Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Antonio P. Mallarino


Two studies were conducted in Iowa to better understand how potassium (K) fertilization affects grain yield, vegetative growth, and K concentration of different parts of modern corn (Zea mays L.) hybrids and soybean (Glycine max L. Merr.) varieties. One study was based on one- to three-year strip-trials (63 site-years) conducted from 2001 to 2007 to evaluate the effects of K fertilization on corn and soybean grain yield and post-harvest soil-test K (STK) measured with the ammonium acetate extractant. Replicated treatments were 0 and 168 kg K ha-1. Potassium increased yield as evaluated by strip averages in 21 of 63 site-years, and only when STK of significant portions of a field tested ≤ 171 mg K kg-1. Yield responses for field areas testing within different STK interpretation classes showed a differential response in seven site-years with no strip-average response. Determined critical concentrations by the linear-plateau model ranged from 178 to 200 mg K kg-1 for corn and 155 to 246 mg K kg-1 for soybean for four data handling methods. The other study was based on 20, two-year field trials conducted from 2003 to 2006 to evaluate the effect of K fertilization on corn and soybean early growth; K concentration in young plants, mature leaves, and grain; and grain K removal. Replicated treatments were 0, 28, 56, 112, and 168 kg K ha-1 broadcast before the first crop. Potassium increased first-year crop yield at eight sites, and residual effects increased yield in eight sites when STK was ≤ 171 mg K kg-1. On average across yield responsive sites, rates of 91 and 103 kg K ha-1 maximized yield of corn and soybean, respectively, at first-year sites while 168 kg K ha-1 maximized yield of both crops at second-year sites. The magnitude and frequency of responses to K were highest for (1) plant K concentration and uptake, (2) grain yield and K removal, (3) grain K concentration, and lowest for (4) early plant growth. The research demonstrated that new technologies are useful to improve assessments of K fertility in highly variable fields, and that large amounts of K absorbed early often is in excess of K needed to maximize crop yield, have little influence on K removed with grain harvest but would significantly affect K removal with harvest of vegetative plant parts.


Copyright Owner

Matthew William Clover



Date Available


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109 pages