Relationships among Anaerobically Mineralized Nitrogen, Chemical Indexes, and Nitrogen Availability to Corn

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Abstract
An accurate method for predicting the N supplying capability of a soil must be available in order to increase N fertilizer efficiency in humid areas with a mixed agriculture that includes N inputs from livestock manure and forage legumes. Previous work had shown that a number of chemical N availability tests were not well correlated (r < 0.4) with the soil N available to field grown corn (Zea mays L.) in 67 experiments throughout Pennsylvania over a 6‐y period. Soils from these same sites were anaerobically incubated for 7 d to determine if the N mineralized during this period would be better correlated with field‐measured N availability than the chemical indexes. The anaerobically mineralized N was no better correlated with field N availability than were the chemical indexes. Anaerobically mineralized N was most closely correlated with total soil N content (r = 0.79) and boiling 0.01M CaCl2 extractable N (r = 0.74). By restricting the experimental sites to well‐drained soils where a legume (primarily alfalfa) had not been growing in either of the 2 years preceding the experiment, the correlation coefficients between two of the indexes, the UV absorbance of a boiling 0.01M CaCl2 extract and NO3 content of the surface 40 cm of soil at planting, and the field estimate of N availability were increased to 0.65 and 0.59, respectively. A stepwise multiple regression analysis using field N availability selected these two estimates with a resulting coefficient of determination (R2) of 0.50. These results are encouraging but are not high enough to use in a routine soil testing laboratory. Restricting the soils as described did not improve the correlation between anaerobically mineralized N and field‐measured N availability. Nitrogen fertilizer recommendations for corn based on current Pennsylvania guidelines that consider crop yield goals and management history were on the average 41 kg/ha (SD = 53 kg/ha) higher than would have been needed to achieve economic optimum yields in these experiments. This is much less precise than desired, but recommendations in effect prior to changes induced by results from these N fertilizer response experiments would have been 94 kg/ha (SD = 53 kg/ha) more than needed for economically optimum yields.
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