I have grown potatoes on top of the ground, literally , and down as much as 12 some inches deep.
Those on top under mulch, produce a reduced yield of smaller sized potatoes.
(This ALL depends on variety planted as to potato size regardless of planting method.)
The deep ones will produce large potatoes at multiple levels; even planting deep, some varieties will have potatoes on the surface so I cover my potatoes, always, with leaf mulch from (depending on amount available) 10 inches deep to 14 some inches deep on average.
I hilled twice in my decades of growing potatoes (my father always did and was amazed when I just dug hole and put a potato in deep) and found it to be a lot more effort with results lacking from the amount of effort.
My potatoes, always turn into a mass of huge greens that trying to separate to maintian rows is like peeing into the wind.
I have grown many types of finger potatoes due to the better taste, but some can turn (Peruvian Purple is the absolute worst ) into a pain in the buttocks taking off like Morning Glory and spreading timy potatoes all over the garden , which, if you do not harvest EVERY single one can become a weed for years, if just planted in a handy empty place that looked good at the time.
Sending vines out four feet from where the seed potato is, rerooting and taking off from there again.
Most are like regular potatoes .
The only absolute failures I have had, have come from planting in too warm ground, and Colorado Potato Beetles.
I always till my gardens as far down as the roto-tiller blades allow; when I dig a hole for the potatoes, I dig down approx. two sand shovel depths, 16 inches plus or minus, and fill part of the bottom of the hole with loose soil -- (on rare occasions, I will put some variety of top line bagged soil , pre-fertilized, into the bottom of the hole, just to see how those compare to those without.)
The past year I got rid of bags of aging fertilizer by broadcasting it across the gardens.
Not sure it helped with the yield , but the greens were very , very healthy.
Last note, every year I buy new varieties on-line; last year two varieties purches on line had a 70 percent failure, did not grow at all.
They were planted in two different gardens, with same result so some times you are damned if you do and damned if you do not.
Potato ROOTS, prefer acidic , fertile, well drained -- loose -- soil.
How well your soil is- is prepared for the roots, deteremines what you will get to the greatest degree.
