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Nitrogen (N) applied in bands across cornfields often induces differences in plant height, leaf color, and growth stage of corn (Zea mays L.). Especially during wet springs, plants growing immediately over the bands are often noticeably taller and greener for a short period. Plants growing between the bands experience N deficiency until their roots reach the bands. The impacts of such short periods of N deficiency on plant early growth have received little attention. We studied the effects of tracks left by fertilizer applicator on corn growth stage, plant height, and leaf chlorophyll meter readings (CMRs) in a field where conditions seemed favorable for a fertilizer-induced advancement in growth stage. Measurements showed that the reduced plant height or leaf color attributable to a temporary N deficiency was mainly associated with the delay of growth stages and might have little influence on final grain yield.

The normal practice of using chlorophyll meters involves measuring the youngest fully developed leaf of a corn plant (Zea mays L.) before silk emergence and then switching to the ear leaf. A discontinuity exists of chlorophyll meter measurements associated with leaf change prior to the silking stage. The lag of silk emergence within a cornfield prior to silk emergence makes it practically impossible to take chlorophyll measurements on leaves at the same growth stage. Therefore, the change in leaves being measured introduces errors when chlorophyll meters are used prior to silk emergence to diagnose nitrogen (N) deficiencies. We report results from three field studies to assess the difference in chlorophyll measurements between the uppermost fully developed leaf and the ear leaf near the time of silk emergence and identify possible problems associated with the difference when chlorophyll meters are used to estimate the need for in‐season fertilization in cornfields.

Land application of animal manure is known to alter rates of nitrogen (N) mineralization in soils, but quantitative information concerning intensity and duration of these effects has been difficult to obtain under field conditions. We estimated net effects of manure on N mineralization in soils under field conditions in a completely randomized design, at six field sites, by comparing liquid swine (Sus scrofa) manure treatments to plots receiving no manure. Soil samples were collected immediately after manure application to determine inorganic N concentrations, and those samples were also incubated 28 d in the laboratory to determine amounts of N mineralized from the soil. Analyses and incubation were repeated on a second set of samples collected after various times, depending on the site. Differences in inorganic N concentrations were significant among treatments at all six locations for the first sampling and five of the six locations for the second sampling. In comparison, significant differences in inorganic N concentrations measured after 28 d of laboratory incubation were observed for only two of the six sites for each sampling time. Our results illustrate how to distinguish between the effects manure has on rates of N mineralization in soils and rates at which manure N is mineralized.

Soybean (Glycine max Merr.) is extensively grown in areas of the US Corn Belt where soils often range from relatively acid (pH

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 ofN 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.

Chlorophyll meters have been widely used to diagnose N deficiencies in research trials and to help refine N fertilizer recommendations in production agriculture. The diagnoses are based on the assumption that chlorophyll meters can detect a wide range of N deficiencies under field conditions according to relationships between chlorophyll meter readings (CMRs) and yield responses to N fertilization that are established from small-plot trials. We conducted a field-scale study to evaluate the sensitivity of chlorophyll meters when they are used to detect N deficiencies in corn (Zea mays L.) production. The multiple year-location data were gathered in three cornfields where N fertilizer was applied at different rates in strips that crossed several soil map units. Grain yields were related to CMRs taken on corn leaves in June, July, and August of 1998-1999. Results showed that the chlorophyll meters could detect severe N deficiencies early in the season, but small and moderate deficiencies of N could not be diagnosed with reasonable certainty until it was too late to make in-season fertilization. Two types of discontinuity are discussed in this paper as likely causes for the limited sensitivity of the chlorophyll meters. Our study suggests that problems associated with the use of chlorophyll meters to diagnose deficiencies of N in production agriculture are much greater than their use to help interpret results of well-controlled research trials where yields are also measured.

