The compost tube is a 6-inch diameter cylinder running down the center of the Garden Tower. The tube has numerous holes running down the entire length that allow composting worms to travel between the compost tube and the soil. The worms feed on the kitchen scraps and leave rich worm castings behind. As you water the soil, this water collects vital nutrients as it passes through compost-worm castings and collects and drains at the bottom of the unit. Garden Tower Project recommends the use of red wiggler composting worms in the compost tube; however, the addition of other types of composting worms (such as Uncle Jim's composting worms which contain several species of composting worm) will increase nutrient transport and cycling in the tower. Night crawlers, for example, will travel throughout the soil and compost creating pathways for air and water while moving nutrients through the system.