First off, you're not going to see white mold simply because you are in continuous soybeans. You will have a greater potential of it if you plant in very narrow rows combined with wet weather. There is always a greater risk of diseases as the pathogen does not get a break from it's host. If you manage it right, and give the crop what it needs, it will have a better immune system and be able to fight them off.

About the organic matter,

Soybeans will not leave behind as much organic matter as Corn or wheat. The mass is just not there. Having said that, I do believe that soybeans are a woody plant and therfore break down slower. The key ( to me at least ) is to work it in a little. Not too deep. If left on top, there will always be part of the organic matter that is burned up by the sunlight and lost. I don't have any proof, just my opinion.

 I'm not going to give away all my secrets, but here is a couple of books you can read which give you more insight on how things work.

 How to grow super soybeans.

Science in agriculture

Good Luck