Do I use field soil, potting mix or a combination in the 5-Gallon Grounder?

A good soilless mix works best. Because there is little contact between the soil outside and the mix inside, a mix should be used. This is a distinct contrast to the Knit Fabric container where there is a great deal of contact between inside and outside through the thousands of small openings. Remember that the soils must drain well for a soilless mix in the Grounder™ to work well.