Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

R. D. Voss

Second Advisor

J. R. Webb


The objectives of these studies were to (1) determine the maximum yield of corn (Zea mays L.) under prevailing climatic conditions in eastern and central Iowa and (2) determine what combination of controlled factors of N, P and K fertilization, plant density, row spacing, hybrid and timing of N application would give maximum yields. Eight experiments were conducted in eastern and central Iowa in 1982 through 1984;Weather had a moderating effect in all but one experiment by either delaying planting providing severe moisture stress. Mean experiment grain yields ranged from 6.49 Mg ha[superscript] -1 in 1983 to 11.2 Mg ha[superscript] -1 in 1984. Combinations of the controlled variables that gave maximum yield varied with site year. There was no yield response to N fertilizer above the lowest rates which ranged from 67 to 168 Kg N ha [superscript] -1. Soil P levels were sufficient to produce maximum yields even when no fertilizer P was applied. Grain yields responded to the top K fertilizer rate of 279 Kg K ha[superscript] -1 in 1982 and the intermediate rate of 186 Kg K ha[superscript] -1 in 1984. However, in 1983, added P and K fertilizer resulted in a negative yield response;In all experiments, the lowest plant densities, which ranged from 49,420 to 64,246 plants ha[superscript] -1, gave maximum yields. The only deviation from this pattern was a 1984 interactive effect in which the 88,888 plants ha[superscript] -1 plant density gave maximum yields when K was adequate. The row spacing variable resulted in a yield difference only in 1982 when 50-cm rows had an advantage over 75-cm rows. Hybrid had a significant effect on yield in all but one experiment;Although varying the levels of the controlled variables in these experiments did not increase yields greatly, the following conclusions were reached: (1) Weather and hybrid selection were critical factors in determining maximum yield. (2) The plant density that produced the maximum yield was affected by hybrid selection, mineral nutrition and moisture and temperature stresses. (3) Nitrogen fertilizer rates of 67 to 168 Kg ha[superscript] -1 and soil P levels of 29 to 32 mg Kg[superscript] -1 were sufficient to produce maximum yields in these experiments. However, these levels may not be adequate in periods of more favorable weather. (4) Added fertilizer K had positive effects on grain yield and on plant lodging when plant densities and N fertilizer rates were increased.



Digital Repository @ Iowa State University,

Copyright Owner

Thomas A. Polito



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165 pages