I have never planted soybeans with fertilizer. I guess I have never seen the need to, the only time I use fertilizer in the planter is the starter fertilizer I use with the corn. I know its old school, but I really like its results. It is impossible for you to supplement all the nutrients being taken up by the soybean plant with just putting it on with the drill, so I guess my point is why take the risk. You are going to have to make a trip across the fields spreading your mineral fertilizers anyway. Maybe try some check strips to see if you get a yield advantage.