Wondering if your garden's soil is healthy? Check for earthworms. The more earthworms, the healthier the soil. Earthworms create tiny tunnels in the soil, which allow air and moisture to pass through it and create a healthy growing environment for plants. They also create "castings" (erm, worm poop), which add needed nutrients. To encourage worms in your garden, simply add additional organic matter, such as mulch, compost, or dead plant matter.