if plants can grow in it and he can dig in it , i'd think whatever dirt is there would be a good enough substrate ..... as far as plants , i'd say some kind of grass that grows in clumps and won't get so big as to overtake the entire enclosure , in my case it's weeds , but there are some small ornamental grasses .... turtles like these like to make depressions at the base of these clumps of grass and it's where they'll spend the nights ........
