Illinois
Fertilizer Conference Proceedings
January 24-26, 2005
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R.G. Hoeft, E.D. Nafziger, L.C. Gonzini, T.K. Lehman, Matias Ruffo, and A. Gulso
1
Research has demonstrated that use of the Illinois Soil N Test (ISNT) offers potential to identify fields that have adequate N for optimum crop production without the use of supplemental nitrogen and may allow producers and their advisors a guide to adjust N above or below those currently recommended (Khan et al., 2001). 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. The objectives of this project were:
Central Laboratory
To insure that the ISNT results are consistent across projects in Illinois and neighboring
states, a central laboratory has been established. This laboratory is operated under strict quality control standards.
Scientists from within Illinois as well as from other Midwestern states that are a part of a North Central regional research
project that has been designed to evaluate the potential for the ISNT submit samples to this central laboratory.
Temporal Variation
For the 2002 and 2003 crop year, scientists and industrial agronomists collected samples from
selected fields. The previous management history was documented for each field. Samples were collected on a weekly bases at
each site to two depths– 0–6" and 6–12", frozen and delivered to the laboratory for processing.
Where possible, sample collection was initiated in fall soon after harvest and continued through late spring. States participating
in this phase of the project included Illinois, Iowa, Minnesota, and Wisconsin.
Spatial Sample Variation
Fields ranging in size from 40 to 160 acres have been sampled on a 2.5 acre grid to depths
of 0–6" and 6–12". The frozen samples were delivered to the Central Laboratory, dried, ground, and analyzed for ISNT.
ISNT Correlation and Calibration: Three separate, but related studies have been conducted over the past three years to expand the correlation and calibration data available for the ISNT.
Small Plot
Nitrogen rate studies have been conducted at a total of 55 sites (14 in 2001, 15 in 2002, and 26 in 2003).
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 (1 sample per replication) for pH, P, and K analysis as well as for the ISNT 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. Soon after emergence, nitrogen was applied in 30 pound
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 × 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 ISNT, and at some locations for the determination of nitrate-N content in the top 4-foot of the profile.
Water Plots
Staff at the Illinois Department of Agriculture identified 9 fields in 2001, 17 fields in 2002, 16 fields in
2003, and 13 in 2004 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 200
lb N/acre in 50 pound increments. Plot size was a strip through the field, except for the 0 lb N/acre which was in some cases shorter.
A randomized complete block design with 2 replications was used at each location. Soil samples were collected to a 12 inch depth in
6– inch increments in early April and were kept frozen until they could be dried and processed for ISNT analysi. In addition, the 0–6
inch samples were analyzed for pH, P, and K. At maturity, the plots were machine harvested by the producer and the data were transmitted
to the University of Illinois for statistical analysis.
Whole Field Plot Studies
Dr. Matias Ruffo, German Bollero, and Don Bullock conducted N rate studies in 16 Central
Illinois fields in 2002 and 2003. Each of the 40-acre fields was divided into 13 to 20 sections depending on field shape.
Within each of these sections 5 N rates consisting of UI recommendation (UIN), UIN–50, UIN–25, UIN +25, and UIN +50 lb N/acre
were applied with commercial fertilizer equipment to individual plots that were two passes of the fertilizer applicator wide
and a minimum of 230 feet long. Soil samples were collected from within each section and analyzed for ISNT. Corn grain yields
were determined by using calibrated, commercial yield monitors mounted on combines equipped with DGPS.
Temporal Variation
While not consistent across locations, the ISNT values tended to be high in early spring, decrease through early summer and then
increase by fall back to levels observed in early spring. The calendar dates at which ISNT decreased and increased were later in
the Northern climates. In Illinois, the major decrease in ISNT began in early to mid–April. In contrast, this decrease
started in late May in Minnesota and Iowa. Since the ISNT appears to be a measure of easily mineralizable organic N, one would
expect it to decrease once microbial activity commences in the spring. Based upon these results, it is logical to assume that
the best time for sampling for the ISNT will be late fall or early spring.
Spatial Sample Variation
Over the three years of the study, 13 fields have been sampled to determine ISNT variation across the field. More than 70 percent
of the samples collected from fields in east–central Illinois with no history of manure application were within 10 percent of
the mean value for the field. However, fields having a recent (<5 years) history of manure application had substantially more
variation in ISNT values. Depending on the field, from 40 to 60 percent of the samples varied by more than 10 percent from the mean.
A high percentage of the samples collected from fields having a manure history had values in excess of the previously established threshold
of 230 ppm N (Figure 1).
