Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Alfred M. Blackmer


It is widely recognized that corn grown in rotation with soybean often yields more at optimum rates of N fertilization and often requires less fertilizer N to attain these yields than does corn grown in monoculture. Information is needed regarding these benefits of rotation and regarding refinements in recommendations for the late-spring soil NO[superscript]-[subscript]3 test to assist growers in determining optimal rates of N fertilization;Seventeen N response trials were conducted during 1990 to 1992 to determine (i) yields and economic optimum N rates for corn after corn and corn after soybean and (ii) the influence of early season rainfall on critical concentrations of late-spring soil NO[superscript]-[subscript]3. Data previously collected (1987 to 1989) was included to provide a six-year analysis for much of the work;Mean plateau yields for corn after soybean averaged 12% higher than the corresponding means for corn after corn. The yield advantage for corn after soybean was greater than this in the drought affected 1988 season and less than this in the 1989 season, possibly due to reduced moisture availability for the corn after soybean. Nitrogen requirements for each rotation and the difference in N requirements between rotations varied considerably among years and locations. Estimates of optimal amounts of N fertilizer were shown to be profoundly influenced by the model used to describe the relationships between yields and N rates applied. Assumed corn and N fertilizer prices were also shown to influence optimal N rates. Use of an "ideal" N recommendation system example where each site received the rate determined to be optimum at that site illustrated the potential for reduced N rates and greater net returns to fertilization for a system based on site-specific information compared to other recommendation systems;Although critical concentrations of late-spring soil NO[superscript]-[subscript]3 for corn after soybean were sometimes slightly lower than for corn after corn, the difference did not appear large enough to be of practical importance. Early season rainfall did have an effect on critical concentrations of late-spring soil NO[superscript]-[subscript]3, but a simple adjustment in soil NO[superscript]-[subscript]3 concentrations for rainfall did not appear to be justified.



Digital Repository @ Iowa State University,

Copyright Owner

Brian George Meese



Proquest ID


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File Size

86 pages