Can my chicks just get grit from dirt or do I really need to buy it

I’ve put a dust bath in the brooder with my two-week-old chicks. It’s wood ash, DE and dry sandy dirt from the yard. They’re having a blast and pecking away at tiny rocks. Is that good enough? Is chick grit a grift? We’d like to start giving them small treats so I was going to buy some later today. But I’d rather save the money if it’s not needed.