I have to respectfully differ with ams3651. Don't avoid the clay. Clay is not a bad thing at all -- it has some very GOOD points, such as good water-holding qualities (i.e needs less frequent watering) and generally excellent fertility.
You just have to (as OaklandCityFarmer is already doing) work with it to prevent its bad points from surfacing. Those being, that it typically has dreadful drainage, is very cold and late to become workable in the spring, and can dry like bricks - cracked bricks! - in the summer. The solution to almost all of those problems is just what OaklandCityFarmer is doing, dig in lots and lotsa compost.
Do
not be seduced by those who say add sand. Unless you add an unrealistically enormous amount of sand, what you will create is concrete that's even worse than the original clay. (If you're going to add an unrealistically enormous amount of sand, which is not really the best solution, make it
very coarse sand, not the find builders' or play sand).
It is sometimes suggested that you add gypsum to help flocculate the clay into bigger less homogeneous particles. I have read, however, and am willing to believe, that this DOES NOT APPLY TO EASTERN OR MIDWESTERN clays... only *certain* Western clays. Due to soil chemistry stuff. Definitely anyone contemplating gypsum should research the subject in some depth first.
You do need to be attentive to site drainage on clay soils. There may be some parts of your property where you can't plant (well, can't plant whatever you want -- there is *something* that will grow *anywhere*) unless you can divert surface water and/or drain subsurface water. French drains, soakaways and tiling are sometimes suggested. However my observation is that they are only suitable for *some* sites, and some clays can be so clay-ey that subsurface drainage is not highly pointful.
If you cannot improve drainage enough for what you wanna plant, your only alternatives are a) raised bed or b) don't plant it there. With a raised bed on clay, I would advocate coarsely, but deeply, breaking up the native clay soil underneath the raised bed (like, turn it up in big clods with a shovel) before putting your fill soil on top.
The only other things I know of are the relatively-obvious... don't work nor even walk on the soil when it is at all wet (can be formed into interesting play-doh style shapes with your hands), and never let it completely dry out hard in the summer. The more organic material you have in it, the less likely hard drying is.
But don't knock clay. Many excellent gardens are built on clay, not despite it but *because* of it. Much as many of us like to whine about it (and believe me, on a property that floods like this one, I can whine about clay like nobody's business if you get me started
), clay is actually a fairly good substrate for a garden as long as you WORK WITH IT and also RESPECT ITs peculiarities
Good luck,
Pat
