Most people have seen earthworms on the ground after a strong rain. Worms even appear on sidewalks and parking lots when they are wet from a heavy rain. Some of these folks even wonder why earthworms come out of the ground after a storm.
The oxygen they need to survive enters through the worm’s skin. In fact, the earthworm’s outer skin must be moist, so the worm doesn’t suffer from wet ground. (They won’t survive in just water though.) Consequently, they leave the ground after a heavy rain or storm because they can move so easily through the wet earth. It may be true that worms leave the ground when the oxygen level reaches a certain point, but they can survive in very moist earth.
Worms also seem to know that they won’t dry out when they are laying on the surface during or after a rain. They won’t survive in the direct sun, so that’s why they don’t appear on hot, dry days. This is why they will not often be seen during the day. Some studies have speculated that worms also use this moist atmosphere to move around and find mates.
Close observation has shown that there are thousands of earthworms in each acre of ground. If the earth is sufficiently moist it may host up to 50,000 worms in one acre. Having this number of worms is a great benefit to the earth itself, to plants as well as to farmers and gardeners. Earthworms contribute to the health of the soil by bringing organic material in and leaving it as they move through the soil.
Worms eat and digest leaves and other vegetable material, leaving behind nutrients that are beneficial to natural plants and cultivated plants. These elements include phosphorus, calcium, nitrogen and so on. Earthworms are generally considered to be one of the major contributors to fertile soil. In soil that is sufficiently moist and provides a home for thousands of worms, the ground may have added tons of rich material.
Aside from the nutrients and amount of soil contributed by earthworms, these small and important animals also help aerate the soil. As they create burrows, the roots of trees and other plants are able to push through more easily. The soil is just naturally looser where earthworms are present.
One of the questions that scientists ask about earthworms and their habit of coming to the surface during and after rains is this: How do they know? Worms have no eyes but they are very sensitive to the feel of soil and to the air when they come to the surface. They have bristles on their outer skin that help them feel and move in the ground.