hi, I definitely recommend a sand bath for your hamster it should be about 1 inch deep at least and quite large and dwarf hamsters do prefer to have a larger sand bath due to where they come from and it is a good idea to put a small hide In it also so it has an enclosed area to groom itself. it may also use this as a toilet making spot cleaning easy, however I would not recommend soil as it isn't beneficial and may have bugs etc and if your hamster swallows it can cause more harm than good however hay is a great idea in moderation just get soft hay as if its sharp and ur hamster stuffs it in its cheek pouches it can cut the tissue causing extreme pain so be careful on the type you use if you don't want to risk it I wouldn't recommend
hope this helped