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Degree Name

Doctor of Philosophy




Critical nutrient percentage is defined as the nutrient concentration associated with maximum yield with respect to the nutrient. Objectives of this research were: (1) to compare three methods for calculating maximum yields and associated critical % N and to determine effects of time of leaf sampling at or near silking on critical % N; (2) to determine effects of P fertilizer rates on critical % N and of N rates on critical % P; and (3) to determine effects of crop sequence, plant density, and moisture stress on critical % N and % P in both grain and leaf. Data were from 39 fertilizer experiments at 4 outlying research centers in Iowa;The direct regression method (yield on % N in the plant part) was generally poorer for estimating critical % N in the grain and leaf than the two-step method (yield and % N on N rates) or graphical method (highest yield based on significance and associated % N). The latter two were similar. Time of leaf sampling and weather factors prior to and during silking affected % N of treatments and critical % N;The P rates increased critical % N in grain as much as 0.12% and that in the leaf as much as 0.16%. As N rates increased, critical % P decreased as much as 0.028% in the grain and increased as much as 0.058% in the leaf. The critical % N and % P in the grain or leaf can evaluate N and P fertility status of corn better if levels of both are considered;The critical % N and % P in both grain and leaf were slightly less in C2 than in C1 in a C1-C2-soybean rotation. Increased stand level decreased critical % N slightly;Moderate-severe moisture stress increased critical % N 0.13% in the grain and decreased it 0.20% in the leaf in an N rate experiment (7 years). In an NP experiment (5 years), similar stress increased grain critical % N 0.14 to 0.23%, decreased leaf critical % N 0.07 to 0.17%, decreased grain critical % P as much as 0.017% and decreased leaf critical % P by 0.019%;In regressions of critical % N from 26 site-years, moisture stress prior to silking increased critical % N in the grain as much as 0.34%. In regressions from 21 site-years, stress after silking decreased leaf critical % N as much as 0.43%. In summary, several factors significantly affected critical N and P percentages in corn grain and leaf.



Digital Repository @ Iowa State University,

Copyright Owner

Luiz Eugenio Coelho de Miranda



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328 pages