What do these symptoms mean?

[McNally]

A horse at work first developed an itchy patch on his side- this became worse til he was chewing himself madly. Now sorted with medicated shampoo and skin gel. Its no longer itchy and is healing. Today i noticed he's not grazing but digging tiny holes (2" deep) and just stands licking the soil.
Made me wonder if its a deficiancy of something which could also be related to the skin problem?
I'm sure soil eating is a sign of something?!

 

[Nugget La Poneh]

I'd be getting a vet to run tests - could be something simple like worms, could be a mineral imbalance or lack of fibre. Could be related to the itchy patch, but might just be coincedence

 

[Rose Folly]

Perhaps get your vet to just check him out. But I wouldn't woory too much. It's a natural thing, and horses self-medicate to quite a large degree.