Illinois Fertilizer
Conference Proceedings
January 26-28, 2004
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R.G. Hoeft, R.L. Mulvaney, S. Khan, E.D. Nafziger, J.J. Warren,
L.C. Gonzini, T. K. Lehman, and A. Gulso1
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.
Before farmers and their advisors can utilize this test, several questions need to be answered. These include:
When is the optimum time to collect soil samples? In order to answer this question, samples will need to be collected from several sites over time to correlate the impact of soil temperature and moisture on the change in ISNT values. Since the ISNT provides a relative measure of amino sugar-N, a form of N that is apparently easily mineralized, it is likely that the test will vary during the time period that mineralization is rapid.
How many soil samples need to be collected from a given land area to accurately characterize the concentration of N obtained from the ISNT? In order to answer this question, samples will need to be collected from several fields representing major soil types found in Illinois, differing N management systems, differing manure histories, differing crop rotations, etc. Preliminary results indicate that ISNT values may vary substantially across a field, particularly if the field has a history of manure application. If this holds true, the ISNT may be a guide to use in planning variable rate N application.
Can the ISNT be used to determine rate of N application necessary for optimum yield? To answer this question, field experiments will need to be done under many different environmental and management situations.
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 Illinois as well as 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 the central laboratory.
Several industrial agronomists agreed to collect samples following the NC-218 protocol for temporal variation. That protocol is as follows:
Identify 2–3 sites having markedly different management histories (possibilities include manured, no manure history, excessive N fertilization, N fertilization history at or below recommended rate).
Install permanent marker or GPS location.
Collect soil samples (5 cores per sample) to a depth of 0–6 inches and 6–12 inches once per week starting Oct. 1 until soils are frozen and again from first thaw until Sept. 30.
Freeze samples and deliver to R.G. Hoeft, Dept. of Crop Sciences., Univ. of IL, 1102 S. Goodwin Ave., Urbana, IL 61801.
Obtain past crop and fertilizer history.
The samples were frozen until they could be dried, ground, and analyzed for ISNT concentration. A total of eleven experiments were conducted in 2002, three in Illinois, two in Iowa, one in Wisconsin, and four in Minnesota.
A number of private consultants, both individuals and company agronomists agreed to follow the NC-218 protocol for spatial sampling for ISNT analysis. That protocol is as follows:
Identify one or more 20–40 acre fields. If more than one, select fields that have differing N management histories (possibilities include manured- corn-soybean rotation; manured- cont. corn; nonmanured – corn soybean rotation; nonmanured continuous corn; alfalfa- 1 and 2 years after plowing; frequently manured history, but no manure in last 5 years on continuous corn or corn-soybean rotation.)
Obtain detailed soil type map.
Obtain past crop and fertilizer history, including crops grown, fertilizer N rate, and manure history from 1997 to present.
Collect soil samples 0–6 inches and 6–12 inches depth on a 1 acre grid from a 10-foot radius around a GPS identified point (8–10 cores per sample).
Freeze samples immediately and deliver frozen samples to R.G. Hoeft, Dept. of Crop Sciences, Univ. of IL, 1102 S. Goodwin Ave., Urbana, IL 61801.
The samples were kept frozen until they could be dried, ground, and analyzed for ISNT concentration.
Two separate studies (small plot and WATER) have been conducted over the past two 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 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 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 nine fields in 2001, 17 fields in 2002, and 16 fields in 2003 across 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 two 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 analysis for the ISNT. 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.
The ISNT concentrations decreased from early April through late May at two of the three locations in Illinois (Figure 1) and at two of the four locations in Minnesota (Figure 2). At the other two Illinois locations (Figure 1, center lines) and Minnesota (Figure 2), levels fluctuated over time, but did not show a significant trend. At the Iowa location, ISNT values increased from mid-March to early May and tended down during the summer months (Figure 3). No significant trend in ISNT values was observed at the Wisconsin location (Figure 4). Based upon these experiments, it appears that the best time for soil sampling will be early spring (Boast et al., 2003).
In 2002, the ISNT 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.
Spatial variability of the ISNT within seven Illinois fields was assessed in 2003. On fields located in east-central Illinois, with no record of manure application for at least 20 years, 100 percent of the samples had ISNT values within 10 percent of the mean. Fields with a manure history in east-central and north-west Illinois had much greater variability (20 to 28 percent from the mean) and less than 60 percent of the samples were within the range of 10 percent above or below the mean. One field in northwest Illinois with a long term manure history, including manure application with the last two years, had ISNT values that ranged from 149 to 482 ppm. More than 65 percent of the samples were more than 10 percent above or below the mean. There appeared to be little pattern to the variability across the field. Approximately 50 percent of the samples in the previously manured field had ISNT values above the critical level of 230 ppm. Work by Ruffo, 2003 demonstrated that ISNT values varied across fields, with like values occurring in relatively large areas within the field (Figure 5).
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 6). 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. Further work is needed to confirm situations where the test will not be expected to work.
Results collected from the farmer conducted WATER plots were less consistent than those from the small plot studies. Over the three years of the WATER plot studies, corn grown at 23 of the sites identified as nonresponders by the ISNT (value > 230 ppm) responded to fertilizer N (Figure 7). Frequency of occurrence of incorrect prediction of response was higher in 2002 and 2003 than in 2001. In 2002, climatic conditions were not conducive to microbial activity as many areas of the state had heavy rains early in the spring and early summer followed by very dry soils through most of the rest of the growing season. In 2003, the spring and early summer temperatures were below normal, and this was followed by an extensive dry period. The N that had been mineralized may not have moved down into the zone of maximum root uptake activity.
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. One site in the WATER study appears to have had a manure history as the P test at this site was very high, >500 lb P/acre. Response to applied N did not occur at this site in any of the three years of the study.
The relationship between N required for optimum yield and ISNT (Figure 8), N required for optimum yield and yield at optimum N (Figure 9), and N required for optimum yield and the unfertilized check yield (Figure 10) are nearly mirror images of each other, with none of them appearing to be good predictors of the amount of N required to attain optimum corn yield. Additional research is necessary to identify those factors that must be considered in developing a reliable calibration of the ISNT.
Figure 1. Change in ISNT concentration over time - Illinois.
Figure 2. Change in ISNT concentration over time - MN.
Figure 3. Change in ISNT concentration over time - IA.
Figure 4. Change in ISNT concentration over time - WI
Figure 5. ISNT Values for central Illinois field - Ruffo, 2003.
Figure 6. Relationship between yield response to applied N and ISNT - small plots, 2001-2003.
Figure 7. Relationship between yield response to applied N and ISNT - WATER plots, 2001-2003.
Figure 8. Relationship between optimum N and ISNT - WATER plots.
Figure 9. Relationship between optimum N and yield at optimum N - WATER plots.
Figure 10. Relationship between optimum N and unfertilized corn yield - WATER plots.
1 R.G. Hoeft is a professor, Dept. of Crop Sciences, University of Illinois: R.L. Mulvaney is a professor, Dept. of Natural Resources and Environmental Sciences; S. Khan is a research specialist in Agriculture, Dept. of Natural Resources and Environmental Sciences; E.D. Nafziger is a professor, Dept. of Crop Sciences; J.J. Warren and L.C. Gonzini are senior research specialists, Dept. of Crop Sciences; T.K. Lehman is a research specialist, 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 Proceedings 2003 (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 fertilizers. Soil Sci. Soc. of Amer. J. 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 Proceedings 2004 (R.G. Hoeft, ed.)
Ruffo, Matias. 2003. Personal communication.
