Illinois Fertilizer
Conference Proceedings
January 25-27, 1993
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F.E. Below, P.S. Brandau, and D.G. Bullock1
A major obstacle to improving N fertilizer management of corn is accurately predicting a soil's capacity to supply N to the crop. This problem occurs because of the complex cycle of N in the environment, and the large number of cultural and environmental factors (and their interactions) that can alter N availability. Although soil N can become available as the result of natural and biological processes, the principle means of increasing N availability in soils is by application of N fertilizers. Because N deficiency can seriously decrease yields, corn growers have a strong incentive to supply adequate levels through fertilization. In the U.S., N fertilizer recommendations for corn are usually based on cropping history and expected yield goal; and to a lessor extent on formulas to estimate the capacity for N mineralization. While generally sound, problems can arise if the yield goal is unrealistic, or if growers fail to accurately assess the capacity of the soil to supply N. The tendency to over, or under, fertilize could be reduced if corn growers had a reliable method to rapidly assess plant N status, and still have time to correct deficiencies. This need has led to several new technologies to estimate plant-available N including the 'late-spring nitrate test' and the 'SPAD leaf chlorophyll meter'.
The 'late spring nitrate test' takes some of the uncertainty associated with N cycling into account, as soil samples are not taken until after the crop is established (plants are six inches tall) and the potential for N loss is lessened. Based on soil analysis and yield response to applied N, a soil N03-N concentration in excess of 20 to 25 ppm is considered adequate for maximum yield; whereas lower values require additional fertilizer (Binford et al., 1992; Blackmer et al., 1989; Fox et al., 1989). Although good at identifying situations where no fertilizer N is required, the test does not work as well in predicting the degree of responsiveness to fertilizer applications; or when a high percentage of the soil N is available as ammonium. In addition, this technique cannot be used if all the N is applied preplant, or if the N is knifed in as anhydrous ammonia. As a result, other plant-based tests, like leaf chlorophyll, are also being evaluated as a way of rapidly assessing plant N status and responsiveness to fertilizer applied N (Piekielek and Fox, 1992; Schepers et al., 1992; Wood et al., 1992. An advantage of plant measurements is that they integrate the effects of N availability and uptake into a single value which is more likely to reflect the impact of N availability on yield. Although these new techniques offer promise for predicting N fertilizer needs and improving production efficiency, there is a lack of supporting data under Illinois conditions.
Thus, the objective of this research is to find ways to integrate the potential
benefits of soil and plant measurements to improve the management of fertilizer
N. Specifically, this work is attempting to relate levels of residual soil N,
with the anticipated yield response to applications of fertilizer N. Tissue
samples from plants grown with varying rates of fertilizer N are being used
in conjunction with soil samples to determine the plant's ability to utilize
residual and fertilizer applied N. We hope to find ways that soil and/or plant
measurements can be used to identify management variables which can be changed
to optimize the use of fertilizer N in Illinois.
.
Two genetically distinct corn hybrids (B73 x LH51 and LBE136 x LH82) were evaluated for response to applied fertilizer N at four diverse locations in Illinois over a three year period (1990-92). The individual locations and some of their cultural and soil characteristics are presented in Table 1. Briefly, the locations include: 1) the Agronomy-Plant Pathology South Farm at the University of Illinois, Champaign, which is devoted to agricultural research and has irrigation capabilities; 2) a grain farm in Tremont (near Peoria) which is part of a high-management corn/soybean rotation; 3) a livestock/grain operation in Geneseo (Geneseo 1) that receives known amounts of hog manure and is in continuous corn; and 4) a separate site in Geneseo (Geneseo 2), that is part of a com/soybean rotation and does not receive manure.
Treatments consisted of each hybrid grown with varying levels of soil applied N. At all locations, the experiment was arranged in a split-plot design with three to four replications. At Champaign, N rates were main plots and hybrids the subplots, while at the other three locations hybrids were the main plots and N rates the subplots. An experimental unit consisted of eight rows of the respective hybrid that were either 20 feet (Champaign and Tremont) or 40 feet (both Geneseo locations) in length. At Tremont, rows were spaced 36 inches apart, while the row spacing was 30 inches at the other locations. At all locations, the stand density was 26,000 plants per acre. N rates evaluated are summarized in Table 1 and ranged from 0 to 2501bs N acre. The N treatments were established when plants were at the V2 to V3 growth stage by applying varying amounts of solid urea or ammonium nitrate and immediately incorporating with cultivation. Six of the eight rows received the fertilizer treatment and the two outside rows served as borders.
