I have 12 HUGE plastic pots in my garden (18-20 inches in diameter and 24-30 inches high). There are no bottoms. Over the years the soil has settled, and i need to add more. However, I know there are earthworms in the pots and I don't want to suffocate them. How much organic composted manure or organic potting soil can I add each day to be sure my worms have a chance to tunnel to the surface before I add another layer, I'd really don't have the time to dump the dirt out of each pot and sort through the soil for worms. Thank you for your help! Laura Borghi