Correlation and Calibration:
Small Plot Studies
Mulvaney et al., 2004, reported that the ISNT did an excellent job of separating responding from nonresponding at the 105 sites in the small
plot experiments conducted on farmer fields around Illinois (Figure 2). Using the previously established threshold of 230 ppm N, the test
accurately identified 95 percent of the nonresponding sites in the study. Approximately 19 percent of the predicted nonresponding sites
did respond to fertilizer application. It is not clear why these sites respond to N, but possible explanations include low soil pH, dry
surface soils during the mineralization season, cool soil temperatures, spatial variability, and variability with depth, etc.
Water Plots
Results collected from the farmer conducted WATER plots were less consistent than those from the small plot studies. Over the four years
of the WATER plot studies, corn grown at 28 of the sites identified as nonresponders by the ISNT (value > 230 ppm) responded to fertilizer
N (Figure 3). Only 2 of the 49 sites included in the 4–years of the study had ISNT levels greater than 230
ppm and showed no significant increase in yield with increasing N rates.
When properly fertilized, yields were high at all locations (Figure 4). Yields were somewhat below normal in 2002
at many locations due to heavy rains in the spring and early summer followed by very dry soils throughout the most of the rest of the
growing season. Corn grown in 2003 and 2004 took advantage of the exceptional growing conditions resulting in above normal yields
when the crop was properly fertilized. Corn grown without supplemental N in 2004 following exceptional high corn yield in 2003 or
following abnormally low soybean yields was lower in yield than that experienced at the same sites in the previous years
(Figure 5). The relationship between ISNT and yield increase (Figure 3),
yield at optimum N (Figure 4), yield of control treatment (Figure 5),
and optimum N (Figure 6) was not significant.
Lack of yield response to fertilizer N occurred on a much higher proportion of the small plot experiments conducted on farmer fields
than occurred on the WATER plots conducted by farmers. This may have been due in part to the difference in previous management.
Nearly 50 percent of the fields used in the small plot studies had a history of prior manure application and in some cases, manure was
applied for the corn crop in the year of the study. Since most of the WATER plot studies were conducted in fields located adjacent to
a major highway, often an Interstate highway, the probability that these fields had received manure in the recent past was very low.
Whole Field Studies
The work by Ruffo 2004 demonstrated that the site-specific characteristics that affected corn yield and response to applied N tended
to vary among fields, but some trends were consistent. He observed a consistent interaction of ISNT with nitrogen rate on corn yield.
AS ISNT increased the response of corn to nitrogen fertilizer decreased, indicating that ISNT could be considered a surrogate for potential
soil N mineralization. Combining ISNT with terrain attributes further enhanced the predictability of nitrogen rate for optimum crop
production. The data collected from a central Illinois field provides promise that the ISNT might be useful for site specific application
of N (Fig. 7).
Figure 1. Variation in ISNT values (ppm N) from a field with a manure history.
Figure 2. Relationship between yield response to applied N and ISNT— small plots. 2001–2004
Figure 3. Relationship between yield response to applied N and ISNT– WATER plots. 2001-2004
Figure 4. Relationship between INST and yield at optimum N.
Figure 5. Relationship between ISNT and yield of unfertilized control
Figure 6. Relationship between ISNT and optimum N.
Figure 7. Nitrogen response functions related to ISNT at selected sites within a production field.
1 R.G. Hoeft and E.D. Nafziger are Professors, Dept. of Crop Sciences, University of Illinois; L.C. Gonzini is Senior Research Specialists; T.K. Lehman is Research Specialist, Dept of Crop Sciences; Matias Ruffo is Research Assistant, Dept. of Crop Sciences; and Alan Gulso is Program Manager, Illinois Department of Agriculture.
Boast, C. W., T. R. Ellsworth, T. J. Smith, R. L. Mulvaney, S. A. Khan, E. M. El-Naggar, and R. G. Hoeft. 2003. Spatial and temporal variability in the Illinois N test. In: Illinois Fertilizer Conference 2003 Proceedings (R. G. Hoeft, ed.). pp. 15-19.Khan, S. A., R. L. Mulvaney, and R. G. Hoeft. 2001. A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Science Society of America Journal, 65:1751–1760.
Mulvaney, R. L., S. A. Khan, J. J. Warren, L. C. Gonzini, T. J. Smith, and R. G. Hoeft. 2004. Potential of the Illinois soil nitrogen test to improve nitrogen fertilizer management for corn production. In: Illinois Fertilizer Conference 2004 Proceedings (R. G. Hoeft, ed.).
Ruffo, Matias. 2004. Personal communication.
