Seasonal canopy photosynthesis and light interception are critical to soybean [Glycine max L. (Merr.)] yield. Early planting dates allow for greater seasonal canopy photosynthesis and greater plant populations at later planting dates increase the rate of light interception, potentially increasing seasonal canopy photosynthesis. The objective of this research was to determine the yield benefit from early planting and evaluate the need to increase the seeding population at later planting dates. Studies were conducted at three locations in Iowa for two years using four planting dates beginning the last week of April through June and four seeding rates ranging from 185000 to 556000 seeds ha^-1. At all locations, planting early increased yield while there was no yield difference between plant populations at any of the planting dates. Grain production decreased 41.6 kg ha^-1 day^-1 following May 1 through the first week of June at high yield potential environments in eastern and western Iowa. At the central location where yield potential was considerably reduced because of the presence of soybean cyst nematode, seedling diseases, and sudden death syndrome, grain production decreased 41.6 kg ha^-1 day^-1 from the first to the third week of May with no further reduction. Seed mass increased 4.5% as plant population increased and was probably a compensatory effect caused by reduced seed number plant^-1 at higher populations and did not improve grain yield. This data indicates that regardless of environmental yield potential, farmers in Iowa should make early planting a management goal and should not increase the plant population as planting is delayed.