Fabric pots: breathable fabrics available in many sizes and shapes.
We use these as well.
This type of system allows for vertical expansion. you can plant 20 heads of lettuce in one tower. Spacing between towers really depends on the crop and its growth patterns. Lettuce only needs about 2 ft' spacing between towers. These towers tend to dry out fast depending on the stages of vegetative growth, automatic watering systems are essential. We use these at Natural Nomad Farms and love the tower's versatility and productivity.