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Richard Mulvaney Richard Mulvaney
Professor
Department of Natural Resourses and Environmental Sciences
(217) 333-9467;mulvaney@illinois.edu
Saeed Khan Saeed Khan
Research Specialist in Agriculture
Department of Natural Resourses and Environmental Sciences
(217) 244-7592; s-ahmad1@illinois.edu

Can Nitrogen Fertilizer Recommendations be Improved With a New Soil Test?

Since 1975, N fertilizer recommendations for corn production in Illinois have been made according to the so-called proven-yield (PY) method, whereby an expected yield goal is multiplied by 1.2 and the product is then corrected by applying estimated credits for previous cropping or the recent use of manure. This method, which was developed by averaging N-response data collected over multiple years and locations (after excluding trials in which N fertilization was ineffective for increasing grain yield), is actually based on the assumption of steady-state conditions, such that no net change occurs in soil N availability during the growing season. The validity of this assumption is clearly questionable, considering that check-plot yields are never zero in N-response experiments, or even in long-term trials on sites such as the Morrow Plots, which have been cropped continuously since 1876 and yet yielded 68 bu/acre in 2003 where fertilizer has never been applied. Clearly, soil N is utilized for plant growth.

When soils are fertilized with N, a substantial proportion (typically between 10 and 30%) is incorporated during synthesis of microbial biomass. As the microbes die and decay, some of the biomass N is released as NH4+ through the process of mineralization. Manure and other organic amendments have historically been recognized as a means to build up soil organic N; however, contrary to current recommendations by the PY method, there is growing evidence to suggest that inorganic N fertilization can also contribute to this buildup, and thereby promotes soil N availability in subsequent growing seasons. Support for the latter possibility has been provided by a recent study of 298 N-response experiments in five midwestern states (Illinois, Minnesota, Missouri, Pennsylvania, and Wisconsin). Even after excluding all cases where no significant yield response was observed to N fertilization, the recommended N rate exceeded the experimentally determined optimum rate for 96% of the remaining 193 studies (by up to 203 lb/acre, 80 lb/acre on average). The same trend is evident from Table 1, which summarizes results from 105 N-response studies conducted during six growing seasons throughout Illinois. Table 1 raises serious questions about the validity of present N management practices in a state where farmers annually apply approximately 1 million tons of N, at a cost of 400 to 500 million dollars.

The only hope of improving N fertilizer recommendations for corn production in a humid region such as Illinois is to account for a soil's capacity to supply plant-available N through mineralization. The Illinois soil N test (ISNT) was developed for precisely this purpose, so as to detect sites where corn will not respond to N fertilization. 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. Failures of the ISNT have usually occurred because of spatial variability among field plots and/or a marked decline in test values with depth, which emphasizes the importance of adequate soil sampling.

 
Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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