Assessing economic and environmental impacts of nitrogen (N) fertilization in production agriculture is important for preventing unnecessary application of N fertilizer and avoiding losses of this N into water body. Chlorophyll meters are often used to evaluate N management practices and diagnose deficiencies of N in corn (Zea mays L.). Chlorophyll meter readings (CMRs) are usually interpreted relative to reference readings taken from plants having ample N to maximize rates of growth. Although measurements taken with chlorophyll meters provide a scale for estimating the sufficiency of N for corn growth, effects of above-optimal supplies of N on the measurements have not been studied when diagnosing N deficiencies during vegetative growth stages. Here, temporal trends in chlorophyll measurements were monitored in trials where various rates of N were applied soon after planting and (or) after symptoms of N deficiency had developed due to the changes of N status in soil and demand for N during corn growth. Divergence of chlorophyll measurements from the reference readings occurred in situations where plants having too little N were compared with plants having adequate N. In contrast, convergence of chlorophyll measurements with the reference readings after application of fertilizer N during the growing season indicated partial or complete recovery of the plants from the deficiency of N. The recovery can be explained by considering that luxury production of chlorophyll occurred at higher rates of fertilization or by interactions of N with soil water and other nutrients supplied during corn growth. Observations that plants can partially or completely recover from periods with inadequate N for chlorophyll production suggest that the leaf chlorophyll measurements taken early in the season should not be always expected to highly correlate with final yields of grain. Therefore, it is important to recognize the possible recovery of chlorophyll production and to avoid N losses to the environment.

The end-of-season test for cornstalk nitrate gives site-specific information about crop N sufficiency and N excess. It is a very valuable decision tool for increasing the sustainability of N fertilizer practices. However, little information is available about the possibility of simplifying this tissue test and the extent to which results of the test are influenced by deviations from recommended sampling procedures. This information would be very valuable to promote the acceptance and use of the test by maize growers. Here we analyzed stalk nitrate concentrations from stalks collected in several experimental maize plots receiving different rates of N fertilizer ranging from deficient (zero nitrogen) to excess (350 kg N ha−1). Our results show that nitrate concentrations of 2.5-cm segments of stalk with and without nodes showed no significant differences, and no special care during stalk sampling must be taken. In addition, although NO3-N concentrations in pith tissue are, on average, 29% higher than in the rind, the relative amounts of nitrate in pith (18%) and rind (82%) revealed little need for efforts to ensure that the ratios of pith and rind were not altered during sampling or analysis. Nitrate concentrations tended to decrease by 4% for each cm of deviation above the height normally sampled. Overall, the results presented indicate the robustness of the test to small deviations during sampling procedures and that a minimum of 10 stalk segments are necessary to adequately assess N sufficiency. Finally, it is feasible to decrease the length of the sampled stalk to segments of 4 cm if they are centered about 25 cm from the ground, to avoid large deviation due to the stem nitrate gradient observed. Our findings can help to promote and simplify the cornstalk nitrate test, improving N management and decreasing the negative environmental impact of excessive fertilizer applications.

Discussions of N fertilizer needs for corn (Zea mays L.) usually are based on the assumption that economic optimum rates (EORs) of N provide the best benchmark to indicate the rates most likely to maximize profits for producers. We describe methods for calculating some additional benchmarks and illustrate how efforts to improve N management could be advanced by recognizing several alternative benchmarks simultaneously. We analyzed data from 54 small-plot trials with a new procedure that disaggregates problems associated with systematic model bias and variability in crop yield responses to fertilizer N. We compared the commonly used method for calculating EORs to a method that is described as discrete marginal analysis at near-optimal rates of N fertilization. We described methods for calculating the incremental break-even rate, the incremental rate that gives no crop response, and the incremental rate of N that gives the desired level of profit. Discussions of the relative merits of each benchmark suggest that problems associated with calculating and using the concept of EORs can be avoided by presenting the alternative benchmarks as proposed in this paper.

Soybean [Glycine max (L.) Merrill] often shows symptoms of iron deficiency chlorosis (IDC) on high pH, calcareous soils of the U.S. Midwest. The objective of this study was to assess the variation in soybean yield that could be explained by soil pH and carbonate concentration in Iowa fields. Color aerial images of soybean canopy taken from 2000 to 2002 from 12 fields having acid to calcareous soils were used to select 10 to 28 sampling areas 10 to 25 m 2 in size to encompass significant variability in early soybean growth and IDC symptoms in each field. Representative areas 0.93 m 2 in size were identified through field observations to collect soil samples and measure grain yield. Soil pH measured to a 15-cm depth across fields ranged from 5.6 to 8.2 and calcium carbonate equivalent (CCE) ranged from 0 to 30%. Soil CCE varied from 2.5 to 30% as pH ranged from 7.7 to 8.2. Grain yield decreased with increasing pH and CCE in 9 and 11 fields, respectively. Soil pH and CCE explained 30 and 41% of the variability in relative yield across sites, respectively. An alkalinity stress index (ASI) that combined both measurements (pH + 0.14CCE) was developed based on the relative effects of each measurement on yield and explained 45% of the yield variability across sites. The index developed was a better predictor of soybean yield in fields with high-pH soils than each measurement alone.

