(THE CONVERSATION) Most of us don’t think much about soil, let alone its health. But as Earth Day approaches, it’s time to recommend some skin care for Mother Nature. Restoring soil fertility is one of humanity’s best options for making progress on three daunting challenges: Feeding everyone, weathering climate change and conserving biodiversity.

Widespread mechanization and adoption of chemical fertilizers and pesticides revolutionized agriculture. But it took a hidden toll on the soil. Farmers around the world have already degraded and abandoned one-third of the world’s cropland. In the Unit...