Illinois
Fertilizer Conference Proceedings
January 27-29, 2003
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R.L. Mulvaney, S.A. Kahn, R.G. Hoeft, J.J. Warren, and L.C.
Gonzini1
Estimation of plant-available N is complicated enormously by the dynamic nature of soil N, owing largely to the effects of temperature and moisture supply on N-cycle processes. Numerous biological and chemical methods have been proposed as an index of soil N availability (Bremner, 1965; Keeney, 1982; Stanford, 1982; Bundy and Meisinger, 1994), but none has been adopted widely for soil testing. Biological methods are necessarily time-consuming because of the need for incubation, and the results are of questionable value since net mineralization is being measured in the absence of plant N uptake. Chemical methods of estimating potentially mineralizable soil N have been based on an empirical approach, and their use has been very limited due to low correlations with crop N uptake and/or the production of mineral N during soil incubations.
Soil testing for NO3- is currently considered the best option for identifying sites where N fertilization will be ineffective in producing a yield response by corn (Bundy and Meisinger, 1994). Two soil NO3- tests have been developed that differ in the time and depth of sampling. With the preplant NO3- test (PPNT), profile samples are collected in the early spring to a depth of 2 or 3 feet, to account for carryover of mineral N from previous cropping (e.g., Bundy and Malone, 1988; Roth and Fox, 1990; Schmitt and Randall, 1994). With the presidedress NO3- test (PSNT), soil sampling is done to a depth of 1 foot in late spring, so that soil N mineralization can be taken into account and supplemented, if necessary, by sidedressing (e.g., Magdoff et al., 1984; Blackmer et al., 1989; Meisinger et al., 1992). The PSNT has been recommended more widely than the PPNT in the eastern U.S., but usage has been limited by the need to collect soil samples during the growing season, and by the fact that N fertilization must be postponed until after testing and can be ineffective if adverse weather conditions delay sidedressing. Besides logistical problems, an inherent limitation with the PPNT and PSNT arises from the extensive spatial and temporal variability in soil NO3- concentrations, which depend on numerous N-cycle processes, including mineralization, immobilization, nitrification, denitrification, leaching, and plant uptake. Consequently, a one-time test for soil NO3- is apt to be of little value for predicting crop N availability throughout the growing season, particularly in a humid region where these processes occur extensively.
In Illinois, soil testing is used routinely to guide agricultural applications of limestone, P, and K, whereas N applications for corn production are based on a realistic yield goal, with adjustments to account for other N inputs, such as legumes or manure. A yield-based recommendation may have merit on a long-term basis, but under- or overfertilization is apt to occur in any given growing season, since soil N availability is not taken into account. Insufficient application of N can have serious economic consequences for the farmer, whereas excessive fertilization increases the risk of environmental pollution.
Reports that corn is sometimes nonresponsive to N fertilization (e.g., Bundy and Malone, 1988; Blackmer et al., 1989; Fox et al., 1989; Roth and Fox, 1990; Meisinger et al., 1992; Brown et al., 1993; Schmitt and Randall, 1994) have stimulated recent work to identify a specific fraction of soil organic N that mineralizes readily, as a means to detect sites where N fertilization is unnecessary. After eliminating major defects in the methodology to fractionate the N in soil hydrolysates (Mulvaney and Khan, 2001), studies showed a much higher concentration of amino sugar-N for nonresponsive than for responsive soils, whereas no consistent difference was detected in their concentrations of total hydrolyzable N, hydrolyzable NH4+-N, or amino acid-N (Mulvaney et al., 2001). In subsequent incubation experiments, nonresponsive soils produced a much larger quantity of mineral N than did responsive soils, and mineralization was accompanied by a net decrease in amino sugar-N but not in amino acid-N (Mulvaney et al., 2001).
Based on these findings, a simple soil test has been developed to estimate
amino sugar-N, as a means of detecting sites where corn is unlikely to respond
to N fertilization (Khan et al., 2001). Our objectives here are to report on
recent laboratory developments concerning this new soil test, clarify the measurement
of labile organic N as opposed to soil organic matter, and summarize preliminary
findings from current field studies to check the reliability of the test and
establish protocol for soil sampling.
The Illinois N test requires gentle heating of alkalized samples on a griddle, so as to promote decomposition of amino sugars and other alkali-labile forms of organic N. Experience has shown that data integrity depends on uniform heating, and that not all commercial griddles provide adequate uniformity. Among currently available units, soil test values have been most reproducible when using a model (no. 76220) that was recently introduced by the West Bend Co., West Bend, WI <www.westbend.com>. Although this griddle is large enough to accommodate 15 of the 1-pint jars required, no more than 12 of these jars should be heated concurrently, and the jars should be placed toward the center of the griddle.
