Sweet potatoes are good for the soil. All summer long their vines and heart shaped leaves keep the soil covered. The mulch below the vines breaks down in cool comfort. The swelling potatoes gently move the soil repairing any compaction issues that existed before them. I find the soil below the potato vines to be black, well aggregated (lumpy) and loaded with worm castings. The weeds that are coming up around my transplants are easy to slip out of the soil where the potatoes were. The new crops are growing as if all of their soil needs are being met, and the soil is holding moisture much longer than the native soil.