Enhanced with alfalfa, kelp & feather meal. With Myco-tone mycorrhizae. Fill the raised bed with Espoma Organic Raised Bed Mix. Water thoroughly after planting, then as needed until plants are established. Begin regular fertilizer program 30 days after planting. How Many Bags Do I Need?. Area (Sq. Ft. 6″ Deep Bed 12″ Deep Bed 20″ Deep Bed. 2 ft x 2 ft Bed 2 Bags 3 Bags 5 Bags. 4 ft x 4 ft Bed 6 Bags 11 Bags 18 Bags. 4 ft x 8 ft Bed 11 Bags 22 Bags 36 Bags