Eating Sand

My non verbal 3yo won't stop eating sand. It has been the case since like forever. Somedays more than others - from licking a bit to eating a mouthful. It's like he gets an urge when hands touch sand/earth, his tongue automatically comes out. We have tried iron supplements to rule out pica. But it remains the same. Few times he has eaten other stuff as well, like raw rice, grains, leaves, flowers. For this reason it's hard to take him to playgrounds and gardens. Any suggestions will be much appreciated.