Prior to planting, the soil at each site was sampled (to a depth of 12 inches) for analysis of available NO3 and NH4. Other samples were taken from plots that did not receive fertilizer N but that contained six inch plants. At flowering and physiological maturity, the above ground portions of four representative plants were harvested from each plot, separated into leaves, stalk (including leaf sheaths), grain, and a reproductive support fraction consisting of husk, shank, tassel and cob. After drying (80°C) to constant weight, all fractions were weighed, ground, and analyzed for total N. In this report, plant N data is presented only for 1990 and 1991, because chemical analysis of 1992 samples is not yet complete.
Once the grain had field-dried to an acceptable moisture content, the center
two rows of each plot were combine harvested for the two Geneseo locations and
hand harvested for the other two locations in order to estimate grain yield.
Yield is expressed as bushels per acre at 15.5% moisture.
Grain yields of both hybrids were markedly increased by application of fertilizer N at three of the four locations in 1990 and at two locations in 1991 and 1992 (Fig 1). These included Champaign and Tremont in all three years and one of the Geneseo locations (Geneseo 2) in 1990. For these N responsive sites, maximum yields were obtained with an N rate of approximately 200 lbs N/acre, while plants relying solely on soil residual N produced from 62 to 82% of the maximum yield. As expected, the response to fertilizer N exhibited a pattern of decreasing increments of yield increase with successive N rates (Fig 1). Because yields had not plateaued at the Tremont or Geneseo 2 sites in 1990, or Champaign in 1992, it is unclear if additional N would have resulted in even higher yields. However, increasing the high N rate by 251bs at Geneseo 2 and 501bs at Tremont did not result in higher yields in 1991 or 1992. The Geneseo 1 site, which was nonresponsive to fertilizer N in all years, has regularly received large amounts of swine manure and has a relatively high level of residual soil N (Table 2).
Regardless of N rate, the hybrid B73 x LH51 outyielded LBE136 x LH82 at three of the four locations in 1990 (Champaign, Geneseo 1 and 2) and 1992 (Champaign, Tremont, and Geneseo 2), while LHE136 x LH82 outyielded B73 x LH51 at three locations in 1991 (Champaign, Geneseo 1 and 2) and one location (Tremont) in 1990 (Fig. 1). Although the reason for this difference is unclear, these hybrids were selected for study based on anticipated differences in their utilization of N. In this regard, LHE136 x LH82, which is generally less responsive to N, did better in the lower yielding environment of 1991, while the N responsive hybrid B73 x LH51 performed best in the higher yielding environments of 1990 and 1992. Collectively, these data demonstrate the variable effect of management practices, hybrid selection, and environment on yield responsiveness of corn to fertilizer applied N.
Soil analysis revealed differences among the locations in the level of residual soil N, as well as differences in the predominant form of N (Table 2). With the exception of the Tremont location in 1992, total N levels did not change appreciably between years and sampling times, while the predominant form of soil N did. In 1990, an average of 53% (range 46 to 66%) of the residual soil N was present as NH4-N, compared to 37% (range 28 to 48%) in 1991 and 33% (range 27 to 39%) in 1992. The predominant N form also changed between sampling dates. For example, at Tremont in 1991, the majority (67%) of residual soil N was present as NH4-N prior to planting, while NO3-N was the principle form (72%) at the six inch stage (Table 2). Although the Geneseo 1 location had a relatively high level of total soil N, a large, and variable, proportion of this N (32 to 70%) was present as NH4-N. As a result, soil NO3-N levels were often below the threshold considered for N sufficiency, indicating a need for additional fertilizer N. Lack of yield response to N at this site (Fig. 1) indicates an important limitation of the soil nitrate test when NH4-N constitutes a large proportion of the residual soil N.
