What kind of Magnolia did you plant? The soulangiana(sp?) is deciduous and would normally be losing it's leaves this time of year. The healthy leaves on this one are usually a soft mid green color and are somewhat flexible.

The grandiflora is an evergreen that should not be losing a lot of it's leaves at one time. The healthy leaves on this one are large, hard, shiny/waxy and deep green, often with a velvety brown underside. If this is the variety you have, I would be talking to the folks at the place where you bought it.

Beacheturtle,
I had a similar problem with my magnolia up here in Atlanta, but we have red clay soil. I found out that the hole I dug to put it in didn't have adequate drainage and was holding water as if in a bowl. I redug it out on the downhill side and built a drainage slough to keep it from holding water. Once I fixed the drainage problem the tree has been doing well.

Perryrip