Efforts to calculate economic optimum rates (EORs) of N fertilization for corn (Zea mays L.) have been hampered by a lack of methods for disaggregating problems caused by model bias and variability in crop responses to N. We illustrate how the concepts of ex post and ex ante analyses can be used in a multistep procedure to disaggregate these problems when calculating ex post EORs when large amounts of data are available. Five models were used to describe yield responses from a collection of 54 small-plot trials that included seven rates of N. The multistep procedure included steps to reduce model bias and steps to reduce unexplained variability by forming categories based on information available at the time of fertilization. The concepts of ex post and ex ante analyses were used to clarify what information is important at each stage of the procedure and to ensure that information generated in early steps is used effectively in later steps. Analyses illustrate that calculated values for EORs should be expected to vary with the amounts of information available and that the new procedure can be described as a systematic search for the best EOR that can be calculated with existing information. Although this procedure may have little practical use when data are collected in traditional small-plot trials, illustration of this method by using data collected in such trials revealed the problems of model bias and variability in yield response as well as the potential for solving these problems in production systems where advances in technology make it practical to collect unprecedented amounts of data.

The sequential growth of winter wheat and corn within one year is the main cropping rotation system in northern China, excess nitrogen (N) fertilizer application to this system is a common problem. Four field experiments with four or five N rates were conducted in Beijing City and Henan province to study the optimal N rate for this system. The results of this study clearly indicate that the high rates of fertilization recommended by the quadratic model were less profitable than the lower rates recommended by the LRP and QRP models. The amounts of N in above-ground plant tissue tended to increase with increase in rate of N application but the percentage recovery of N in each increment decreased with each additional increment of fertilizer applied, and higher percentage recoveries were observed in wheat than in corn. Soil nitrate concentrations increased with increasing rates of N application. Average of 30% of the soil mineral N deficit can be accounted for uptake of N by wheat, but linear regress...

Nitrogen fertilizer needs for corn (Zea mays L.) in fields already treated with animal manure can be estimated by using general guidelines or soil testing for inorganic N. Although the soil-testing approach has been extensively evaluated for ability to predict yield responses to applied N under field conditions, the general-guideline approach has not been subjected to comparable performance-based evaluations. Fertilizer response trials were conducted in 205 manured fields to (i) compare the two approaches for ability to predict corn yield responses to fertilizer N applied after animal manure, (ii) identify reasons for differences in predictive ability, and (iii) explore the benefits of performance-based comparisons of the alternative approaches. Analyses showed that 34% of the observed variability in response could be explained by inorganic N concentrations whereas less than 5% of this variability could be explained by the general-guideline approach. The soil-testing approach, therefore, had greater ability to integrate the effects of all factors affecting yield responses across the range of conditions studied. Mean yield responses (0.55 Mg ha -1 ) were smaller than are usually detectable in individual trials, but they were great enough to prompt farmers to fertilize. Results of this study indicate that the most commonly accepted approach to estimating N fertilizer needs is less reliable than generally believed and, therefore, that superior approaches are likely to remain unrecognized unless the performance of the commonly accepted approach is objectively evaluated under realistic field conditions.

Additions of 15 N-labeled fertilizers to soils often induce mineralization of nonlabeled N, and such mineralization poses problems when interpreting the results of isotopic analyses. We studied the effects of added 15 NO 3 - on turnover of N during the decomposition of corn (Zea mays L.) residues in soils. Samples of a Galva silty clay loam (fine-silty, mixed, mesic Typic Hapludoll) were treated with various levels of corn stover and various levels of 15 N-labeled NO 3 - and were incubated for 90 d. Evolution of CO 2 and concentrations and isotopic composition of NO 3 - were monitored. Increases in rates of NO 3 - addition increased initial rates of immobilization of labeled N and subsequent rates of mineralization of nonlabeled N. The sequential immobilization of labeled N and mineralization of nonlabeled N resulted in substantial replacement of labeled NO 3 - by nonlabeled NO 3 - . Early added-N-induced immobilization of labeled N often was offset by subsequent added-N-induced mineralization of nonlabeled N before the end of the study. Sequential immobilization and mineralization should be recognized as a potential source of error in 15 N-tracer studies because sequential processes violate the commonly held assumptions that mineralization and immobilization occur simultaneously. This error could be avoided by recognizing that sequential immobilization and mineralization can occur and that the sequential processes result in a different distribution of isotopes than does simultaneous mineralization-immobilization turnover.

