Why do your birds dust bath? Does the dirt help absorb oils or something? Unrelated but my cat likes to also roll around in dirt, but he's just a stange boy.
Absorb is probably not the right word for it.
Dust bathing does 3 main things. a) provides gritty material to aid in removing feather sheaths b) aids in mechanical (mechanical here meaning physical forces acting upon mass, not technological mechanical) removal of stale lipids, and c) aids in the mechanical removal of ectoparasites.
Feathers grow in ensconced in a flaky sheath of keratin. This is soft and pliable as it comes out of the skin, but will dry and become flaky, for easy removal. If left too long, or if made with bad nutrition, these sheaths can become very hard/stiff and may even become impossible for the bird to remove without assistance. Dust bathing is a mechanical method of removing sheaths, by rubbing at them and speeding up the flaking process.
Birds apply preen gland oil to their feathers, and it becomes stale over time the same way your hair can. Rather than soap, they are able to again mechanically rub off stale oils in a dust bath, effectively cleaning the feathers and allowing for proper feather alignment again.
Dust baths don't kill ectoparasites, but they can "remove" them the same way you might remove a spider crawling on your leg- by physically brushing it off. This doesn't prevent infestations, it doesn't treat infestations, it merely provides some temporary relief until the parasites climb back aboard or hatch more. It won't even remove all of them! It just reduces the load to be more manageable.
