Crushed oyster shells may add trace amounts of calcium to the soil but will not hurt your camellias - unless they hide the soil and your forget to water your potted shrubs. Camellias have shallow roots and don't like to dry out or be in the hot afternoon sun. They love to grow on the north or east side of a house. Peatmoss or wood chips in the soil is also good for camellias. Keep growing, Marianne Binetti