This year I placed the potato eyes, Onion sets and garlic on the surface of the soil then I covered them all with 6" of potting coil.
I planted White Potatoes and Red Potatoes, in the same row in 2 sections. I got some very nice size potatoes even though it do not rain for 3 months and the temperature was in the 100s for most of the spring and summer. The potatoes must have cross pollinated some of the while potatoes have red streaks running through the white potato.
I planted red, white and yellow potatoes in the same row in 3 sections. They did great too with no rain and 100 degree temperatures for 3 months.
Garlic was planted last year about Sept. I got about 100 very nice size garlics. Wow, I as set for garlic for a very long time.
I bought 1 scoop of what they call potting soil at the garden center. They claim it is left over soil from mushroom farms mixed with sand and soil $35 per load about 1500 lbs in a pickup truck bed.
None of my crop was as large as what can be bought at the grocery store but it is the best I have ever grown in Tennessee.