Illinois Fertilizer
Conference Proceedings
January 24-26, 1994
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R.G. Hoeft, H.M. Brown, M.J. Mainz, G.A. Raines, J.W. Warren, and L. Gonzini1
Results obtained in two recent FREC funded projects have prompted concern about the ultimate fate of residual N. In one project, there appeared to be some accumulation of inorganic N at depths of 3 feet or deeper in medium to heavy textured soils in central Illinois. In the other project, several sites were identified where grain yield of 150 bushel per acre or greater were obtained without any supplemental fertilizer N. In the latter case, currently available soil testing procedures did not identify these sites as being non-responsive.
In the spring of 1991, soil samples were collected in one foot increments to a depth of 5 feet on plots that had long term history of variable N rates under different cropping sequences at the University of Illinois Off and Northwestern Illinois Research Centers. At the Northwestern Illinois location, there was an accumulation of NO3-N at the 3 and 4 foot depths under both continuous corn and corn following soybeans at the 180 and 240 lb N/acre rates of application (Mainz and Raines, 1993). When soybeans followed corn, the accumulation of NO3-N was less than for continuous corn at the two highest N rates applied to corn the prior year. At the Off Center, there was an accumulation of NO3-N at the 3 and 4 foot increments under continuous corn. However, following soybeans there was little indication of any NO3-N accumulation at any depth. Precipitation in the year preceding the collection of the samples was above normal for both locations. However, the two years preceding that were abnormally low. Since these results may have been atypically influenced by the low precipitation patterns, the objective of this follow-up project was to evaluate the influence of crop rotation and N rate on the inorganic N status of soils with depth following a period of higher precipitation.
The recently competed project, "Evaluation of soil profile NO3-N
for prediction of N fertilizer requirements under Illinois conditions, identified
29 sites at which corn did not respond to applied N (Brown et al, 1993). Of
these, 18 could be explained by the application of manure during the cropping
year, forage legume as the previous crop, or drought. However, approximately
12 percent of all sites could not be explained by any of the above factors and
the currently available soil test procedures did not identify any of these sites
as non-responders. Several of the unexplained, non-responding sites had high
P and K soil tests indicating a high probability of manure application or high
N rates in the past. If that were the case, it is probable that there may have
been an accumulation of organic N that is now being mineralized at a rate adequate
to meet crop N needs. The objective of this phase of the project was to determine
the reliability of various organic N extractants for predicting N availability
on sites for which current soil test procedures fail to predict lack of response
to applied N.
Soil samples were collected in one-foot increments to a five foot depth on N rate plots following continuous corn, corn in a corn soybean rotation, soybeans in a corn soybean rotation, and corn in a corn-soybean-wheat rotation at the Orr Research and Extension Center and the Northwestern Illinois Research and Extension Center prior to N application in the spring of 1993. The N rate by crop rotation studies were established in 1980 and 1982 at the Orr and Northwestern Illinois sites, respectively. At the Orr location, 5 equally spaced N rates from 0 to 3201b N/acre were applied each year that corn was grown from 1980 to 1990. Commencing in 1990, N rates were reduced to levels of 0, 60, 120, 180, and 240 lb N/acre. When wheat was included in the rotation, rates ranged from 0 to 120 lb N/acre. Crop rotations at the Orr site included continuous corn and corn-soybean-wheat. At the Northwestern Illinois site, the 5 equally spaced rates ranged from 0 to 240 lb N/acre when corn was in the rotation. The rotations included continuous corn and corn-soybean. Soil types were an Ipava sil at the Northwestern Illinois site and Herrick at the Orr site.
Bulk soil samples were collected from 7 of the unexplained-nonresponding and
5 responding sites identified in the earlier study conducted by Brown et al.
The samples were dried, ground and analyzed by two procedures that were selected
to determine readily available organic N on the 12 soil samples collected from
responding and non-responding sites. The "heated KCl" procedure involved
digesting soil samples with 2M KCl at 212 °F for four hours (Gianello and
Bremner, 1986). The KCl/soil mixture was then steam distilled for determination
of NH4-N. The amount of NH4-N initially present in the sample was determined
by following the same procedure, but without heating. The difference between
the two procedures was assumed to be the NH4-N liberated from an organic source.
Oxidative release of readily available organic N was the other procedure used.
This procedure was based on a correlation between easily oxidizable organic
matter and the mineralization of organic N. Soils were extracted with 0.1N KMnO4/1N
H2SO4 (Stanford and Smith, 1978). An aliquot of the extractant was steam distilled
for determination of NH4-N. The difference between samples extracted with the
0.1N KMnO4/1N H2SO4 mixture and those extracted with with 1N H2SO4 alone was
considered to represent the amount of NH4-N released from oxidation of organic
matter.
