N from the CMRs on plants having higher rates of N is actually evidence for luxury production of chlorophyll in the latter when their final yields of grain are not different.

Hidden Problem of a Not-Well-Defined Reference and Assumption
The common practice of expressing CMRs as a percentage of those observed with the highest rate of N application exacerbates problems associated with distinguishing between toxicities and luxury production of chlorophyll. This can be illustrated by merely considering the problem expected if estimates of fertilizer need were based on tissue tests that are greatly influenced by luxury uptake of nutrients (Houles et al., 2007), and if the phenomenon of luxury uptake was not recognized (Schepers et al., 1992; Peterson et al., 1993; Blackmer and Schepers, 1994). The fact that Zhang et al. (2008b) identified the poor sensitivity of chlorophyll meters may be explained in part by relating CMRs to yields at near-optimal supplies of N.

Problems caused by luxury production of chlorophyll tend to be greater early in the season than late in the season. This trend can be explained by recognizing that N translocates from vegetative parts to grain during the second half of the season, so the N content of the vegetation better represents the sufficiency of N for grain production. Therefore, luxury production of chlorophyll could be a more serious problem when chlorophyll meters are used to estimate the need for in-season fertilization than to provide feedback that can be used to refine N recommendations in future years.

The tendency for the time of appearance of N deficiency symptoms to be inversely related to the magnitude of the deficiency of N (i.e., the shortage of N in the soil) should be expected to produce a significant error in diagnoses of small deficiencies of N. The reason is that the relationship between yields and CMRs taken at any given time early in the season might show curvature as N rates approach optimal (Dwyer et al., 1995). This curvature could occur because N deficiencies that will affect grain yields are producing symptoms only in treatments where N deficiencies are great. A wrong assumption of linearity or failure to recognize this curvature can result in an overestimate of fertilizer needs. Lack of adequate distinction between deficiencies of N (i.e., shortages of N that limit final yields of grain) and luxury production of chlorophyll should be expected to introduce errors into diagnoses made in fields having supplies of N that range from near-optimal to above-optimal.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The poor sensitivity of chlorophyll measurements should be recognized when they are used to estimate the need for in-season fertilization, at least under similar conditions of this study. Observations in this study give reasons to question the assumption that CMRs are not significantly affected by above-optimal supplies of N. This assumption is not consistent with the finding that additions of N during the season cause the CMRs taken from plants showing marked symptoms of N deficiency to converge with the CMRs taken from plants always having adequate N. This assumption is not consistent with the observations that different rates of N application often produce different temporal patterns in CMRs even when identical yields of grain are observed. These observations indicate that increases of N application rates often promote an increased production of chlorophyll that is not accompanied by an increase in grain yields. In situations where corn is grown for grain, such increases in rates of N application should be considered to cause luxury chlorophyll production.

Small amounts of luxury chlorophyll production necessarily limit the ability of chlorophyll meters to diagnose minimum deficiencies of N during the growing season. Due to occurrence of this phenomenon, the ratio of deficiency symptoms (i.e., relative CMRs) to actual N deficiency changes with the stage of plant development, and symptoms of small N deficiencies do not appear early in the growing season. This problem is exacerbated by the difficulty of distinguishing luxury production of chlorophyll from symptoms of N deficiencies before yields are measured and optimal supplies of N can be defined. Failure to recognize that luxury chlorophyll production could occur in corn would result in overestimates of fertilizer needs.


   ACKNOWLEDGMENTS
 
This project and publication was financially supported by the Case New Holland International (formerly Case IH) and the Iowa Soybean Association. We would like to express our appreciation to Larry Hendrickson (currently at John Deere Ag Management Solutions) for his coordination of this project; to Maureen Schaber at the Pacific Agri-Food Research Centre, BC, Canada for polishing late versions of the manuscript; to Kenneth J. Koehler for his supervisory consultation of statistical methods employed in the study; and to Matt Dickson, Joseph Sines, and Heather Kenyon-Clark for their generous help with field data collection. The corresponding author fully acknowledges the valuable supervision and contributions of his major professor, Dr. Alfred Blackmer (1943–2006).

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 Agron. J.Home page
J. Zhang, A. M. Blackmer, J. W. Ellsworth, and K. J. Koehler
Sensitivity of Chlorophyll Meters for Diagnosing Nitrogen Deficiencies of Corn in Production Agriculture
Agron. J., May 7, 2008; 100(3): 543 - 550.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Agricola
Right arrow Articles by Zhang, J.
Right arrow Articles by Blackmer, T. M.
Related Collections
Right arrow Nitrogen
Right arrow Field-Scale Studies
Right arrow Nutrients
Right arrow Maize
Right arrow Nutrient Management
Right arrow Plant Nutrition
Right arrow Maize Management

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome