UNLIKE CORN SOYBEANS ARE A DAYLENGTH SENSITIVE CROP
Soybeans are photoperiod dependant crop.What this means is that once daylengths start to shorten after June 21 soybeans are triggered into flowering.If you have several different soybean maturities planted you will find that they will flower at different times.Your earliset maturing varieties will flower first followed bu your later maturing varieties.The key to maximizing the genetic yield potential bred into any variety is to maximize its vegetative growth before flowering occurs.What this means is that you should attempt to have 6 True Trifoliates established before flowering begins.Larger vegetative plants have more carbohydrate reserve to reduce early pod abortion when plant stresses set in.This ultimately leads to MORE YIELD.

Lets consider what it takes to make 6 True Trifoliates happen before first flowering occurs.To put it into perspective I'll use a timeline chart :

Grower A – Plants His Soybeans May 5

2 weeks to emerge – May 19 ( VE )
5 days for full development of Unifoliates – May 24 ( VC )
6 days for 1st true trifoliate ( V1) May 30 } Nodules are forming
6 days for 2nd fully developed Trifoliate ( V2 ) June 5 } Limy looking beans
6 days for 3rd fully developed trifoliate ( V3 ) June 11
6 days for 4th fully developed trifoliate ( V4 ) June 17
6 days for 5th fully developed trifoliate ( V5 ) June 23
6 days for 6th fully developed trifoliate ( V6 ) June 29

Voila! To maximize soybean yields means you have to start thinking about planting soybeans when you start planting corn.