Soybean [Glycine max (L.) Merr.] yield response to increasing plant population density can be significantly affected by factors such as maturity, row spacing, and environment, which can lead to conflicting row spacing and seeding rate recommendations. To address this issue, we conducted experiments evaluating maturity group (MG) 00 through VI soybean sown at densities of 10, 20, 40, 60, or 100 seed m^-2 at Fayetteville, AR in 2001, 2002, and 2003. Soybean yield had an asymptotic response to increased plant population density, but responsiveness to increased plant population density varied among MG and years of the experiment. Soybean yield also displayed an asymptotic response to increased cumulative intercepted photosynthetically active radiation (CIPAR), and responsiveness to increased CIPAR was similar among years of the experiment. Using this relationship we determined that 90% of asymptotic soybean yield could be obtained by intercepting 605 MJ m^-2 of CIPAR from emergence to R6. The critical value of 605 MJ m^-2 of CIPAR was then used in a mechanistic approach to optimizing soybean seeding density using seeding density and time from emergence to R6.