Alfred M. Blackmer

Optimal rates of nitrogen fertilization for a winter wheat-corn cropping system in Northern China

Abstract 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 regression analysis showed no significant relationship between soil mineral N deficit and uptake N by corn. The relationships between soil mineral N deficit and mineral N in the soil before planting indicated that soil mineral N deficits were directly proportional to the amounts of mineral N initially in the soil. Good relationships were observed between relative yields and amounts of mineral N in the soil before planting, and the r 2 values for the relationships with nitrate to 40 cm were similar to those observed with nitrate to 100 cm. The need to sample no deeper than 40 cm increases the practicality of soil testing and adjusting rates of N fertilization.

The Ex Post Relationship between Growing Conditions and Optimal Fertilizer Levels

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.

Performance-based evaluations of guidelines for nitrogen fertilizer application after animal manure

guidelines that estimate amounts of N needed by the crop and amounts of N supplied by the manure and the Nitrogen fertilizer needs for corn (Zea mays L.) in fields already soil (Midwest Planning Service Livestock Waste Subtreated with animal manure can be estimated by using general guidecommittee, 1985; Miller, 1986; Killorn, 1995; Schmitt et al., lines or soil testing for inorganic N. Although the soil-testing approach has been extensively evaluated for ability to predict yield responses 1997; Killorn and Lorimor, 1999; USDA Nat. Resour. to applied N under field conditions, the general-guideline approach Conserv. Serv., 1999, 2001; Iowa Dep. of Nat. Resour., has not been subjected to comparable performance-based evaluations. 2000; Jackson et al., 2000). Estimates of N need by the Fertilizer response trials were conducted in 205 manured fields to (i) crop are based on expected N removal by the crop, compare the two approaches for ability to predict corn yield responses which is based on expected yield level or published yield to fertilizer N applied after animal manure, (ii) identify reasons for potentials of soil map units. Amounts of N supplied by differences in predictive ability, and (iii) explore the benefits of perforthe manure are estimated by analyzing the manure for mance-based comparisons of the alternative approaches. Analyses N content and adjusting for expected losses of N by showed that 34% of the observed variability in response could be NH3 volatilization soon after application and for perexplained by inorganic N concentrations whereas less than 5% of this centages of organic N expected to be mineralized. variability could be explained by the general-guideline approach. The soil-testing approach, therefore, had greater ability to integrate the Soil testing for inorganic N when plants are 15 to effects of all factors affecting yield responses across the range of 30 cm tall (i.e., in late spring) offers an alternative apconditions studied. Mean yield responses (0.55 Mg ha 1) were smaller proach for selecting rates of N fertilization (Magdoff et than are usually detectable in individual trials, but they were great al., 1984; Blackmer et al., 1989; Fox et al., 1989; Magdoff, enough to prompt farmers to fertilize. Results of this study indicate 1990; Binford et al., 1992; Bundy and Meisinger, 1994). that the most commonly accepted approach to estimating N fertilizer Such testing gives site-specific estimates of the suffineeds is less reliable than generally believed and, therefore, that ciency of N for plant growth where sufficiency indicates superior approaches are likely to remain unrecognized unless the supply relative to needs of the plants on a scale ranging performance of the commonly accepted approach is objectively evalufrom below to above optimal (Blackmer, 2000). Balkated under realistic field conditions. com et al. (2003) recently showed that testing soils for inorganic N after fertilization offers an effective way to evaluate N management practices and guidelines given L application of animal manure provides N needed to farmers. for corn production, but there is great uncertainty Performance of the soil-testing approach to estimating in the amount of N a given application of animal manure fertilizer need is usually evaluated by considering ability will supply for plant growth (Bouldin et al., 1984; Boulto predict yield responses (often expressed as relative din and Klausner, 1998; Sharpley et al., 1998; Klausner yields) to fertilizer N under field conditions. Such evaluet al., 1994; Blackmer, 2000). Schepers and Fox (1989) ations are reasonable because fertilizers are applied to attributed this uncertainty to (i) inaccurate and vague increase yields. The problem addressed in this paper estimates by farmers concerning amounts of manure is that we can find no published studies that provide applied, (ii) extreme variation in N concentrations in comparable evaluations of the performance of the genmanure, (iii) variable amounts of N lost by NH3 volatileral-guideline approach (i.e., estimating N fertilizer needs ization following unincorporated surface applications, by following general guidelines that do not include soil (iv) uncertainty concerning the proportion of the matesting for NO3). There is need for such evaluations benure N that will become available for plant uptake, and cause reports indicate that most farmers make little or (v) the possibility that manure additions will increase no downward adjustment in rates of N fertilization for N losses due to denitrification. N already applied as animal manure (Duffy and White, This uncertainty causes problems when selecting rates 1998; Nowak et al., 1998; Balkcom et al., 2003). Balkat which commercially prepared fertilizer N should be com et al. (2003) found that farmers may have valid applied after the manure. These problems usually are reasons for not making these adjustments. The reliabiladdressed by encouraging farmers to follow general ity of methods for estimating N fertilizer needs is more important than ever before because land application of D.J. Hansen, Dep. of Plant and Soil Sci., Univ. of Delaware Res. and animal manure and fertilizer N has been identified as Educ. Cent., 16684 County Seat Hwy., Georgetown, DE 19947; and a major source of NO3 in rivers, and therefore, many A.M. Blackmer, A.P. Mallarino, and M.A. Wuebker, Dep. of Agron., farmers are now being required to develop nutrient Iowa State Univ., Ames, IA 50011. Journal paper of the Iowa Agric. management plans (Jackson et al., 2000; Kalkhoff et al., and Home Econ. Exp. Stn., Ames, Project no. 4003. Received 10 July 2003. *Corresponding author (ablackmr@iastate.edu). 2000; USEPA, 2001). Our objectives in this paper are (i) to compare the soilPublished in Agron. J. 96:34–41 (2004). testing and the general-guideline approaches for ability  American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA to predict corn yield responses to fertilizer N applied af-

