March is blustery days and bright sunshine and rain and the smell of warming soil. It seems a magical thing—soil is always there, so how come the right combination of sun and rain and warmth lets us know that it’s time once again to garden? The technical name for the way the soil smells is geosmin—literally earth smell, but magic is close enough. Geosmin is a magic all gardeners are well aware of and accept without question, no matter what you call it. In reality, the smell is caused by a type of bacteria that grows in the soil, called...