I'm not sure why soil/water would be any worse for an aluminum pole than concrete would be. Concrete isn't exactly a good environment for aluminum. Most aluminum poles I know of are not designed for EITHER. They typically mount above ground level to something set in the ground (either a steel pipe or to bolts embedded in concrete). If I could find a piece of PVC that my pole fit into tightly, I might try that.

Based on my shopping on line, electrical supply houses, and big box stores, most poles for residences are aluminum today and they are advertised as "direct burial". Concrete also has corrosion problems.

I would place the pole in a PVC sleeve. Last summer, we installed three post lamps throughout our yard. (1 in front yard, 2 in the back.) We set the aluminum post lamp poles in PVC-sleeved, cement-filled holes. The PVC protrudes from the soil about 2 inches, doesn't really show, and also protects the finish from damage from the string trimmer when the lawn is mowed.

I wired all three lamps with low-voltage landscape wire so we wouldn't have to dig deep trenches to bury 120v electrical conduit. Strung the lamps to a low-voltage transformer and inserted 12 VAC l.e.d. light bulbs.

The lamps look great, use less electricity, the l.e.d. bulbs should last for years, and we didn't have to destroy the yard (many trees!) to bury conduit.