I have a cross border collie and he has recently started to lick and eat dirt in the garden. Can anyone give me any reason why? thank you

Some dogs will do this if they feel sick and want to make themselves sick.  I would get him checked over by your vet.
Neeva

They also do it if you've shouted at them for having a poo somewhere where they shouldn't have to conceal the evidence.  Try feeding some pineapple it should stop him eating it, also clear his stooles up as soon as he's had them.

Warm regards Susan