There is such a things as Lasagna gardening and you might consider this in filling your beds. A layer of straw, a layer of chicken cleanings, and/or horse manure a layer of top soil, and so on. The last layer could be a 1 1/2 Cubic of garden soil. Cover the entire thing with black garden cloth or cardboard. Cut holes and stick your plants in . I buy top soil or garden soil when it goes on sale. I have never filled an entire raised bed with purchased soil only. I usually use them on the top layer. Everything will break down as time goes by and then you can just mix it to you hearts content.