To improve data quality, samples should be tested in duplicate, and adjacent jars should be exchanged after heating for approximately 1.5 h, so as to compensate for the somewhat lower temperatures that tend to occur in corner positions. The benefit of the latter manipulation is readily apparent from Figure 1, which shows a decrease of more than 70 percent in the coefficient of variation (CV) for 12 replicate analyses of a single soil sample. Experience in our laboratory has shown that thermal variation can be easily detected by touching the lids after jars have been heated for at least one h, and that corner positions may vary in temperature, depending on ambient air currents.
A new laboratory has been established in the Department of Crop Sciences at
the University of Illinois for the sole purpose of conducting the Illinois N
test in conjunction with field evaluation research.
In some states, fertilizer N recommendations have been made on the basis of soil organic matter or total N content, on the assumption that a fixed proportion (typically 1 or 2 percent) of soil N mineralizes during the growing season. This same assumption has been employed extensively in modeling the soil N cycle to estimate the impact of terrestrial NO3- production on the quality of surface waters (e.g., Goolsby et al., 2001). An inherent limitation arises, however, in that soil organic matter is extremely heterogeneous in chemical composition, with the majority being essentially inert toward microbial decomposition.
The Illinois N test was not designed to measure soil organic matter content, but a labile fraction of organic N that supplies the plant through mineralization. Figure 2 demonstrates that this test is consistent with the yield response of corn to N fertilization, in that higher N test values were obtained for six nonresponsive sites than for six responsive sites. In contrast, these sites were not distinguishable on the basis of organic matter content, which was higher for some of the responsive soils than for others that were nonresponsive. Of particular interest is that organic matter content was higher for the lowest testing responsive soil (Varna) than for five of the six nonresponsive soils. Not surprisingly, the correlation between these two parameters was virtually nonexistent.
If the Illinois N test measures a mineralizable form of soil N, then test values should vary considerably with time. A decrease would be expected during the growing season because of crop N uptake, followed by an increase associated with production of microbial biomass in the absence of plant competition for mineral N.
Figure 3 provides evidence that soil N test values decreased during the 2001 growing season, while Figure 4 compares soil test values for samples collected in late November 2001 and early April 2002 from five sites under continuous corn. Test values were 3.5 to 12.6 percent higher for spring sampling, presumably owing to microbial decomposition of crop residues during a mild winter. To reduce the risk that a responsive soil could be erroneously identified as nonresponsive on the basis of an elevated test value, current indications are that soil sampling for the Illinois N test is best done in the fall after harvest.
Depth of sampling is an important consideration for reliable soil testing, so as to ensure valid calibration of plant response relative to nutrient supply within the soil profile. In the case of the Illinois N test, the quantity of interest is a labile fraction of soil N associated with organic matter, and because of limited leaching, accumulation would be expected near the soil surface following addition of aboveground plant residues or manure. This is confirmed by Figure 5, which shows that this test was much more effective for differentiating responsive from nonresponsive sites on the basis of surface samples (0 to 6 or 0 to 12 inches), as compared to profile sampling (0 to 24 inches). Figure 5 provides further evidence that the new N test would be compatible with routine soil testing for pH, P, and K, because nonresponsive sites were detected equally well by testing 6- instead of 12-inch samples, provided the critical test level was increased by 30 ppm. As indicated by a lack of correlation between soil N test values and crop N response, a serious loss of resolution occurred when sampling was done to 24 inches, which can be attributed to profile dilution of the labile N fraction.
The value of the Illinois N test for detecting sites where corn does not respond to N fertilization is evident from Figure 6, in which soil test values are plotted versus percentage yield response for 55 research sites throughout Illinois representing a range of soil types and management practices in 1990 to 1992, 2001, or 2002. Assuming a critical value of 225 to 235 ppm, the test was 96 percent accurate in identifying these sites as being either responsive or nonresponsive to N fertilization. Two failures occurred in 2001, owing to insect infestation or localized drought.
The Illinois soil N test estimates potentially mineralizable N to detect sites
where corn does not respond to N fertilization. This test does not measure total
organic matter, but a fraction of soil organic N that decomposes when heated
under strongly alkaline conditions. The depth and time of soil sampling affect
the test values obtained. Although this test was originally developed using
soil samples collected to a depth of 1 foot, subsequent studies have shown 6-inch
samples to be equally satisfactory for detecting nonresponsive sites, provided
the critical test level is increased. Somewhat higher test values have been
obtained for spring than for fall sampling, which is consistent with crop removal
during the growing season. The sensitivity of this test is apparent from the
range in test values obtained for sites under different cropping and management
practices.
Appreciation is expressed to Cecilia Azpiroz Gutierrez for assistance in carrying
out soil N tests for some of the work reported.
Figure 4. Comparison of Illinois N test values for fall versus spring sampling (0 to 12 inches)
1 R.L. Mulvaney is a professor
and S.A. Khan is a research specialist, Department of Natural Resources and
Environmental Sciences, University of Illinois. R.G. Hoeft is a professor and
J.J. Warren and L.C. Gonzini are senior research specialists, Department of
Crop Sciences, University of Illinois.
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