thanks christacular and melissa17s! It has happened a couple times at day care, and I have noticed it when we're out at the park. I don't think it's anything as serious as Pica; I supose I just wanted to know if it was somehow related to the fact that we don't feed him meat, and so he may be eating sand as a substitute for something he is lacking.
I hope (obviously) he grows out of it, but I worried I may be depriving him of some type of nutrition his body is requiring.
He has a balanced diet of healthy food (whole grains, beans, veggies, fruit, dairy, etc) -- and my dr. told us that he may not be absorbing the iron in the veggies properly. So, I guess that got me a bit worried that I wasn't doing everything I could to make sure he was healthy, YKWIM?