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I found several photos and drawing showing how potatoes grow. The eye sends roots down into the soil, the plant grows up and all the pototoes grow off the side of the plant. It looks to me like the taller the plant gets the more potatoes it will grow. The plant sends out runners in all directions and the potatoes grow on the runners.

So maybe that is why my car tire potatoe beds do better than planting in the soil with no tires. I plant the potatoes in the soil in the center of the tire. As the plants gets taller and taller I fill the tire with more and more soil. When the tire will not hold any more soil I put on another tire and keep filling it up with more soil. By the time the plant turns yellow and dies the soil is piled up about 22" high.

I am not a big fan of raised bed but today I made 3 raised beds for potatoes. I bought a scoop of garden soil from the garden center it is a mix of mushroom compost, sand, river bottom soil, and composted leaves. The raised beds have about 6" of soil in them I intend to plant the potato eyes in the garden soil then cover the plants with the compost soil.

I am going to do another car tire potato bed experement too.

I also have 3 wooden boxes 12" deep that will be potato beds.

I might try a few more experements too. Maybe wire cage. I want to know for sure how much soil depth has to do with how many potatoes the plants produce and if the soft compost soil makes larger potatoes than regular soil.

I am only planting Red Pontic potatoes this year they always do better than white potatoes. I bought 2 lbs of seed potatoes from 3 different places that might make a difference too.

Somebody here posted some good information about the way potatoes grow. I think it was JALUT. It convinced me that it wasn't beneficial to pile on huge amounts of soil because the root tubers only occur a few inches above the original eye. I think that I found that to be true as I harvested the last couple of years. The highest layers of soil contained no tubers and I had to dig down to the base of the plant to find them.

That is not to say that if I had piled on even more soil, there wouldn't have been more tubers below the top of that pile. I really don't know and I would be very interested to hear the results of your experiment. Not having to find more good soil to pile up was a convenient excuse for me, but I cannot say for certain that I wouldn't benefit from using more.

I have not gone to the extreme of using tires/cages to mound up potatoes, but the way I was taught by my dad and grandparents was to plant the seed potatoes at the bottom of a shallow trench, and once they sprouted up, start filling the trench and mounding up to cover the seed potatoes in a greater depth. This only amounts to maybe 6-8 inches of soil over the seed, nothing like the experiment. Very interest to see what your results are.

My grandfather taught me to plant potatoes when I was in grade school in Illinois 50 years ago. We did it the same way plant the seed potatoes at the bottom of a shallow trench, and once they sprouted up, start filling the trench and mounding up to cover the seed potatoes to a greater depth. We always had a good crop of potatoes in Illinois and we didn't even try very hard.

I have lived in Tennessee 35 years and I have never been able to grow a good crop of potatoes here. I am determined to learn the reason why. Soil here is pretty bad lots of clay in the top soil and if you dig down about 5 to 6 inches you hit some clay that looks a lot like pottery clay. In the rainy seasons the clay is soft and in the dry season it is hard as cement.

A few years ago I did an experement where I kept shoveling on soil about once a week to keep the potatoes covered up. Every time it rained it washed the soil away and I had to shovel it back. By the time the vines turned yellow and died the soil was hilled up about 12". I had a lot of potatoes that year but nothing larger than a golf ball.

This year I need to be a little more observant and keep notes. I have never used much fertilizer on potatoes so this year I am going to feed them well. This years crop of potatoes may cost $20 per lb.