Most all soil that has not been chemically fertilized has Mychorrizae. Unfortunately the soil in nursery pots has usually had chemical fertilizer, so it may be deficient in Mychorrizae. I am not an organic gardener, but reseach has shown this problem. Fortunately the best rhododendron and azalea fertilizer, Hollytone, is organic. Actually I seldom use fertilizer. Rhododendrons don't need much.

It is much more important when planting to prepare a large hole with a good rich acidic humous soil. Excellent drainage is extremely important, but in your area sandy soil is common so that may not be a concern. Then open up the root structure of the plant. Potted plants end up with their roots wrapped around the inside of the pot. If planted this way they will very likely die in a few years when they become root bound. Spread the roots out in the hole and cover up with good moist acidic soil. Never plant deeper than it was originally. It is best to plant a little higher. Be careful to not damage the very fine roots. They are the important ones. Water thoroughly. Now mulch with a couple inches of organic mulch, not stone.

If you want a real treat, go to Rarefind Nursery in Jackson, NJ. They are the premier rhododendron nursery on the East Coast and have many other nice plants. They know what they sell. Hank Schannen is the proprietor and has an excellent staff. You will need directions. They have a website at:

Rarefind Nursery in Jackson, NJ

Here is a link that might be useful: How to Grow Rhododendrons and Azaleas

From what I understand, rhodies/azaleas have extremely specialized mycorrhizae, not the common sort available commercially.

Thanks for the feedback.

I found a product in a nursery called "Soil Amendment" from Monrovia.

http://www.monrovia.com/learn/soil_story/soils_and_amendments.php?page=2

Supposedly contains compost and the microbial Mychorrhiza. I will also mix hummus with the existing soil. I will plant the base of the plant 1- 2 inches above soil level.