Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Alfred M. Blackmer


Evaluation of plant nutrient status is an essential step in developing or improving recommendations concerning the amounts of fertilizers needed for crop production. Although many methods have been used to evaluate the nitrogen (N) status of corn (Zea mays L.), information regarding the sensitivity of these methods is not adequate to address economic and environmental concerns about existing or proposed fertilization practices. This dissertation describes rigorous evaluations of various methods commonly used to assess the N status of corn. The studies included field experiments having 10 rates of N at 18 site-years in Iowa;A comparison of models commonly used for describing corn yield response to N fertilizer showed marked discrepancies among models when used to identify economic optimum rates of fertilization after yield response data are collected. Mean optimum rates of fertilization as indicated by various models ranged from 128 to 379 kg ha[superscript]-1. Statistical analyses indicated that the most commonly used model, the quadratic model, tended to identify optimal rates of fertilization that were too high;Evaluations of the relationships between yields and concentrations of N in corn tissues (ear leaf at silking and grain at harvest) showed that these commonly used indicators of N status of corn lack the sensitivity needed by modern corn producers. Although statistically significant relationships between yields and tissue composition were observed, near- and above-optimal supplies of available N resulted in similar concentrations of N. Use of these tissue analyses probably promotes application of more fertilizer N than required to attain maximum yields;Studies indicated that use of nitrapyrin, a commonly used nitrification inhibitor, did not increase yields and did not increase amounts of fertilizer N recovered in soil or plant. Unlike most previous studies to evaluate the efficacy of this fertilizer additive, lack of favorable response to nitrapyrin cannot be attributed to lack of experimental sensitivity;Overall, the results indicate that recognizing the limitations of methods now being used to evaluate the N status of corn probably is a vital step in improving N fertilization practices in the Corn Belt.



Digital Repository @ Iowa State University,

Copyright Owner

Manuel Enrique Cerrato



Proquest ID


File Format


File Size

137 pages