Degree Type


Date of Award


Degree Name

Doctor of Philosophy




A crop rotation and nitrogen (N) fertilization experiment was begun in 1957 at the Northwest Iowa (Galva-Primghar) Research Center near Sutherland, Iowa. The experiment was designed to compare crop rotations and to determine the response of corn to nitrogen fertilization in the various crop rotations. Sixteen years of corn yield data from the experiment are summarized and analyzed in this study;The experiment was a split-plot with rotations as whole-plot treatments and N fertilization as subplot treatments. The crop rotations were: corn-oats-meadow-meadow (COMM), corn-corn-oats-meadow (CCOM), sorghum-corn-oats-meadow (SgCOM), corn-soybeans (CSb), continuous corn harvested for grain only (C), and continuous corn with the stover and grain removed (C(,s)). In rotations including meadow, corn was fertilized with 0, 20, 40, and 80 lb N/acre. In other rotations, the rates were 0, 40, 80, and 120 lb N/acre. Treatments of 0, 50, 100, and 150 lb potassium (K)/acre were applied to C(,s) plots during the last 5 years of the experiment;Because of the split-plot design and because the experiment was conducted over time, yields for different treatment combinations were correlated. The corn yield data were transformed to produce nearly uncorrelated errors with homogeneous variance before estimating functions for the response of corn yields to N and K fertilizer;The C(,s) grain yields for 1974-1978 were fitted to a grafted polynomial function of K. The response to K was much greater at higher levels of N fertilization;Functions for the response of corn yield to N fertilizer in the various rotations were proposed and estimated. The preferred function for the response of corn to N fertilizer was an exponential (Mitscherlich) equation which included terms for a time trend in the yield of continuous corn. The analysis showed that the maximum yield obtained under continuous corn was less than the maximum yield obtained in rotations containing legumes.



Digital Repository @ Iowa State University,

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Ronald Paul Mowers



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139 pages