Reading over you original post again -

If your heart is set on Winter wheat for a fall planting - I wouldn't plant turnips or rapeseed in the spring. Because in my experience - they don't get used at all in the summer months. They might make a good green fertilizer - but so will a weedy clover plot. Your better off keeping it in a clover (even if its weedy) - spraying in July and Planting winter wheat in late summer.

I still like corn though - even if for one year - You'll never get more natural N in the soil (except for maybe with Alfalfa) - than with a 4-5 year old clover plot.

FH