Is my soil good for the plants?

So I was wondering if my soil is actually any good for growing any kind of plant. It looks like having a lot of clay, and it has a tendency to turn into big rocks... Any chance to make it good finally?

So I've stumbled over this article, describing what is a good soil:

http://gardening.about.com/od/gardenprimer/a/Amending_Soil.htm

I just need to add the right amounts of nutrients, some humus, and ... hope to get it in better shape. Will let you know if I've made it ;-)