Yes, we had a clay soil that was not used for almost 15 years and had to add manure, grass, hay, sand and compost now for 4 years to get it right. ALSO, something I learned from a neighbor, as your onions get growing to about the size of a pingpong ball--start digging around them a bit and get the dirt AWAY from the onions themselves!Not very large, but I think the clay soil is too hard. I'm going to rework an area and add lots of sand and top soil to make the ground easier for growth.
The ROOTS need soil and water to grow...but the onions do not need to FIGHT against the hard soil/clay. They will grow better and bigger if they do not have to waste energy fighting against the hard ground. I only leave about an inch of earth around the base of the onions and just leave the roots covered.
BTW: a video on making your own fertilisers: