This dissertation is comprised of three separate studies in soil science and fertility. The first component concerns the use of swine manure on a leguminous crop. Recent environmental pressure has been placed to limit the practice of applying animal manures to fields planned for soybean [Glycine max (L.) Merr] production because of the perceived hazard of excessive NO3 movement offsite. The objective is to evaluate the impact of manure application to soybean fields on soil NO3-N, N uptake and soybean crop productivity. A field experiment was established in the spring of 2007 and 2008. Liquid swine (Sus scrofa domestica) manure was applied at three different N rates (based upon manure analysis and estimated availability) using two application methods (surface application or injection). Commercial fertilizer treatments (same equivalent rates) were also included as positive controls. Soil samples to 60 cm and tissue samples were collected throughout the growing season. Even though N application did result in higher soil NO3- levels for some treatments, applications rates that were less than or equal to 135 kg ha-1 represented a lower risk of possible NO3-N movement. Nitrogen supplied via manure and commercial fertilizer resulted in larger N uptake than the controls, but grain yield was not improved. This reveals that despite the fact that soybeans can fix their own N, soybeans will absorb soil inorganic-N as a net N sink.
The second chapter of the dissertation explores the interactions of foliar-applied manganese and the herbicide glyphosate on glyphosate-resistant weeds. Glyphosate is a metal chelating agent that interacts with cations essential to plant growth, and previous studies have shown interactions with Mn resulting in decreased herbicide efficacy and manganese metabolism issues. This study measured the effects of different formulations of glyphosate and manganese combinations as well as timing of the application. Of six site-years, one instance of manganese application showed a positive grain yield response, regardless of glyphosate rate and timing. One site-year showed a negative yield response from the addition of manganese, suggesting that when manganese is not deficient, the toxic limit is easily reached and may account for yield depression.
The final chapter of the dissertation tests the effects of N rate and timing on corn (Zea mays, L.) yield components. As part of a multi-state study using normalized difference vegetative indices in precision placement of N fertilizers, effects of delayed N nutrition by growth stage and rate on the physiology of grain filling could lend greater insight to the logistics of precision placement. Four site-years of data were analyzed in a factorial design comparing five N rates (0, 67, 135, 201, and 269 kg ha-1) at each of the growth stages of preplant, V4, V8, and V12. Results were not consistent over four site-years of data, though the yield data suggest that any N application at V8 and V12 was not able to overcome the permanent yield losses caused by N deficiencies compared to an unlimited control treatment.