Other wrong predictions of N requirement based on soil NO3-N levels are also apparent in this data, which are not related to the level of soil NH4-N. Specifically, high NO3-N and total N levels (at the six inch growth stage) at Tremont in 1992 (Table 2), are counter to yield increases obtained at this location from N fertilization (Fig. 1). Similarly, low soil NO3-N levels at Geneseo 2 in 1991 and 1992 indicated the need for additional N, yet yields were relatively nonresponsive to fertilizer N. Thus, these data demonstrate some problems associated with the soil NO3-N test that could limit its effectiveness in improving N fertilizer management.
Although yield was not responsive to N fertilizer at all sites (Fig. 1), in all cases, adequate accumulation of N by the plant was required to maximize yield (Fig. 2). For the nonresponsive Geneseo sites (Geneseo 1 in 1990 and 1991 and Geneseo 2 in 1991), approximately 200 lbs N/acre was absorbed from the soil regardless of the level of fertilizer applied N (Fig. 2). Similarly, for the N responsive sites, plant N accumulation increased with successive additions of fertilizer, and yields were generally maximized at the N rate that resulted in 200 lbs of plant N/acre (Fig. 2). Thus, these data demonstrate the requisite need for N to optimize productivity, but show that it can come from the soil (i.e. residual N or mineralization) or from fertilizer.
With regard to predicting the need for fertilizer N, the concentration of leaf
N (leaf N status) at flowering was the plant parameter most closely related
to yield. As shown in Figure 3, changes
in leaf N with N supply mirrored those of grain yield. For locations not responsive
to N, leaf N concentration was high (in excess of 30 mg per gram) without fertilizer
N, and remained relatively unchanged with fertilizer additions (Fig.
3). In contrast, at N responsive sites, the concentration of leaf N was
initially low and increased with additions of fertilizer N. These data are in
agreement with the concept of tissue testing at flowering to determine nutrient
sufficiency (Melsted et al., 1969). Unfortunately, the usefulness of this technology
is primarily as a diagnostic tool, as there is limited opportunity to correct
any deficiencies or excesses.
As public awareness focuses on environmental quality there are increasing pressures for corn growers and fertilizer dealers to justify the levels of N fertilizer they are recommending or using. This concern is prompted by the discovery of nitrates in groundwater and a public perception that growers over-apply fertilizer N. The research being conducted in this project is prompted by these concerns. Reliable and up to date information on how corn plants use N will add to information required to improve N management of Illinois soils, and will help to minimize the adverse environmental impacts of N fertilizer use.
The findings obtained so far are not supportive of the late-spring nitrate test, as we observed a range of responsiveness to fertilizer N which was poorly related to residual soil NO3-N. This lack of relationship is partly the result of a variable (but potentially large) proportion of the soil N as NH4-N. The data also show the crucial interaction of management practices, hybrid selection, and environment in determining the need for fertilizer N. However, regardless of the responsiveness to fertilizer N, adequate accumulation of N by the plant (either from the soil or from fertilizer) was a requirement for optimal productivity. Thus, despite limitations of the soil nitrate test, we still believe that N management could be improved by the ability to better predict the plant's need for fertilizer N.
Binford, G.D., A.M. Blackmer, and M.E. Cerato. 1992. Relationships between corn yields and soil nitrate in late spring. Agron. J. 84:53-59.
Blackmer, A.M., D. Pottker, M.E. Cerato, and J. Webb. 1989. Correlations between soil nitrate concentrations in late spring and corn yields in Iowa. J. Prod. Agric. 2:103-109.
Fox, R.H., G.W. Roth, K.V. Iverson, and W.P. Piekielek. 1989. Soil and tissue nitrate tests compared for predicting soil nitrogen availability to corn. Agron. J. 81:971-974.
Melsted, S.W., H.L. Motto, and T.R. Peck. 1969. Critical plant nutrient composition values useful in interpreting plant analysis data. Agron. J. 61:17-20.
Piekielek, W.P., and R.H. Fox. 1992. Use of a chlorophyll meter to predict sidedress nitrogen requirements for maize. Agron. J. 84:59-65.
Schepers, J.S., D.D. Francis, M.F. Vigil, and F.E. Below. 1992. Comparison
of corn leaf nitrogen concentrations and chlorophyll meter readings. Commum.
Soil Sci. Plant Anal. (in press).
Wood., C.W., D.W. Reeves, R.R. Duffield, and K.L. Edmisten. 1992. Field chlorophyll measurements for evaluation of corn nitrogen status. J. Plant Nutrit. 15:487-500.
