I know that heavy clay. We found it along a local creek while working on a slope stabilization project. My coworker was super excited about it as they had spent some time making pottery.
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Your average fruit tree has a more distributed root system that spreads out. The bulk of their roots are in the first 18 inches of soil. As an example of a tap root a mango tree has a proper tap root that can dig 30 feet down. A paw paw also has a tap root, where the babies in that picture are already exceeding the average dept of roots on a 50 year old apple tree. Obviously soil composition and water affects the root depth but trees have certain tendencies that makes them better or worse suited to a given soil.
In hort school, we called them striker roots. I know the general def, but wasn’t sure what yours was since you seemed to be suggesting that clay soils encourage them.
I would think that plants would more likely have a deep taproot on porous sandy soils than on clay even if the clay is well drained. Sand is where they need to go deep for water not clay. But then there’s a lot I don’t know.
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Don’t worry FN, I can support your claim with the experience you mention. Been in direct contact with variations of same species root growth most of my ancient life. I’ve had nurseries in heavy clay, clay loam and a light silty loam (on my own property) where a high percentage of trees were and are managed as bare roots and dug with my own hands and hand tools.
Lighter soils encourage more expansive and deeper root systems, but they are still much easier to hand dig trees in when digging them up bare root. I’ve learned I can make it even easier by strategically placing sheets of plastic about 14" down under whips as I set them in my nursery. It catches water and stops the roots from going deep.
However, most of my experience is with “dry farming”. Roots grow much differently when water reliably comes from drip irrigation. My trees have depended on rain for the 30 years I’ve been doing this in the NE.
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Just like the foliage on top, tree roots have a phenotype that while the soil has a way to influence how it is expressed, it still adheres to the way the species like to conduct their business. For example an apple tree under the most perfect of circumstances will not attempt to shoot a root 30 feet down, while a mango tree on the worst of circumstances will do its darnest to pull that stunt.
Some trees like to spread out, some like to dig deep. Heck a tree like the columnar Norwegian maple does not play well with others; it sends extensive shallow roots that suck up everything it can. Not even weeds grow under them. Trees like most tall oaks could not care less what grows near them, their roots run deep.