Nitrogen fertilization is widely recognized for its potential to help maintain soil organic matter (SOM) concentrations by increasing plant growth and amounts of plant material decomposed in soils. We studied possible mechanisms by which annual additions of more N than needed to maximize yields of corn (Zea mays L.) could cause losses of SOM. Various amounts of stover and NO − 3 were added to a Galva silty clay loam (fine-silty, mixed, mesic Typic Hapludoll), and NO − 3 concentrations and CO 2 evolution were monitored for periods up to 90 d at 24°C in the laboratory. The added NO − 3 suppressed mineralization of C from SOM and stimulated mineralization of C frome stover. Adjustment of rates of stover decomposition to temperature regimes normally encountered in Iowa fields after harvest showed that stover decomposition would not be complete within 1 yr and that increases in NO − 3 availability decreased the amount of stover C remaining in the soil. This effect of NO − 3 could explain how additions of unneeded N could decrease concentrations of SOM in long-term studies in which residues and NO − 3 are added in annual cycles. These observations suggest that practices that reduce unnecessary fertilization could help conserve SOM and reduce net amounts of CO 2 released to the atmosphere

Alfalfa (Medicago sativa L.) contributes substantial N to grain crops that follow, but there is uncertainty concerning the amounts. The objective of this study was to determine optimal rates of N fertilization for first-year corn (Zea mays L.) grown after alfalfa. Fertilizer N was applied at seven rates, ranging from 0 to 200 Ib/acre, at 29 trials conducted over 4 yr in northeast Iowa. Concentrations of soil nitrate in late spring and concentration of nitrate in cornstalks at the end of the season were used to help determine optimal N rates. Fertilizer significantly increased yields at six of the 29 trials. The rate of N fertilization most profitable when applied across the 29 fields was mainly determined by cost of fertilization and value of grain. At prices prevailing in the Corn Belt, applications of 0 and 25 Ib N/acre were the most profitable [...]

Nitrogen applied to soils as ammonium fertilizers often is found as nonexchangeable, or fixed, ammonium soon after application. The importance of ammonium fixation under field conditions has not been well established, and studies to assess the importance of ammonium fixation are hampered by lack of information about the effects of air drying of soil samples on ammonium fixation. The work reported here examines the effects of air drying on fixation and release of 15N‐labeled ammonium in soil samples collected at 10, 45, and 90 days after labeled anhydrous ammonia was applied to two soils in an Iowa cornfield. Comparisons of field‐moist and air‐dried samples revealed that air drying induced fixation of ammonium in samples collected soon after fertilization and induced release of fixed ammonium in samples collected after exchangeable ammonium concentrations had been depleted by nitrification or other processes. The magnitude of the effects were proportional to the amount of fertilizer N present as f...

Precision in tissue testing to characterize the nitrogen (N) status of corn (Zea mays L.) is more important than ever before because of current concerns about the effects of N fertilizers on environmental quality. This study was conducted to assess the importance of errors associated with differences in physiological age of plants when plots receiving various rates of N fertilizer are sampled at a single time during the period of silk emergence. Ear leaf samples were collected at four times between early and late silking from five experiments at different locations in Iowa during 1987. Nitrogen deficiencies delayed silking by less than 2 days. After adjustments for delays in the onset of silking, the rates of silk emergence were similar for all N rates. Rapid rates of silk emergence indicated that it is practically impossible to sample leaves at a specific silking percentage such as 50 or 75%. Leaf N concentrations were not significantly influenced by time of sampling during the period of silk em...

