Answer: The flies are probably attracted to the odor of manure. If you allow the top to dry out between waterings, it should not be so attractive to the flies. If they are really annoying you, cover over the top of the soil/mulch/manure with several layers of newspapers being sure to anchor them down so they won't blow away. This physical barrier will keep the flies from accessing the organic matter and from laying eggs in it. At the end of the season you can dig everything into your garden bed, newspapers and all to help amend your soil. Best wishes with your garden.
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