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A New Nitrogen Soil Test That Works

Richard Mulvaney Richard Mulvaney
Professor of Soil Fertility
Department of Natural Resources and Environmental Sciences
(217) 333-9467
  Saeed Khan Saeed Khan
Research Specialist in Agriculture
Department of Natural Resources and Environmental Sciences
(217) 244-7592

In Illinois, nitrogen (N) fertilizer recommendations for corn production are estimated on the basis of a realistic yield goal, which is multiplied by 1.2 to calculate the pounds of N required per acre, less any adjustment for N derived from other sources, such as manure or legumes. 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.

Relationship between soil test-N and N-fertilizer response
Figure 1. Relationship between soil test-N and N-fertilizer response
for surface (0-30 cm) soils from 25 N-response sites.

Public concern that excessive N fertilization may contribute to nitrate enrichment of ground and surface water has stimulated interest in soil testing to improve the accuracy of N fertilizer recommendations for corn. For many years, a preplant nitrate test (PPNT) has been used in western Canada and the Great Plains region of the U.S. to account for carryover of mineral N from previous cropping. A more recent option is the presidedress nitrate test (PSNT), in which case soil sampling is postponed until corn is 6 to 12 inches tall, so as to estimate plantavailable nitrate as closely as possible to peak uptake by the crop. If the test indicates less than 20 to 30 ppm of nitrate-N in the surface 12 inches, supplemental N is applied as a sidedressing.

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, when farmers are occupied with many other tasks. Fertilization must be postponed until after testing, which can lead to crop N deficiency if adverse weather conditions prevent sidedressing. Neither test is currently recommended in Illinois.

Besides logistical problems, an inherent limitation with soil testing for nitrate arises from the dynamic nature of N-cycle processes, most of which affect soil nitrate concentrations. Ideally, a soil test for N would estimate a labile organic fraction that supplies the plant through mineralization. This approach would have an advantage in that soil test levels would depend on fewer N-cycle processes. Thus, these levels would be less prone to temporal and spatial variability, so that N availability potentially could be predicted on the basis of a one-time test, regardless of soil type or management. Further, the time of sampling would be much less critical than with nitrate, and samples could be stored for later analysis.

The search for a soil N test that can estimate mineralization has been underway for more than a century, but with little progress. Recent work at the University of Illinois has provided new reason for hope by identifying a particular form of soil organic N that mineralizes readily and thereby reduces the yield response by corn to N fertilization. Amino sugars are the key.

A simple soil test has been developed to estimate amino sugar-N as a means of detecting sites that do not need N fertilization. In this test, the soil sample is treated with sodium hydroxide solution in a Mason jar, then heated for five hours on a griddle at 48–50º C to convert amino sugar-N to gaseous ammonia. The ammonia is collected in boric acid-indicator solution and determined by titration. By design, nitrate-N is not recovered, so as to reduce soil test variability and eliminate the need for profile sampling and special care in sample processing. On the basis of soil test-N, 25 Illinois soils (0–12 inch samples) were classified correctly as being responsive (< 225 mg kg-1) or nonresponsive (> 235 mg kg-1) to N fertilization for corn production.

The new soil N test can help improve N-fertilizer efficiency, increase profitability, and reduce the adverse environmental effects of excessive N fertilization.

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