Dirt or Dandruff on Labrador's Coat?

When I brush my lab he ends up covered in what looks like sand? Am I digging dirt up out of his coat or does he have dry skin? I do not bathe him very often at all because I read it was bad for the natural oils in their coats. So it could be dirt. I’d appreciate any help. Also, any ideas on how to get it out of his coat once it’s out?

 

I’m attempting to post a picture.

 

If it’s white it’s likely to be dry skin/dandruff. You could try adding an oil to the dog food.

 

I have this too, mainly when main sheeding season starts