I wonder how well rotted the manure is. Six inches is a one off amount to improve poor soil and kill weeds, normally I recommend one to two inches. So perhaps there is still a lot of litter like stray, paper or even wood?
Or if the manure is reasonably well rotted and going to soft, dark humus then you can sow into it. Have a go, and for broad beans I suggest using a dibber to make holes two to three inches deep.