Illinois
Fertilizer Conference Proceedings
January 27-29, 2003
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C.W. Boast, T.R. Ellsworth, T.J. Smith, R.L. Mulvaney, S.A.
Khan, E.M. El-Naggar, and R.G. Hoeft1
The Illinois N test was designed as a relatively simple procedure for estimating amino sugar N in soil (Mulvaney and Khan, 2001). The goal in designing the test was to identify a portion of soil organic N that is capable of rapid mineralization and thereby serves as a source of plantavailable N during crop growth. In fact, the Illinois N test has been shown to be capable of identifying soils that are responsive and nonresponsive to N fertilization (Khan, et al., 2001, Mulvaney, et al., 2001). Thus, there is a potential that the test will enable improved fertilizer recommendations by providing a reasonable estimate of soil N availability, and studies are underway to test for this capability.
Since the Illinois N test is designed to measure a portion of soil organic
N that is capable of relatively rapid mineralization, one could anticipate that
test values would vary with time, for example, from season to season or even
more rapidly. Preliminary results like those reported elsewhere in this volume
(Mulvaney, et al., 2003) show higher test values in the spring and lower values
in the fall. If the Illinois N test were to be used to separate N-responsive
from N-nonresponsive soils or for N fertilizer recommendations, such temporal
variability would need to be either avoided (for example, it might be important
to always sample at one time of the year) or corrected for. In addition, when
soil samples are taken at different times, at even slightly different locations,
the observed variations could be due, at least in part, to spatial variability
in soil test values. Thus, an intensive study of the temporal and spatial variability
of Illinois N test values was undertaken, beginning in November 2001.
Three fields near Mansfield, Illinois were chosen for the spatial and temporal variability study. One field (designated "manure") receives manure annually and is in a long-term cornsoybean rotation. The other two fields have been free of manure applications for at least 17 years: one (designated "north") is in a long-term cornsoybean rotation, and another (designated "south") is under long-term continuous corn. The soil at the south field is a Sabina silt loam, a light-colored, low organic matter Alfisol. The manure site is located on a Drummer silty clay loam, and the north site is on an Ipava silt loam, both Mollisols. Within each field, four 100-m2 (25 feet by 43 feet) plots were established, two that were fertilized with N and two that were not. Approximately 160 pounds of N per acre were applied as anhydrous ammonia to the fertilized plots at the manure and north fields on October 22 and November 1, 2001, respectively, whereas the south field was fertilized on May 24, 2002, with 28 percent N solution to supply approximately 160 pounds N per acre.
Starting on November 9, 2001, soil samples were collected weekly to three depths (0 to 3 inches, 3 to 7 inches, and 7 to 12 inches) from each of the 12 plots and analyzed using the Illinois N test. The sample for each depth in a given plot consisted of a composite of four subsamples obtained as follows. Each plot was first divided into four 11-feet-by-20-feet subplots on a 2 x 2 pattern, and then a 7 x 8 grid of locations was marked with twine in each of the four subplots. On a given sampling day, one of the 56 (7 x 8) locations was chosen randomly, without replacement, for sampling. For each depth, a soil sample was taken from this location in each of the four subplots within a given plot, and the four samples were composited. Although complicated, this sampling scheme was designed to approximate a conventional five-composite sampling scheme, such that repeated sampling could be performed within each plot at randomly chosen locations for an entire calendar year.
On March 21, 2002, soil samples were collected to assess the spatial variability
of the Illinois N test: 140 samples from the two unfertilized plots at the north
field, 140 samples from the two unfertilized plots in the manure field, and
236 samples from the four unfertilized plots in the south field (see Figure
2 for the sampling pattern). These soil samples differ from the weekly soil
samples in three ways: There was no compositing of samples, only one depth range
was sampled (0 to 7 inches), and the sampling was done on a regular grid.
