Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Antonio P. Mallarino


Crop production removes P and other essential nutrients from the soil that need to be replaced by fertilizer application in order to maintain higher yields. The overall objective of this research was to assess the impact of P fertilization methods on crop and soil-test P responses to P. Two distinct studies were conducted to achieve this general objective.;A starter fertilization study involved strip trials on seven cornfields. Treatments applied were no-starter and liquid starter with or without spring tillage. Results from this study showed that grain yield, early plant growth, and early N and P uptake often were greater with tillage than with no-tillage. Yield responses to starter fertilization occurred mainly in areas with soil-test P optimum for crop or lower (<21 mg kg-1, Bray-P 1). Responses were less frequent and smaller than for tillage, and did not substitute for tillage effects on yield. Early growth and nutrient uptake responses to starter were large and occurred independently of soil-test values or other soil properties. Across all sites, starter fertilization increased yield by 1.3%, early growth by 29%, P uptake by 30% and N uptake by 30%. Unfrequent yield responses in areas with high soil-test P could partly be attributed to either the P or N in the starter.;A variable-rate P response study involved six fields managed with a corn-soybean rotation. Treatments were a check, a variable-rate application method based on soil-test P, and a uniform-rate method based on field-average soil-test P. Results from this study showed that grain yield responses were statistically significant in fields where soil-test P was optimum for crop or lower. Variable-rate and uniform-rate application methods seldom differed. The variable-rate method increased yield more than uniform-rate in one field, but the reverse result was observed in another field. The variable-rate method reduced the total amount of fertilizer applied in nine site-years (35% on average) and increased it in three site-years (21% on average).;Overall, the results of this research showed that use of starter fertilization in addition to broadcast fertilization and use of variable-rate fertilization instead of uniform-rate fertilization would manage P better and could potentially increase yield because significant field areas tested low in soil-test P.



Digital Repository @ Iowa State University,

Copyright Owner

Manuel Bermudez



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File Size

82 pages