Studies were conducted in South Dakota to assess mechanisms of intraspecific competition between corn (Zea mays) plants. Treatments were two plant populations (74,500 and 149,000 plants ha-1), three levels of shade (0, 40, and 60%) on the low plant population, two water treatments (natural precipitation and natural + irrigation), and two N rates (0 and 228 kg N ha-1). In-season leaf chlorophyll content was measured. At harvest, grain and stover yields were quantified with grain 13C-discrimination (Δ) and N percent determined. Soil samples at harvest were collected and analyzed for soil water and inorganic N. Grain yield per plant decreased when shaded or when planted at high population. If similar mechanisms reduced yield in shade and high population treatments, then we expected that plant and soil responses would be similar. However, differences in several key measurements were noted and suggest that factors responsible for yield reductions in these treatments differed. These differences included 1) grain N% was minimally impacted at high population, whereas N% increased in shade treatments; 2) chlorophyll levels in plants grown in high N/irrigated treatments with or without shade either increased or were not impacted by treatment whereas chlorophyll levels in plants grown in high populations decreased compared with levels measured in plants in low population; 3) grain from plants grown in high N/irrigated treatments with shade had increased Δ (0.27‰), whereas Δ was not impacted by population level; and 4) population level did not impact the amount of available water or inorganic N remaining in the surface 60 cm of soil at harvest. The mechanism responsible for the yield reduction in shade treatments was attributed to reduced light availability, whereas at high population, reduced yield was attributed to reduced growth of plants.