Posted: 12/11/2013 at 18:54
Dove is bang-on - it's always preferable to improve clay that way. In severe cases lime will help but it's best to do a soil test with a cheap kit first as adding lime to an already alkaline soil isn't a good idea - there are far more plants which prefer neutral to acid soil than those which prefer alkaline conditions. In really bad claggy alkaline clay, adding gypsum (builder's plaster is the cheapest way to buy it) will help it break down without increasing alkalinity. Frost will do a fine job, too - leave big clods of clay on the surface over Winter and they will be broken down a little every time there is a frost.
Clay is naturally very fertile and although it's hard work to improve the structure, once done you will have one of the best soils there is.