When attempting to maximize the crop yield from field-grown soy ( Glycine max (L.) Merr.) by means of improving the light conditions for photosynthesis in the canopy, it is crucial to find the optimal planting density and nitrogen application rate. The soy plants were cultivated in N-dense mutual pairs, and included two cultivars with different leaf shapes; one sported ovate leaves (O-type) and the other lanceolate leaves (L-type). We analyzed the results quantitatively to gauge the effect of the experimental parameters on the yield as well as the photosynthetic light and nitrogen use efficiency. Results indicate that the different leaf shapes were responsible for significant disparities between the photosynthetic utilization of direct and diffuse light. The light interception rate of the L-type canopy was lower than that of the O-type, but its canopy apparent photosynthetic nitrogen and light use efficiency were higher. However, the nitrogen and light use efficiency contributions associated with exposure to diffuse light were greater for the latter than for the former. Our conclusion is that optimizing the yield from densely planted L-type soy hinges on using an appropriate nitrogen application rate, while O-type soy requires optimizing its nitrogen and light use efficiency under conditions of diffuse lighting.