Illinois
Fertilizer Conference Proceedings
January 24-26, 1994
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F.E. Below and P.S. Brandau1
While it is well accepted that N fertilizer is a key component in corn production, each year growers face the dilemma of knowing how much N to apply. Because N deficiency can seriously reduce corn yields, there is a strong incentive to use fertilizers to supply adequate N. However, unused fertilizer N is economically wasteful and can become an environmental hazard if lost from the rooting zone. Fertilizer N use has also been implicated in groundwater contamination, increasing the pressure on fertilizer dealers and corn growers to improve their management of N.
Although an obvious remedy would seem to be less fertilizer N, this approach increases the risks associated with corn production, which could lead to less profits for both the grower and the fertilizer dealer; and ultimately to higher prices for the consumer. Rather, because corn plants can utilize N as either NO3 or NH4, but only NO3 is susceptible to N losses, a -logical approach to improving N management involves altering the predominant form of N available in the soil. In the NO3 form, can be lost from the rooting zone by the physical process of leaching (because the NO3 ion is repelled by negatively charged soil colloids), or by the microbial process of denitrification (conversion of NO3 to gaseous N compounds). Both processes are economically and environmentally undesirable, and perpetuate a large amount of the uncertainty associated with N fertilizer management.
In addition to being the N form responsible for soil N losses, NO3 is often the predominant N form available to the corn plant. This situation occurs because two groups of chemoautotropic soil bacteria rapidly oxidize NH4 to NO3 (nitrification) in warm, well-aerated soils that are favorable for corn growth. The advent of nitrification inhibitors, however, changes this situation by blocking nitrification, and increasing the proportion of soil N as NH4. Use of these inhibitors can reduce losses of fertilizer N, and may provide a means of altering the predominant form of N in the soil (Huffman, 1989). The identification and development of more potent second generation nitrification inhibitors could further improve the ability to manipulate soil N form.
Increasing the supply of NH4 in soils should also enhance plant performance, as numerous plant species (including corn) have been shown to absorb more N and grow more rapidly when supplied with mixtures of NO3 and NH4 than with only N03. Using field-hydroponics, we have consistently observed higher corn yields with mixed N nutrition (average of 12%), across a wide range of environments and genotypes (Below and Gentry, 1987 and 1992; Gentry and Below, 1993). Although these increases were obtained with hydroponics, where greater control over the NO3/NH4 ratio is possible, moderate increases (6 to 11%) have also been reported under production conditions by enhancing the supply of soil N as NH4 (Barber et al, 1992; Smiciklas and Below, 1992). This increase occurs as the result of alterations in several important physiological processes such as reproductive development, N acquisition, dry matter production, and assimilate partitioning. Other work shows that late vegetative and early reproductive development are the most crucial times to supply mixed N (i.e. NO3 and NH4) to the plant (Below and Gentry, 1992). Growers and fertilizer dealers have the ability to alter soil N form during this time period, which should improve improve N fertilizer management. However, there is limited information on the NO3/NH4 ratio that gives optimum productivity, especially under varied tillage practices.
Thus, the overall objective of this work is to characterize the effects of
varying the ratio of NO3 to NH4 in the soil on N use efficiency
and productivity of corn. Specifically, we are evaluating: 1) how effectively
this ratio can be altered in soil by varying the N source, or inhibiting nitrification;
2) under what conditions (cultural and/or environmental) will altering this
ratio affect productivity; and 3) if the optimum ratio is related to the level
of applied N.
Nitrogen use and productivity of corn as a function of N source, N rate, and nitrification inhibitor treatment was determined at three diverse locations in Illinois in 1993. 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 Geneseo with conventional tillage and continuous corn; and 3) a separate site in Geneseo that is under no-till and a corn/soybean rotation. Another site with a sandy soil type (listed in Table 1 as the third Geneseo site) was originally included in the experiment, but was abandoned due to extensive water damage. At all locations, the single-cross hybrid LH132 x LH212 was overplanted and after emergence thinned to a stand density of 26,000 plants per acre.
Treatments consisted of four different sources of fertilizer N (calcium nitrate, ammonium nitrate, urea, or an equal mixture of urea and ammonium nitrate) applied at four rates (0, 60, 120, 180 lbs of N per acre), either with or without inclusion of an experimental nitrification inhibitor (XDE474, DowElanco, Indianapolis, IN). The use of this experimental inhibitor was primarily for its convenience of application, and does not preclude the use of other commercially available nitrification inhibitors to maintain soil N as NH4. At all locations, the experiment was arranged in a split-split plot design with four replications with N rates as main plots, N sources as subplots, and the nitrification inhibitor treatment as the sub-sub plot: An experimental unit consisted of five rows (row 20 feet long spaced 30 inches apart) with the center three rows receiving the N treatment.
The N treatments were established shortly after seedling emergence (about the V2 growth stage) by hand applying varying amounts of solid fertilizer down the center of the rows. For the nitrification inhibitor treatment, the inhibitor was mixed with water and sprayed onto the fertilizer band immediately after application. At the sites with tillage, the fertilizer was spread in a diffuse band (approx. 10 inches) and incorporated with a field cultivator to a depth of 2 to 3 inches: At the no-till site, a narrow furrow (approx. 2 inches wide and 5 inches deep) was made down the center of each row with the passage of a 2-inch straight chisel. point. The fertilizer was poured into this opening and the furrow covered with soil by hand hoeig.
