The New Illinois N Soil Test

Robert Hoeft

Robert Hoeft
Professor of Soil Fertility Extension

Phone: (217) 333-4424


For decades, scientists have been trying to identify a nitrogen soil test that accurately predicts N need. Much of this work has focused on nitrate testing, a procedure that has not worked consistently in the more humid areas of the Corn Belt, particularly in fields that were nonresponsive to applied nitrogen. On-farm research in Illinois demonstrated that 17 percent of the cornfields needed no nitrogen for optimum yield, even though the nitrate level in those fields was below the level that would indicate no response. Since corn requires a substantial amount of inorganic N (usually nitrate), it led us to theorize that there must be a fraction of organic N that was easily mineralized to nitrate during the growing season.

Dr. Richard Mulvaney and Dr. Saeed Khan launched a major project to identify this organic fraction and found that amino sugar-N was highly correlated with lack of response. Whenever amino sugar-N concentration exceeded 240 ppm N, there was no response to applied N. As the amino sugar-N concentration decreased, the response to applied N increased. Amino sugar-N was further confirmed as the readily mineralized organic compound by laboratory incubation of soil samples that showed that the decrease in amino sugar-N was strongly correlated with the increase in nitrate concentration.

Unfortunately, the amino sugar-N test is complicated and time consuming, characteristics that will discourage its adoption by commercial soil testing laboratories. To overcome this, Dr. Mulvaney and Dr. Khan developed the "Illinois N Soil Test," a procedure that is highly correlated to amino sugar-N, but which is simple and uses commonly available chemicals. As with the amino sugar-N test, there was no response to applied N when Illinois N Soil Test values exceeded 240 ppm. When the values fell below 240 ppm, the response to applied N increased.

While this test looks promising, much research needs to be done before it is ready to be used to make recommendations. This work will include calibration of the test; determination of the proper soil sampling technique (ideal soil sampling time, depth, and handling); and number of samples per unit land area.

Soil Test Calibration

As shown, the test reliably predicts no response when values are greater than 240 ppm-N, but there is not adequate data to predict the rate that is needed if they are less than 240 ppm-N. This will require experiments at several locations and across many environments. Such work is underway, and enough data should be collected by the end of the 2002 growing season to give a first cut at calibrating the test. The data in provides strong evidence that the test can be used to adjust rates.

Sampling Time

The initial correlation work was done with samples collected in spring (April). Since this is an organic compound that will change over time due to mineralization, there is a question about when to collect the samples. Preliminary results collected by Dr. Michelle Wander from the Morrow Plots at the University of Illinois indicate that the Illinois N Soil Test values were similar in both fall- and early spring-collected samples. During the summer, the values decreased, as would be expected for a compound that is being converted from organic to nitrate by a microbial system. Sometime during the summer, it appears that the immobilization reaction occurs at a faster rate than the mineralization reaction, and, as such, there is an accumulation of amino sugar-N. Assuming that these results are representative of other fields, it would appear that there is a wide window of opportunity (fall through early spring) for collecting soil samples.

Sampling Depth

The results presented in were based on samples collected to a depth of 12 inches. However, preliminary work implies that a 6-inch sample may provide equally acceptable results, since the 0-12 inch samples and the 0-6 inch samples were both greater for nonresponsive than responsive sites. As expected, the 0-6 inch samples contained a higher Illinois N Soil Test concentration than did the 0-12 inch samples. This will mean that the concentration required to trigger a rate adjustment or recommendation for no N application will be higher for 0-6 inch samples than for 0-12 inch samples. If there is little to no gain in the correlation between yield response and the Illinois N Soil Test concentration at the 0-12 as compared to the 0-6 inch sample, it will be advantageous to use the 0-6 inch (possibly 0-7) sample, the same one as taken for pH, P, and K.

Sample Handling

Under warm, moist conditions, amino sugar-N compounds will likely change relatively rapidly. With this in mind, samples will need to be dried rapidly or kept frozen until the laboratory can process them. The initial calibration was done with air-dried samples. However, most commercial laboratories will not want to take the time or space to air dry samples, which will likely be oven dried instead. Initial data indicates that oven drying will give basically the same result as air drying and that use of field moist samples will give markedly lower values and markedly more variable results.

Sample Numbers

At the time of this writing, only one field has been sampled to determine field variability. Based on that one field, it appears that sample intensity similar to that used for pH, P, and K will be adequate to determine field variability. If the field has previously had variable management in terms of manure, cropping, or N management, it is possible that the test can be used as a guide for variable-rate application.


While the data collected thus far are very encouraging, there is still a lot of work to be done before the procedure is ready for routine use. Even when the questions given above are answered, one must remember that there will be years in which the test will fail. In particular, soils testing high in amino sugar-N may still need an application of N in years in which soils are very dry or very cold in the spring. The one advantage to the Illinois N Soil Test is that abnormal spring weather will be known soon enough in the growing season to allow producers the opportunity to sidedress supplemental N before the plants are too big.

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