Ensure that the soil should not just hold water but also have good drainage. Generally a sandy clay loam soil mixed with compost is best for most plants in raised beds. Soil depth in raised beds depends on the plants that will be grown. For shallow rooted plants such as lettuces and herbs, an 8-12-inch depth is considered a minimum. To be able to grow a wider range of crops, soil depth should be between 18 and 24 inches.