Ginseng definitely likes growing in rocky soil and does well there. When I find seng thriving in the flats of a hollow bottom, it is almost always the case that the soil is rocky, sandy, or shale chip filled. Good drainage and lots of calcium and other minerals in the soil.
The most seng I ever dug in one day was just a little over 3 pounds wet. My partner dug the same... we had 6 lbs 3 oz or geen seng in one trip. The hollow was named rocky hollow and it was just that. Lots of rock outcroppings, and the soil was rock chip filled.
Now light conditions have a big impact on seng too.
Seng will grow and do ok in deep shade (old growth forest)... but will never reach it's full potential if too much shade and will remain smaller, and grow much slower. In places like that I often find 30-40 year old 8-10\" tall 3 prongs with 1/8 oz roots.
But now if someone comes in there and clear cuts the timber on the ridge tops, leaving timber in the hollow bottoms. The seng that is growing in that change over area between the good woods and the clear cut... will all of a sudden start getting more light, and it will grow like crazy, get big a stout.
Locations where you have seng growing in good woods, and just to he east of it gets select cut or clear cut... and that lets a lot of morning sun in to the seng area... it will do extra well there. Seng loves morning sun, but not so much the hotter evening sun.
I hunted a hollow here a few years ago, a long ridge and hollow and I was hunting the north hillside along that ridge and hollow bottom.
At the end of that ridge, They had clear cut the timber on top of the ridge.
There was ginseng growing all down that hollow, but where the ridge top had not been clear cut, the seng was average to small (8-10\" 3 prongers)...good quantity but very small to average plants. Then all of a sudden when I got down that hollow far enough to where the ridge had been clear cut on top, the hillside below that had went nuts with all kinds of plants growing, very weedy, there was even jewel weed and polk salid, and big old briars growing where all that extra light was getting...
And in with all that stuff there was SENG. HUGE 3 and 4 prongers, and a bunch of them.
I am sure what happened there was prior to that ridge top being clear cut, that seng was just like the rest in that hollow... average size, but once the ridge top was cleared and all that extra light started getting into that area... the seng grew like crazy.
I have seen that same thing happen in several hollows around here. It is fairly common for timber companies to clearcut the ridge tops, but leave the hollow bottoms.
Where that allow more light (especially morning light) down into areas where seng is growing already.. boom... it really takes off and grows very well.
Some of that area that you saw Billy (Ky Boy) digging in, had probably been select cut. Initally that is hard on the seng that was growing there, but after a few years when the light conditions get better, the seng will come back and thrive in that extra light situation. It may not be a situation where a lot of reproduction takes place, but those older more mature roots that survived the select cutting a few years, will continue to grow, and grow big and stout once the light conditions get better, even to the point of being more than you would expect seng to grow in.