It sounds crazy, but by knowing how many earthworms are in your garden soil, you can quickly estimate your soil fertility.
The best time to check for earthworm activity is in the spring when the soil temperature has risen to a minimum of 50° F (10° C) and the soil at the surface is moist.
Simply take a trowel or a shovel and dig up an area of soil measuring about 12 inches (30 cm) square.
Put the cube of soil on the ground and break it apart while looking for earthworms.
If your soil is healthy, you'll find at least 10 earthworms per cubic foot (0.092 cubic meter).
If your soil has fewer than 10 worms per cubic foot (0.092 cubic meter) it's time to add more organic matter and work it into your soil.
You can repeat this technique in other areas of your garden such as your lawn or vegetable garden to help you provide a better growing medium for all your plants.