High nitrate (NO 3 -N) concentrations in Iowa rivers have been linked to areas of intensive row crop production, but they have not been experimentally linked to specific management practices used during row crop production. This study demonstrates how the late-spring test for soil NO 3 -N and the end-of-season test for cornstalk NO 3 -N can be used to measure N sufficiency levels across many fields and how the results can be used to evaluate management practices within a watershed. More than 3200 soil and cornstalk samples were collected over a 12-yr period from fields planted to corn (Zea mays L.) and already fertilized by farmers using their normal practices. Results showed that early-season rainfall and associated N losses were major factors affecting N concentrations in soils and cornstalks. Evidence for NO 3 -N movement from fields to rivers was provided by an inverse relationship between annual means for NO 3 -N concentrations in soils and rivers. Because these losses can be avoided by delaying N applications, the practice of applying N several weeks or months before plants grow was linked to inefficient use of fertilizer and manure N by crops. Results of the study demonstrate how aggregate analyses of soil and cornstalk samples collected across many farms and years make it possible to identify the major factors affecting N management outcomes and, therefore, N management practices that are likely to produce the best outcomes within a watershed or region. This approach seems to have unique potential to interrelate the management practices of farmers, the efficiency of N fertilization, and NO 3 -N concentrations in rivers.

Soil temperature at the time of application has been the primary factor used to predict rates of nitrification and assess the risks associated with losses of N applied in the fall as anhydrous ammonia in the Corn Belt. We report studies assessing the importance of soil pH as a factor affecting nitrification rates and losses of this N before corn (Zea Mays L.) begins rapid growth in June. Data were collected in a series of field studies conducted during 4 yr. Anhydrous ammonia was applied in the fall after soils had cooled to 7.5. Significant relationships between soil pH and percentage nitrification were observed each year. Means of measurements made in mid-April (when planting begins) indicated 89% nitrification of fertilizer N in soils having pH > 7.5 and 39% nitrification of this N in soils having pH < 6.0. The finding that soil pH influenced when nitrification occurred helps to explain why the effects of nitrification inhibitors have been variable in this region. Significant relationships between soil pH and recovery of fertilizer N as exchangeable NH + 4 and NO - 3 were observed in years with above-average rainfall before samples were collected in April. The effects of soil pH on nitrification, therefore, influenced the amounts of NO - 3 lost by denitrification or leaching during spring rainfall. The observed effects of pH on nitrification rates suggest that economic and environmental benefits of delaying application of fertilizer N may be greater in higher-pH soils than in lower-pH soils.

This article tests whether ex post optimal nitrogen rates increase with better growing conditions. Results of the tests indicate whether there is statistical support for the rule-of-thumb that farmers should increase their ex ante fertilizer rates to ensure an adequate supply of nutrients if the growing season turns out to be favorable. In addition, the test results indicate if yield goals have a role in a system of fertilizer recommendations. Results indicate: (1) there is scant statistical support for the hypothesis that better growing conditions on a given site increase optimal nitrogen rates; and (2) better site-specific growing conditions seem to increase optimal nitrogen rates.

Zones of compaction and/or high nutrient concentration that exist with selected tillage systems can alter plant root growth. It is hypothesized that these alterations affect water and nutrient uptake patterns. A greenhouse experiment was conducted to study the effects of horizontally and vertically oriented compaction zones and nitrogen (N) fertilizer placement on corn (Zea mays L.) root growth patterns, soil water depletion, and N fertilizer uptake. Compaction treatments included a simulated plow pan, simulated wheel tracks in one or both interrows, and uncompacted soil. N-enriched ammonium nitrate was point injected 0.1-m deep at a rate of 80 kg N ha 1 either under the plant or in a simulated interrow region (0.13 m to the side of the plant); a check treatment (no N) was also included. Compaction and N treatments were factorially combined in five randomized blocks. Corn plants were grown for 26 d without rewatering. Measurements taken at 26 d included: shoot and root growth, plant N content, and soil water content, and distribution. Compaction treatments did not significantly affect total root growth, but N fertilization tended to decrease it. Root growth compensation, both enhancement in zones of favorable nutrient supply and redirection of growth to zones of more favorable physical conditions, was observed. Nitrogen placement had a greater impact on N uptake when compaction zones were present than when they were not present. Water extraction from a given zone was directly related to root density in that zone. Additional Index Words: Fertilizer use efficiency, Point fertilizer placement, Compensatory root growth, Plow pan. View complete article To view this complete article, insert Disc 5 then click button8

A simple method is described for the separation of mixtures of CH₄, N₂O, CO₂, N₂, and O₂ in soil atmospheres using a single sample. This method uses two columns of Porapak Q at different temperatures but requires only one detector and recorder. Total analysis time is 23 min, but samples can be injected every 17 min without conflicting peaks.