About the Illinois Soil Nitrogen Test

How did the Illinois Soil Nitrogen Test come about?

Phase 1 (1990-1993)

During the 1980s, there was growing concern that excessive N fertilization was contributing to nitrate pollution, along with increasing reports that corn was sometimes nonresponsive to N fertilization. This led to a project funded at the outset of the FREC program, in which 75 N-response experiments were conducted throughout Illinois during 1990-1992, for the purpose of evaluating three N recommendation systems for corn yield and residual soil nitrate. Surprisingly, a significant N response was observed in only 42 of these experiments, while the proven-yield (PY) method proved ineffective for detecting many of the 33 nonresponsive sites, as was also the case when soil testing was done for nitrate, either before or after planting (http://frec.cropsci.uiuc.edu/1993/report6/index.htm). The latter finding can likely be attributed to the dynamic nature of soil nitrate, which reflects the influence of weather conditions on numerous N-cycle processes.

Phase 2 (1995-1998)

The widespread occurrence of nonresponsive sites noted in Phase 1 motivated a further FREC project designed to ascertain whether long-term N fertilization contributes to a buildup of labile organic N that subsequently supplies plant-available N through mineralization, and thereby reduces crop responsiveness to N fertilization. This was indeed confirmed using 15N-labeled fertilizer, through studies that also demonstrated more extensive uptake of soil than fertilizer N, which was observed even with the highest N rate studied (240 lb N/A) (http://frec.cropsci.uiuc.edu/1997/report8/index.htm).

Simultaneously, FREC invested in a 2-year project to improve the methodology for N and 15N analysis. An array of simple Mason-jar diffusion methods were thereby developed for inorganic and total N analysis of soil, water, and plant material (See Technical Notes 97-01b, 97-04b, 97-05b, 99-01b, and 00-01b), which were employed successfully in several projects, including the FREC study mentioned in the preceding paragraph.

Phase 3 (1998-2000)

In an attempt to differentiate responsive and nonresponsive soils collected in Phase 1, classical steam-distillation methods were employed to determine the chemical distribution of acid-hydrolyzable organic N. No consistent differences were detected, even in comparing soils with and without a history of heavy manuring. Subsequent studies revealed serious defects in methodology, so new methods were developed to fractionate hydrolyzable soil N, by adapting the Mason-jar diffusion methods mentioned previously in connection with Phase 2 (See Technical Note 00-02b and Mulvaney and Khan, 2001, Soil Sci. Soc. Am. J. 65:1284-1292). When the newly developed methods were applied in a comparison of responsive and nonresponsive soils, the latter group was found to have a higher concentration of amino sugar-N (http://frec.cropsci.uiuc.edu/2000/report3/index.htm, and Mulvaney et al., 2001 Soil Sci. Soc. Am. J. 65:1164-1172).

Phase 4 (2001-present)

The Illinois soil N test (ISNT) was developed to provide a more rapid and convenient means of estimating amino sugar-N, without the need for acid hydrolysis (http://frec.cropsci.uiuc.edu/2001/report7/index.htm, and Khan et al. 2001, Soil Sci. Soc. Am. J. 65:1751-1760). In 105 small-plot N-response evaluations to date (see Table 1), the ISNT was 90% effective in detecting nonresponsive sites, whereas approximately 12% of these sites were predicted by the PY method, which often led to under- or overfertilization when evaluated on a site-by-site basis (http://frec.cropsci.uiuc.edu/2004/report5/index.htm). Fertilizer responses not detected by the ISNT have in many cases been more apparent than real, because the N requirement per bushel increase of yield was often too high to be economical. In other cases, several soil and plant parameters were identified that account for the high N requirements thereby observed, among which were moisture stress, soil acidity and a limitation in the availability of P or K. The importance of adequate soil sampling has been clearly indicated when a similar pattern of spatial variability is observed for yield data and ISNT values. There are obvious implications for site-specific N management.