Illinois
Fertilizer Conference Proceedings
January 23-25, 1995
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F.E. Below, P.S. Brandau, and J.A. Yockey1
Nitrogen is the mineral element that corn plants require in the greatest quantity for growth and yield, yet the soil cannot supply enough for maximum productivity. Because N deficiency can seriously reduce corn yields, there is a strong incentive to use fertilizers to supply adequate N. However, in addition to being removed with the crop, N can be lost from the soil in large amounts as the result of leaching, denitrification, volatilization, surface runoff, and soil erosion. The economic implications of these losses are self-evident, especially when they are large enough to limit crop productivity or pollute the environment. Because N fertilizers have been implicated in contamination of ground and surface waters, fertilizer dealers and corn growers are under increasing pressure to improve their management of N.
Since corn plants can utilize N as either NO3 or NH4, but NO3 is the form primarily responsible for N losses, a logical approach to improving N management involves altering the predominant form of N available in the soil. In addition to being susceptible to loss, 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).
Increasing the supply of NH4 in soils may also enhance plant performance, as moderate yield increases (6 to 11 %) have been reported for maize by enhancing the supply of soil N as NH4 (Barber et al, 1992; Smiciklas and Below, 1992). Growers and fertilizer dealers in Illinois currently have the ability to alter soil N form by using different fertilizer N sources and including amendments like nitrification inhibitors. However, there is limited information on the NO3/NH4 ratio that gives optimum productivity, especially under varied soil types and 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 four diverse locations in Illinois in 1994. 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, UrbanaChampaign, which is devoted to agricultural research and has irrigation capabilities; 2) a grain farm in Geneseo with conventional tillage and continuous corn; 3) a grain farm in Geneseo with a sandy soil type, conventional tillage, and a corn/soybean rotation, and 4) another grain farm in Geneseo which is under no-till and a corn/soybean rotation. 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.
Four different sources of fertilizer N (calcium nitrate, ammonium nitrate, urea, and ureaammonium nitrate) applied at four rates (0, 60, 120, 180 lbs of N per acre) were evaluated at each location. Ammonium sulfate was included as an additional N source at two of the locations (Champaign and Geneseo 1). The N sources were selected to supply varying proportions of NO3-N and NH4-N, including all NO3 (calcium nitrate), mixtures of NO3 and NH4 (ammonium nitrate and UAN), and all NH4 (urea and ammonium sulfate). All N treatments were applied either with or without the nitrification inhibitor N-Serve. The purpose of the nitrification inhibitor was to maintain soil-applied 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 just 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 hoeing. At the Champaign site, plots received 1.5 inches of irrigation water immediately following N application.
During vegetative growth, the soil was sampled (to a depth of 12 inches) from all plots at each site for analysis of NO3 and NH4. For all sites, these measurements were taken when the corn crops was at approximately the V12 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 three 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. Data were analyzed over locations using a combined analysis and an LSD (P ≤ 0.10) calculated to compare treatment means. Only soil N and grain yield data from the 1994 growing season are presented in this report. Yield data for the ammonium sulfate N source is not included because it was collected at only two of the four locations. The overall yield at these sites was lower than the other locations, which skewed the average yield response to ammonium sulfate downward.
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 there was no effect at the 60 lb rate, at N rates of 120 and 180 lbs per acre the source of fertilizer N affected the soil N level. This difference was especially apparent when the fertilizer N did not include the nitrification inhibitor (Fig. 1). Including the nitrification inhibitor modestly increased the level of soil N for the high ammonium sources (urea, ammonium sulfate, and UAN), and seemed to minimize the differences in soil N level attributed to fertilizer source. Somewhat surprisingly, fertilizer sources containing high percentages of NO3-N (ammonium nitrate and calcium nitrate) had the highest levels of total soil N, while urea containing materials (urea and UAN) had the lowest (Fig. 1). Environmental conditions at all sites during the 1994 growing season were generally not conducive to NO3 losses (e.g. leaching or denitrification), which could explain the high soil N levels obtained from fertilization with NO3 sources of N. Alternatively, microbial immobilization may have been lower when the bulk of the available soil N was present as NO3.
In addition to affecting the soil N level, the source of fertilizer N also altered the predominant form of N (NO3 or NH4) in the soil (Fig. 1). When the fertilizer N source was calcium nitrate, the vast majority (80% or more) of the available soil N was present as NO3 (Fig. 1). Conversely, when the N source was urea or ammonium sulfate most of the available soil N was present as NH4. Fertilizing with either mixed N source (ammonium nitrate or UAN) resulted in approximately equal mixtures of NO3 and NH4 in the soil (Fig. 1). For all N sources except calcium nitrate, the NO3 to NH4 ratio remained relatively constant over the different N rates. For calcium nitrate, this ratio increased with increasing rates of fertilizer N. Regardless of the fertilizer N source, including the nitrification inhibitor tended to decrease the NO3 to NH4 ratio in the soil; indicative of more N in the NH4 form.
Although the magnitude of response was different, grain yield was increased
by addition of fertilizer N at all four locations (data not shown). Without
the nitrification inhibitor, there was a difference in the yield obtained with
the various N sources (Fig. 2). Supplying
N as urea resulted in the lowest yield, especially at the 60 lb N rate. Yield
obtained with the all (calcium nitrate), or high (ammonium nitrate) NO3
N sources was surprisingly high; which may be related to the high levels of
total available soil N observed with these sources (Fig.
1). When the nitrification inhibitor was included, the yield response to
applied N was similar, regardless of N source (Fig.
2).
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. The level of available soil N can also be altered by N source and inhibitor treatment. Including the nitrification inhibitor minimized differences in soil N and yield among the N sources. Although additional work is needed, these findings suggest improvements in N management can be obtained by altering the source of N used, and the form of N available in the soil.
1Associate Professor of Plant Physiology, Research Specialist in Agriculture, and Graduate Research Assistant, Dept. of Agronomy, Univ. of IL at Urbana-Champaign.
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.
