Trees in the nursery often have a number of roots growing straight down under the trunk. Trees in the previous photo came from this nursery. Note that there are many deep roots on this tree.
These deep roots would be submerged in the perched water table after planting so they could not carry on normal respiration. This can quickly lead to inability to absorb water followed by tree death. Many field nurseries choose well drained soil for their business because it makes year round operations possible. Deep roots can form in well drained nursery soils.