This may occur under the black plastic, as well. Many tree companies have stump removers that saw below the soil line. The old soil line, that once stumped you ,can now have a sense of humus. The most thorough job is an extraction with a bulldozer. Todd, Waterford, CT Any plant benefits with elevated rows , especially vine type plants.
Rows make it easier to cultivate and keep weeds at a minimum.
Weeds attract insects. It is far easier to harvests in rows. Ipomoea batatas is more easily grown in zone 6 in the past years but still risky. They like warm sandy soil. Use a low nitrogen fertilizer.