Nitrogen deficiency and recovery in sustainable corn production as revealed by leaf chlorophyll measurements

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.

Measuring Soil Nitrogen Mineralization under Field Conditions

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.

Fertilizer-Induced Advances in Corn Growth Stage and Quantitative Definitions of Nitrogen Deficiencies

Evidence that nitrogen (N) fertilization tends to accelerate maturation as well as increase rates of growth has received little attention when diagnosing N deficiencies in corn (Zea mays L.). Such a tendency could be a potential source of errors when the diagnosis is solely based on comparing plants with different rates of growth. Whether N fertilization could accelerate rates of growth and maturation was tested in a field study with 12 paired plots representing relatively large variability in soil properties and landscape positions. The plots were located under conditions where preplant N fertilization reduced or avoided temporary N shortages for some plants but did not reduce for other plants early in the season. We measured corn heights to the youngest leaf collar, stages of growth and chlorophyll meter readings (CMRs). The added N advanced growth stages as well as increased corn heights and CMRs at any given time. Fertilization effects on corn heights, growth stages and ear weights were statistically significant (P < 0.05) despite substantial variability associated with landscape. Reductions in growth due to a temporary shortage of N within a growth stage might be partially offset by longer periods of growth within that stage to physiological maturity. Temporary shortages of N, therefore, may produce symptoms of N deficiency in situations where subsequent additions of N should not be expected to increase yields. Recognition of these two somewhat different effects (i.e., increase growth rates and advance growth stages) on corn growth could help to define N deficiency more precisely and to improve the accuracy of diagnosing N status in production agriculture.

Reliability of Chlorophyll Meter Measurements Prior to Corn Silking as Affected by the Leaf Change Problem

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.

Differences in Physiological Age Affect Diagnosis of Nitrogen Deficiencies in Cornfields

Abstract Many studies have shown that chlorophyll meter readings (CMRs) can be used to diagnose deficiencies of nitrogen (N) during the growth of corn ( Zea mays L.) in small-plot trials, but there is need to address additional problems encountered when diagnoses are made in fields of the size managed in production agriculture. A noteworthy difference between small-plot trials and production agriculture is the extent to which the effects of N are confounded with the effects of other factors such as tillage, landscape, soil organic matter and moisture content. We illustrate how some of these factors can cause differences in the physiological age of plants and introduce errors in the diagnoses of N deficiencies. We suggest methods (measuring the height to the youngest leaf collar and assigning leaf numbers by using the first leaf with pubescence and the ear leaf as references to identify growth stages) for minimizing these errors. The simplified method of growth stage identification can be used to select appropriate plants and leaves for making diagnoses in fields and to distinguish the effects of N from the effects of other factors that influence plant growth.

A simplified test of cornstalk nitrate for better N management

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.

Fixation and release of n‐15‐labeled ammonium during soil drying

Abstract 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...