I would halve the peat moss and substitute with bark fines(soil conditioner).
I have used peat but only on hillside gardens where I want to hold the moisture.

Amend with as much organic matter as you can -- compost, manure, etc. That really works wonders. Are there any horse farms near you?

I agree with Alicia, try to go to a horse farm/stable and get a load of cpmposted, or even partially composted horse manure. Spread it about 6 inches thick, wait 6 weeks and you'll be set. If horse manure isn't available, the pine fines (sold as soil conditioner) works great. I also like mushroom compost, or cow manure. I have heard you are not supposed to use sand because it can combine with the clay and form a very hard soil, however I do not have first hand experience. I have also heard that adding some #78 pea gravel or Permatill will really increase drainage. I have just started using the gravel in some of my shrub planting, so I can't really comment yet. The clay is a pain in the neck, but once you get it ammended it isn't so bad. If your soil is really bad, like the scraped off subsoil you get in a lot of newer subdivisions, I would highly suggest lasagna gardening. There are several books on the subject and my MIL has used it with great success.

A very fine gardener in the Raleigh area once told me that he builds berms and amends heavily with permatil. He told me that it is important to have some areation around the roots that can be permanent. I have seen his gardens on TV and they are gorgeous. Adele

I think it's fine to add sand to improve drainage IF you also add soil conditioner. Ideal loam is part clay, part sand, and part silt. By adding soil conditioner and something like cow manure you improve the texture, which improves drainage but also helps hold moisture, and improve soil nutrient level and make the nutrients in the clay particles accessible to plants.

Laura's right, adding sand is OK if you also add organic matter. I haven't used peat moss and have read that it doesn't add much nutrient-wise, and bagged topsoil often packs, which isn't good for aeration around plant roots.

100ft square is pretty big. Search out a garden/landscape shop that sells soil and mulch by the load. Or, do the same and build a berm.

Thanks for all the tips!

I was told once - use 1/3 clay, 1/3 soil conditioner and 1/3 humus (or nutrient rich matter) - It seems to have worked for me in all of my flower borders but my husband planted our shrubs eight years ago in straight clay (no additional additives) and they are tremendous and healthy. I still can't figure that out - Carrie

I'm sure everyone here knows this, but, in case not: amend means tilling the soil first, and incorporating the amendments with the native soil. You can just dump the amendments on top of soil for raised beds, but will have much better success (especially with the hard soil of new houses), if you break up the hard clay first, allowing the plants roots to stretch down.

Didn't see that specific point here, so thought I'd add it. For gardeners new to clay soil, it's an important one.

If you're unlucky enough to have clay subsoil like I did, tilling doesn't help nearly as much as with clay topsoil. We tried tilling in several areas and it just didn't work as well as topdressing. I knew that I was dealing with subsoil because most of my land has around 18" of sandy loam over the clay. The way I had to deal with the problem was to dump wheelbarrow loads from my horse's stalls about 18" to 2' (this was uncomposted) and planting in that.

I'll vote for the deep tilling too. Mine was tilled pretty deeply and heavily amended w/mostly rotted leaf mulch - it's done splendidly, but in some areas I think a deeper till would have been beneficial. It does tend to settle back to its original status - jmo.

phylla's absolutely right about preparing beds in new construction yards. Another thing to consider is that since much of the new construction is on tracts of former farmland, you run into what's known as 'plow pan', 12-18" down. An area of very compacted soil and rocks that needs to be broken through before planting shrubs.
The very act of construction with its attendant heavy machinery, materials storage and parking on the land will create areas of severe compaction of the soil that will lie just below whatever top soil/clay the builder adds before grassing.
So, rule of thumb for new construction yards is to dig deeper and wider than you think necessary to plant.

Now is a good time to prepare the soils. The clay is easier to dig or till when there's more moisture.

Back on the farm we always plowed in tons of horse and cow manure. We had plenty. :0)

botanical_drifter