At the Northwestern Illinois site, N rate, sample depth, crop rotation by N rate, and N rate by sample depth all significantly affected NO3-N levels (Table 1-2). When averaged over all sample depths, N concentrations increased with increasing N rate following corn in both continuous corn and corn-soybean rotations. Interestingly, there was little difference associated with increasing N rate following soybeans (Table 3). Nitrate concentrations were higher at all N rates except for the 240 lb N/acre rate following soybeans as compared to following corn; similar to the results observed in 1990. While the differences were statistically significant, all concentrations were low and the differences were of little practical significance.
When averaged over all crop rotations, the highest NO3-N concentrations were found in the surface foot of soil, except for the 2401b N/acre rate which had the highest concentration in the lower 2 feet of the profile (Table 4). These results are similar to those observed in the 1991 sampling, but the concentrations were substantially lower in 1993 than those observed in 1991. The difference in results was likely due to the fact that the 1993 samples were collected after a two year period of near to above normal rainfall and crop yields as contrast to the 1991 results that were collected after a 2 year period of below normal rainfall followed by a year of near normal rainfall. The crop yields obtained during the below normal rainfall period were significantly below normal, whereas the yields obtained in 1991-92 while being somewhat below normal were well above those obtained in the 1988-89 time period (Table 5).
At the Orr site, N rate, sample depth, and N rate by crop rotation had a significant effect on NO3-N concentration (Table 6-7). When averaged across crop rotations and sample depths, NO3-N levels increased from 1.4 ppm at the 0 and 60 lb N/acre rate to 2.4 ppm at the 240 lb N/acre rate. While the differences were statistically significant, they have little practical significance from either a crop production or environmental contamination standpoint. As at the Northwestern site, the highest concentrations were observed in the first foot of soil. The highest N rate tended to have the highest NO3-N concentration following continuous corn as compared to the other two rotations (Table 8). In contrast, at the low rates of N. application, samples collected following wheat had the highest concentrations of any of the rotations. These differential responses to rate and rotation resulted in the statistically significant rotation by N rate interaction indicated in Table 7. The NO3-N levels detected in 1993 were lower than those detected in 1991. As at the Northwest site, these lower concentrations were likely the result of the improved yield obtained in the 1991-92 time period (Table 9).
Both of the procedures designed to extract a portion of the organic N resulted
in substantially higher N concentrations that were observed with the KCl extraction
(Standard Extractable N) for both the responding and non-responding sites. Unfortunately,
neither of these procedures provided a clear delineation between responding
and non-responding sites. It does appear as if the stronger extractot (Oxid/Acid)
might be of some value for predicting non-responsive sites as samples 4, 5,
and 12 all had concentrations of 45 ppm or greater. Two of these three sites
had very low NO3-N concentrations when KCl was used as the extractant.
Results obtained with the 1993 sampling indicate little if any residual accumulation of NO3-N at any N rate or under any crop rotation at either the Northwest Illinois or Orr Research and Extension Center. The contrast between the data obtained in 1993 and 1991 is most likely due to the fact that the 1991 samples were collected following a period of 2 to 3 years of below normal grain yield whereas, the 1993 samples were collected after a period of more normal precipitation and yield.
Three additional extractants are presently under evaluation for the twelve responding, non-responding samples to determine if one of them will serve as a better predictor of responsiveness to N fertilization. If a procedure shows promise, it will be used on samples from all of the original field experiments.
Table 3: Effect of crop rotation by N rate on NO3-N accumulation
Table 9: Effect of N rate and crop rotation on corn yield. Orr Research and Extension Center
1 R.G. Hoeft is Professor, H.M. Brown is Research Assistant, M.J. Mainz and G.A. Raines are Agronomists and Superintendents of the Northwest and Orr Agricultural Research Centers, respectively, and J.W. Warren and L. Gonzini are Research Specialists with the Department of Agronomy, University of Illinois.
Brown, H.M, R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation
systems for corn yield and residual soil nitrate. in Ill. Fert. Conf. Proc.
(R.G. Hoeft, ed.) p. 43-49.
Gianello, C. and J.M. Bremner. 1986. A simple chemical method of assessing potentially
available organic nitrogen in soil. Commun. in soil Sci. Plant Anal. 17(2):
195-214.
Mainz, M.J. and G.A. Raines. 1993. Residual soil nitrogen levels in 2 nitrogen
rate studies. in III. Fert. Conf. Proc. (R.G. Hoeft ed.). p 7-12.
Stanford, G. and S.J. Smith. 1978. Oxidative release of potentially mineralizable
soil nitrogen by acid permanganate extraction. Soil Sci. 126 (4): 210-218.
