RE: What fertilizer for soybeans?
If your plot is not a high fertility old farm field, it will probably respond to fertilizer. 5-14-42 is a popular alfalfa blend that would work for soybeans, 9-23-30 is a popular corn starter fertilizer that would work. With a soil sample, you could apply the proper amount of P and K (soybeans fix their own N if they are properly innoculated). Without a soil test, 100 lbs/a of either blend should help (200 lbs/a wouldn't hurt, but you may be wasting money). Take a soil test for next year at the very least. Soil can be sampled and tested at any time of the year.