Making poor soil

I have a strip of ground that I want to sow wild flowers in. The soil is quite rich as I added around a ton of top quality soil to the existing soil a few years back when I was going to do something else. I have some Yellow Rattle to sow. Should I add anything to the soil to make it less rich??

 

I grow most of mine in pots with lots of grit and seed compost because my soil is too high nutrient for them, it's loamy clay

 

@luciusmaximus i don't know if this link will help also

http://www.charlesflower-wildflowers.co.uk/images/Guidelines.pdf

 

It's a bit late to comment but in case it's still relevant, I hade this same problem and on advice from the supplier of wild flower turf, I stripped all of the top soil away from what had been a well fed veg patch. The resultant 5o sq mt of wild flowers are well worth the effort.