Illinois Fertilizer
Conference Proceedings
January 27-29, 2003
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R.G. Hoeft, R.L. Mulvaney, S.A. Khan, E.D. Nafziger, J.J. Warren,
L.C. Gonzini, T.K. Lehman, and A. Gulso1
Research has clearly demonstrated that use of the Illinois N Soil Test offers tremendous potential to identify fields that have adequate N for optimum crop production without the use of supplemental N and may allow producers and their advisors a guide to adjust N rates below those currently recommended (Khan et al.). When properly calibrated, this new test will improve the efficiency of corn production, allowing Illinois farmers to more effectively compete in the world market. Additionally, use of the test to adjust N rate more closely to N need offers the potential to reduce the risk of environmental contamination of water supplies. Before farmers and their advisors can utilize this test, several questions need to be answered. These include:
To insure the reliability of Illinois N Soil Test data being collected from
not only the projects proposed for this project but also numerous studies by
other researchers in Illinois and in neighboring states, a central laboratory
has been established. This laboratory is operated under strict quality control
standards and monitored by Saeed Khan, one of the original developers of the
Illinois N Soil Test procedure. Scientists from within Illinois as well as from
other Midwestern states that are a part of a North Central regional research
project designed to evaluate the potential for the Illinois N Soil Test will
submit samples to this central laboratory.
A series of studies were conducted to answerthe above questions.
Temporal variation
Several industrial agronomists agreed to collect samples following the NC-218 protocol for temporal variation. That protocol is as follows:
The samples were kept frozen until they could be dried, ground, and analyzed for Illinois N Soil Test concentration. A total of eight experiments were conducted, four from Illinois and four from Minnesota.
Spatial sample variation
A number of private consultants (individuals and company agronomists) agreed to follow the NC-218 protocol for spatial sampling for Illinois N Soil Test analysis. That protocol is as follows:
The samples were kept frozen until they could be dried, ground, and analyzed for Illinois N Soil Test concentration.
Soil test correlation and calibration
Two separate but related studies have been conducted over the past two years to expand the correlation and calibration data available for the Illinois N Soil Test.
Study 1
Nitrogen rate studies have been conducted at a total of 29 sites (14 in 2001 and 15 in 2002). Sites were identified to provide a range in past nitrogen management history, manure application, soil type, geographic location, and crop rotation. Once the field was identified, the plot area within the field was delineated by flags, and soil samples were collected (one sample per replication) for pH, P, and K analysis as well as for the Illinois N Soil Test analysis. Permanent markers were left in the field to assure that the farmer did not apply nitrogen to the plot area. All field operations other than nitrogen application were conducted by the farmer in the same manner as the rest of the field. As soon as the crop was planted, nitrogen was applied in 30-lb increments at rates ranging from 0 to 210 lb N/acre. The experimental procedure was a randomized complete block with four replications, for a total of 28 plots per experimental location. Individual plots were 10 x 50 feet. At the V-4 stage of growth, two harvest rows were thinned in each plot to uniform plant population. At physiological maturity, the designated harvest rows were hand harvested, with the grain being shelled for yield determination. Soil samples were collected in the fall after harvest for the Illinois N Soil Test, and at some locations for the determination of nitrate-N content in the top 4 feet of the profile.
Study 2
Staff at the Illinois Department of Agriculture identified 23 fields across
the state of Illinois. Each of the fields was located along a major Illinois
highway, with most being located along interstate highways. In each case, the
farmers established nitrogen rate studies with N rates ranging from 0 to 240
lb N/acre in 60-lb increments. Plot size was a strip through the field, except
for the 0 lb N/acre, which was limited to a 300-foot length. A randomized complete
block design with two replications was used at each location. Soil samples were
collected to a 12-inch depth in increments of 0 to 6 and 6 to 12 inches in early
April and were kept frozen until they could be dried and processed for analysis
for the Illinois N Soil Test. In addition, the 0-to-6-inch samples were analyzed
for pH, P, and K. At maturity, the plots were machine harvested, and the data
were transmitted to the University of Illinois for statistical analysis.
Temporal variation
Two of the four Illinois locations followed the pattern observed with the Morrow Plots data; the values decreased from early spring into the summer (Figure 1). At the other two Illinois locations, the values remained constant throughout the sampling period. There was a significant decrease in Illinois N Soil Test values at two of the Minnesota locations (Figure 2), but the decrease was somewhat later in the season than observed at Illinois. If, in fact, the amino sugar levels as measured by the Illinois N Soil Test are reflective of the easily mineralizable N source found in the soil, this decrease in values in early to midsummer would be expected.
The decrease in Illinois N Soil Test values occurred at the same time and at approximately the same magnitude at the 0-to-6-inch and 6-to-12-inch depths (Figure 3). This trend offers promise that a 6-inch soil sample will adequately characterize the available N supply of a field.
Spatial variability
Variation in Illinois N Soil Test values within a field ranged from a low of 35 ppm in one field to as much as 189 ppm in another. In three of the four fields studied, over 70 percent of the samples were within 10 percent of the mean. However, in the field that varied by 189 ppm from low to high sample, only 14 percent of the samples were within 10 percent of the mean. In general, the variation tended to follow a pattern within a field, with the high values being grouped together, most likely indicating an effect of past management.
Correlation and calibration
In 2001, there was little to no response to application of fertilizer at eight of the 14 sites (Figure 4). Seven of those sites had soil test levels greater than 240 ppm. There was a large increase to applied N at one site that had a high soil test level (> 240). In 2002, there was no response to applied N at any site that had a test level greater than 240 ppm (Figure 5). Over the two years, results from these small-plot studies conducted in farmers' fields have confirmed that the test does a good job of predicting nonresponsive sites, with only one failure out of 29 experiments.
Results collected from the farmer-conducted WATER plots were much less consistent than from the small-plot studies discussed above. Over the two years of the WATER plot studies, corn grown at 12 of the sites projected as nonresponders by the Illinois N Soil Test responded to fertilizer N (Figure 6 and 7). Most of these failures occurred in 2002, a year characterized by heavy rains early in the spring and early summer followed by very dry soils through most of the rest of the growing season. These unusual climatic conditions undoubtedly would have had a negative impact on microbial activity necessary to convert the amino sugar N to plant-available inorganic N.
Additional research is necessary to calibrate the Illinois N Soil Test. As
noted in Figure 7, the amount of N necessary to attain
optimum yield varied from 120 to 240 lb N/acre at the same soil test level.
Figure 1. Temporal variation in Illinos N soil test values at four Illinois locations
Figure 2. Temporal variation in Illinos N soil test values at four Minnesota locations
Figure 3. Change in Illinois N soil test over time at the 0-to-6-inch and 6-to-12-inch depths
Figure 4. Yield increase to applied N at differing Illinois N soil test values - 2001
Figure 5. Yield increase to applied N at differing Illinois N soil test values - 2002
Figure 6. Yield increase to applied N at differing Illinois N soil test values - 2001 - WATER plots
Figure 7. Optimum N rate at differing Illinois N soil test values - 2002 - WATER plots
1 R.G. Hoeft is a professor,
Department of Crop Sciences, University of Illinois; R.L. Mulvaney is a professor,
Department of Natural Resources and Environmental Sciences; S.A. Khan is a research
specialist in agriculture, Department of Natural Resources and Environmental
Sciences; E.D. Nafziger is a professor, Department of Crop Sciences; J.J. Warren,
L.C. Gonzini, and T.K. Lehman are senior research specialists, Department of
Crop Sciences; and Alan Gulso is program manager, Illinois Department of Agriculture.
Khan, S.A., R.L. Mulvaney, and R.G. Hoeft. 2001. A simple soil test for detecting
sites that are nonresponsive to nitrogen fertilizers. Soil Sci. Soc. of Amer.
J. 65: 1751-1760.