Spatial Variability
Figure 1 is a histogram of Illinois N test values measured on the 516 samples obtained from the eight unfertilized plots on March 21, 2002. The three fields are clearly separated in Figure 1. The 236 values from the south field (a low organic matter soil) were all lower than 200. At the other extreme, each of the 140 values for the manure field exceeded 387. The 140 north field values were intermediate, ranging from 236 to 383. As might be expected, test values ranged more widely for the manured than for the two non-manured fields, although this difference largely disappeared by expressing values relative to the field average. The coefficients of variation for the three fields were remarkably similar, ranging from 8 to 10 percent.
The 236 locations for spatial sampling of the south field are shown in Figure 2, with each spot representing a single sample. On March 21, 2002, all four plots at the south field (indicated as A to D in Figure 2) remained unfertilized. In plot B, the northwest subplot was sampled more intensively than the remainder of the south field: soil samples were taken 0.4 m south of each of the 56 (7 x 8) locations identified for weekly sampling in this subplot. Twelve samples were collected from each of the other 15 subplots.
Figure 2 shows the spatial distribution of Illinois N test values for the south field, which ranged from 116 to 200 ppm. This distribution is represented by dots of varying intensity but is more evident from the kriged map generated for the south field plot area (Figure 3). Interestingly, soil test values tended to exhibit greater similarity in the north-south than in the eastwest direction, which is consistent with the orientation of crop rows and suggestive of a persistent effect of past residue distribution and/or fertilizer practices. The latter trend will be studied more extensively as ongoing investigations clarify the effect of spatial variability on the temporal variability observed in Illinois N test measurements.
Temporal Variability
Figure 4 shows the weekly Illinois N test values for 38 weeks from early November 2001 to the end of July 2002, and Figure 5 shows the weekly data averaged by month. The monthly averaged data proved to be much less erratic than the weekly data, so long-term temporal trends are more apparent from Figure 5. During the late fall to early winter of 2001, a substantial increase in Illinois N test values was observed for all three fields, after which soil test values remained relatively constant from January through March and then decreased in early spring. The initial increase may be due to accumulation of microbial biomass during decomposition of crop residues, while the subsequent decrease would be expected if mineral N derived from labile organic forms is consumed by crop uptake and/or lost through leaching or denitrification. Knowledge of the timing and extent of these changes will have an important effect as soil sampling guidelines are developed for the Illinois N test.
Figure 4 gives a much more detailed picture of temporal fluctuations in Illinois N test values. For the two higher testing fields studied, weekly values varied by as much as 40 percent during November and December 2001. Beginning in January 2002, test values were less variable, especially for the two non-manured fields. Some of the fluctuations in Figure 4 appear to be systematic (for example, the dip observed for all three fields during late May and early June). To elucidate the causes of these fluctuations, instrumentation has been installed in each field that permits continuous monitoring of soil moisture and temperature, as well as air temperature and precipitation.
Figure 3. Kriged map of Illinoi N test data shown in Figure 2
Figure 5. Illinois N test data for 0-to-12-inch soil samples collected weekly from unfertilized plots averaged by month. 1 C.W. Boast and T.R. Ellsworth
are professors, T.J. Smith is a visiting research specialist, R.L. Mulvaney
is a professor, S.A. Khan is a research specialist, and E.M. El-Naggar is a
graduate assistant, Department of Natural Resources and Environmental Sciences,
University of Illinois; and R.G. Hoeft is a professor in the Department of Crop
Sciences, University of Illinois
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., and S.A. Khan. 2001. Diffusion methods to determine different forms of nitrogen in soil hydrolysates. Soil Science Society of America Journal, 65: 1284-1292.
Mulvaney, R.L., S.A. Khan, R.G. Hoeft, and H.M. Brown. 2001. A soil organic nitrogen fraction that reduces the need for nitrogen fertilization. Soil Science Society of America Journal, 65:1164-1172.
Mulvaney, R.L., S.A. Khan, R.G. Hoeft, J.J. Warren, and L.C. Gonzini. 2003.
Field and laboratory evaluations of the Illinois N test. In 2003 Illinois Fertilizer
Conference Proceedings (R.G. Hoeft, ed).
