Hi Quinn - Sq. Foot Gardening has its own forum here with lots of info and a great set of FAQs available. Be sure to check into all the discussions about the best fill mix recipes to use as they have changed over the years.

http://forums.gardenweb.com/forums/sqfoot

For improving in ground beds additions of high quality compost is the standard recommendation and can be especially helpful with clay soils. You'll also want to add a nutrient source - some form of fertilizer and mix it all in well.

Nothing prohibits you from using Sq Ft planting methods in an existing bed, many do. You just have to monitor the nutrient levels closely since it is such an intensive planting method.

Dave

I think Dave covered it fairly well. I'm one of those who uses the [modified] Sq. Ft. Gardening planting method, but using in-ground beds. Depending where you live, clay can be extremely nutritious soil. In most cases though, as Dave mentioned, fairly heavy additions of compost will really help. My soil here in E. Tn. is mostly clay (all clay in the upper sections of the garden,) and it produces an incredible amount of produce from my 4' x 18' beds.

For this year, I would add the compost, and then come fall, turn about a 6" layer of fall leaves (or any other type of readily decomposable organic materials - such as grass clippings, used coffee grounds, etc... if you wish) right into the soil. If you can shred them before turning them under (many just run them over with a lawn mower,) they will incorporate faster, but even if you turn them in whole, unless you're in an area with very long winters and deeply frozen ground, by spring the worms (and other soil life) will have supplemented your beds with a fair amount of worm castings (a decent fertilizer, and great source of organic matter.) Follow that basic schedule (compost in the spring, organic materials in the fall) for 2 - 3 years, and use an organic mulch during the growing season, you'll soon have a bed that will be easy to maintain with just a little compost and a surface mulch each year. You'll hardly remember it's clay.

the 1/3 compost is not really just compost it can be composted manure

and other composted materials. but you need to read Mel's updated book get that part of info. instead just read excerpts on internet.

Or you could go with the 1981 Mel's mix:

mix thoroughly:

1 bale of peat moss 6 cu ft

1 large bag course vermiculite 4 cu ft

10 pails ( 2.5 gallons size )play sand 3 cu ft

2 pails wood ash or burn't charcoal

10 plails (2.5 gallon)blended compost 3 cu ft

1 quart lime

1 quart organic fertilizers

( the above was modified slightly to make measurements and information easier to understand the info same as the 1981 version of the book on page 65)

I also do square foot gardening in an existing raised bed. I did add some peat moss and vermiculite to the existing soil originally. Now I add as many inches of compost as will fit each year, and in fall I build a small foot tall fence around the bed and dump a bunch of fallen leaves on it. For fertilizer I use either soybean or alfalfa meal. In fall I also bury cleaned pumpkins in the bed.