A sunbaked particle of protein may have no bacterial action - no scent. After a light rain or morning dew, it may be rejuvenated. When the subject's feet step on vegetation or scuff the dirt, new bacteria is turned up in the soil and cells are crushed in the plants. The cells begin to ferment and the ground smells different. A tracking dog can work these vegetable and soil scents the same as a trailing dog does the human scent particles.
Many dogs do both.
Some search dogs are cross-trained to air scent, trail and track. The Dog A search dog has 44 times more olfactory sensory cells than a human. He's a super sniffer. A dog's olfactory lobes take up nearly one-eighth of his brain. He's scent smart.