During late vegetative growth, the soil was sampled (to a depth of 12 inches) from all plots at each site for analysis of available NO3 and NH4. At Champaign, these measurements were taken on July 6, which corresponded to the V12 growth stage; while at Geneseo, soil was sampled on July 15 when the crop was at approximately the V14 growth stage. Leaf chlorophyll levels were determined at the same time using a hand-held SPAD meter. At physiological maturity (the R6 growth stage), the above ground portions of four representative plants were harvested from each plot, separated into stover, 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. For yield estimates, three rows of each plot were combine harvested at the two Geneseo locations, and the center two rows were hand harvested at the Champaign location. Yield is expressed as bushels per acre at 15.5% moisture. In this report, only grain yield and soil N data are presented, because chemical analysis of plant samples is not yet complete.
At all locations, and for all N sources, increasing the rate of applied N resulted: in corresponding increases in the level of total N available in the soil (Fig. 1). While the level of total soil N was relatively unaffected by the fertilizer N source; the predominant form of N in the soil (NO3 or NH4) was markedly altered by N source (Fig. 1). As expected, most of the available soil N was NO3 when the fertilizer N source was calcium nitrate, while most of the soil N was NH4 when the N source was urea. Conversely, fertilizing with either mixed N source (ammonium nitrate or the ammonium nitrate-urea mixture) resulted in approximately equal levels of NO3 and NH4 in the soil (Fig. 1).
Adding the nitrification inhibitor to calcium nitrate fertilizer had no affect on the total N level, or on the proportion of soil N as NH4 at any of the locations (Fig. 1). In contrast, including the nitrification inhibitor with urea fertilizer increased the proportion of soil N as NH4 at all locations, and increased the level of total available soil N at two of the locations (Champaign and Geneseo-conventional till). Adding the nitrification inhibitor to the mixed N sources gave intermediate results, with the ammonium nitrate response being more like that of calcium nitrate and ammonium nitrate-urea similar to that of urea (Fig. 1).
Although the magnitude of response was different, grain yield was increased by addition of fertilizer N at all three locations (Fig. 2). This increase in yield was most .dramatic at Champaign, where plants fertilized with 180 lbs of N yielded approximately 70% more than unfertilized plants; compared to yield increases of 20 to 25% at the Geneseo locations. The differences in responsiveness of the locations to fertilizer N are related to differences in the level of residual N available in the soil (Figs. 1 and 2).
At each location, the yield and pattern of response to applied N was similar
regardless of the source of fertilizer N (Fig.
2). However, the effectiveness of the nitrification inhibitor in increasing
yield was dependent upon the N source, the rate of applied N, and the location.
When the fertilizer N source was calcium nitrate, yield was not affected by
including the nitrification inhibitor at any N rate for any of the locations.
Conversely, when thefertilizer N source was urea, including the nitrification
inhibitor with 60 or 120 1bs of N significantly increased yield at the two locations
that received tillage (Fig. 2). These two
locations were the same ones where the inhibitor increased the level of total
N available in the soil (Fig. 1). Including
the nitrification inhibitor along with the N mixtures had a tendency to increase
yield at the lowest N rate (601bs), but this difference was only significant
for ammonium nitrate + urea at Champaign.
The overall goal of this work is to find strategies that growers and fertilizer dealers can use to improve the efficiency of N use while safeguarding the environment and maintaining profitability. Fertilizer dealers already have the capability to alter soil N form by adjusting the source of fertilizer applied N and by using nitrification inhibitors. While a growing body of evidence suggests that maintaining a mixture of NO3 and NH4 in the soil has a positive effect on N use and yield, it is unclear how to best alter this ratio under variable production and tillage conditions. The findings obtained so far suggest that the predominant form of N in the soil can be readily altered by adjusting the source of fertilizer N, and in some cases by including a nitrification inhibitor. Although N source did not effect the yield response to fertilizer N, it did affect the response to a nitrification inhibitor. The greater the percentage of the fertilizer N as NH4, the greater the likelihood of a yield increase from inhibition of nitrification. Tillage and N rate also affected the response, as the inhibitor treatment was most effective at low N rates at locations which received tillage.
1Associate Professor of Plant Physiology and Research Specialist in Agriculture, Dept, of Agronomy, Univ. of IL at Urbana-Champaign. This study was supported in part by a grant from the Fertilizer Research and Education Council.
Barber, K.L., L.D. Maddux, D.E. Kissel, G.M. Pierzynski, and B.R. Bock. 1992.
Corn, responses to ammonium-and nitrate-nitrogen fertilization. Soil Sci. Soc.
Am. J. 56:1116-1121.
Below, F.E., and L.E. Gentry. 1987. Effect of mixed N nutrition on nutrient
accumulation, partitioning, and productivity of corn. J. Fert. Issues 4:79-85.
Below, F.E., and L.E. Gentry. 1992. Maize productivity as influenced by mixed
nitrogen supplied before or after anthesis. Crop Sci. 32:163-168.
Gentry, L.E., and F.E. Below. 1993. Maize productivity as influenced by form
and availability of nitrogen. Crop Sci. 33:491-497.
Huffman, J.R. 1989. Effects of enhanced ammonium nitrogen availability for corn.
J. Agron. Educ. 18:93-97.
Smiciklas, K.D. and F.E. Below. 1992. Role of nitrogen form in determining yield
of fieldgrown maize. Crop Sci. 32:1220-1225.
