Here is an interesting story about planting soybeans from Farm Progress. In the story, it points out that planting soybeans too early does not give an advantage to plants but actually results in shorter plants with less leaf area. That results in lower yields than soybeans that are planted a little later that grow taller. Planting too early can also result in possible frost damage, seedling disease pressure, increased early season insect pressure, and end-of-season quality issues. The story also notes that optimal planting dates have changed over time due to improvements in soybean genetics.

Soybeans planted in wheat. Source: Maggilautaro, Commons Wikimedia