Sally, granite grit tends to slightly acidify soil, while oyster shells shift it toward alkaline. Oyster shells=calcium carbonate.
I always have a lot of plants in containers and learned of rice hulls for weed control from Julie Borneman of Watermark Woods Nursery and plan to employ this coming year. Quote from online source:
"I have evaluated many mulch materials for weed control in containers. One of the most effective materials for controlling container weeds is rice hulls. Our research has shown that rice hulls provide excellent control of bittercress, creeping woodsorrel, willowherb, and common groundsel. In our experiments, rice hulls at _ to 1 inch depth provided effective weed control through 16 weeks. We found that rice hulls dry very quickly after irrigation. Quick drying time makes weed germination and establishment extremely difficult. We also found that rice hulls decompose very slowly, providing an effective weed control barrier for months."
