Almost every tomato plant, pansy, or succulent you’ve ever purchased first sprouted in something called potting soil, a concoction that often contains neither soil nor compost. Instead, all those bags at the nearest nursery are dirt-less sterile blends of exotic mosses, fibers, and minerals, ingredients that hide lung disease, water waste, and a whopping carbon footprint.

So why use it at all? Demand for potting soil in the United States has exploded; the number of urban gardeners has risen 30 percent in the last 30 years, and nurseries and greenhouses are the two fastest growing agricultural industries. Some lucky gardeners can grow directly in the ground, but dirt is too heavy and becomes too compacted for raised beds and