Any suggestions on the best way to prepare the soil? What type of additives to use, what pH works best, stuff like that. Many thanks.
Go to "Top 100 Garden Sites" Post, in that post Rhodyman gave a nice explaination to your question and a web link to his great site.
Which state are you in? We could be more specific if we what type of native soil you have.....
I am in SE Coastal NC. Azaleas are big around here, in fact we have an Azalea Festival in Wilmington. The soil is fairly sandy, but has a rich dark brown tint to it.
If azaleas are popular in your area, chances are no one is going to extreme measures to alter natural conditions to make them happy. Sometimes how things are growing in your neighbors garden can be a better soil test than the kits you can buy, unless you've done something to change the PH of your soil, like pour large cement patios...
If you are creating a new bed, it never hurts to work in more organic material (compost) whether you have clay or sand. If you don't have your own, you can buy it by the bag (Home Depot here has very well aged, odorless steer manure for just $1 bag, I buy them 20 at a time)...keep in mind that mushroom compost is the wrong PH though and stay away from that one.
Azaleas like a highly organic soil, and you've probably read by now the the tips to loosen rootballs and not plant any deeper than they are growing in their containers. If you've amended and loosened the soil, plant a little high to allow for settling.
The real key to making them happy is consistent moisture (the first year especially), and a mulch over the root zones can make the difference. Helps to keep the root zone cool, and conserves moisture.
The following explaination is how I plant them with my soil, I will try to consider your soil in the customized suggestion.
It seems like you have very well draining soil, as per your description. If so that is the opposite of my clay based with 6 inches of a sandy loam topsoil.
In poorly draining soil like mine, I just saturate an entire bail of dry sorghum peat in a large non rustable wheel barrow. Then I pile it all on top of the ground and make a 6-8 inch high bed. The azaleas are then planted directly into the peat and covered with 4 inches of shredded cedar mulch for the first year. The next and every following spring I add two more inches of peat with out removing any previous years remaining mulch. After that, I cover the two inches of added moistened peat with two inches of very finely shredded pine mulch.
Since your soil apparently drains better than mine, you would most likely be better off mixing a 50 pound bag Black Cow compost with a bale of sorghum peat which you have saturated with water.
Next dig a hole in your soil from 6 to 8 inches deep and at least 4 foot square. If you want to add to your mix some of the virgin soil you just dug out, then add only enough of that to make sure you have enough peat/compost mixture to fill the hole and enough material to mound up the 4'x4' bed by 2-4 inches. That mounding should help as the bed begins to compact and decrease.
Make as many 4' x 4'beds as it takes to fill the area you have calculated the number of azaleas you are planting need.
After planting the entire bed with all your azaleas, just mulch them with 2 inches of fine, not chunky, pine needles or pine mulch. Each following spring if the beds have sunken below ground level, just add 2 inches if sorghum peat and cover with 2 more inches of fine not chunky pine mulch or needles
Only other thing I would consider is your humidity levels. If they are so high that your mulch seems to get moldy, then only spread one inch of the fine pine mulch or needles, and then cover that entire mulched bed with seedless pinecones.
My azaleas planted in straight peat and covered with pine mulch have never needed any more added nutrients. If yours begin to look like they do need more nutrients, that might be due to how well your virgin sandy soil drains. If that is the case then each following Spring do not only add peat; rather instead, simply mix a 50/50 mix of the Black Cow fully processed compost listed above and the saturated with water sorghum peat. Then add enough of that mixture to bring your sunkun beds up to ground level and mulch with the pine as describes above.
The purpose of me adding Black Cow compost to your beds is to compensate for the draining away of nutrients and moisture in your sandy soil. That compost, along with a schedule of appropriate watering, should also retain enough moisture to help keep that sandy free flowing drainage from drying out your Sorghum peat to the point where it will be too difficult to remoisten.