Bedding for Guinea Pigs - Soil and Sand

Question
Hi.  We have two (2) male guinea pigs who live in a 4' x 4' home.  They are bedded with alpine wood chips and tons of hay.

I have made a tray of grass (combo of rye, wheat and oat.  Planned on including barley, but store was out of those seeds).  The tray is approx. 12" x 18"  and was put at the back end, where they rarely go.

They love to lay on the dirt!  After the grass died off, I left it in with them and began a new tray.  They even pulled their igloo on top of it!

Can I bed them in soil?  What about sand for Chinchillas?  They absolutely love the soil!  

Answer
Helly Mysty,
You can actually keep them in soil. I can see why they would love soil. It keeps them cool and warm. :) If I was guinea pig I'd put my house over a soil bed anytime. I don't think sand for chinchillas is a good idea though. All the dust being around them constantly could cause some harm to their respiratory system. The dirt however would be a good idea. :) It would probably make getting rid of the bedding easier since you can just dump it outside. :)