Ask-a-Dave's-Gardener: Hibiscus planted 3 months ago yellowing leaves, 1 by Onthefly71

Onthefly71 wrote:
Hello! I\'m in PHX Arizona. Hope someone can shed light on an issue I\'m having!

I put some hibiscus plants into two pots and three into the ground.

The two in large pots have periodic bouts of yellowing leaves that fall off.

The rest of the plant seems totally healthy.

Is it the soil? We have very hard water and I wondered if it could be salt burn.
The soil is kept moist, but not wet.

Thanks in advance!