This lesson is part of a series called, Too Much? Too Little? created to introduce middle school students to the connection between soil nutrients and the food they eat. The lessons consist of a series of demonstrations and hands-on experiments that show that plants require nutrients in certain quantities. The lesson series allows students to investigate soil properties, learn how to properly prepare fertilizer nutrient solutions, identify deficiencies in plant nutrients using a key, and much more. Other related lessons include:

• Plant Parts and Functions: Anatomy and physiology of a plant.
• Digging Into Nutrients: How plants obtain nutrients from the soil.
• The Right Solution: How fertilizer solution is properly calculated and applied.
• Can We Have Too Much of a Good Thing?: Effects of applying too much or too little fertilizer.
• The Right Diet for Your Plants: Read fertilizer labels and choose the best fertilizer.
• Let's Vote On It: How soil nutrients effect local communities and economies.
• It's a Dirty Job: How earthworms benefit soil.

Although they often go unnoticed, earthworms play a significant role in our lives. As decomposers, worms have the important job of recycling waste material into nutrients that plants can use to grow and provide food for people and other animals. As worms burrow through the soil, they ingest soil particles and obtain nutrition by digesting any dead plant or animal remains, fungi, and bacteria in the soil. Through this process, the worms mix up the soil, providing air spaces for roots and water to penetrate. Worm excrement, or castings, provide excellent fertilizer. This is why many home gardeners put worms into their compost piles.

If you have ever held an earthworm, you probably noticed that it was moist. Earthworms do not have lungs, they breathe through their skin, which must be moist in order for oxygen to dissolve and pass into their bloodstream. Earthworms need a cool, dark, moist environment. They do not have eyes but do have special receptor cells to sense ground vibrations and light. The rings on an earthworm’s body are called annuli. Tiny bristles called setae help the worms pull themselves through the soil. The lighter colored “bump” in the middle of the earthworm is the clitellum, the structure responsible for secreting mucous required for the worm to breathe, reproduce, and for protection as the worm burrows through rough soil. Worms have a special organ called a gizzard to help them grind up and digest the soil that they eat.

Earthworms come in a variety of sizes. The largest known earthworm species in Australia can reach nearly ten feet in length! Some worms live close to the surface of the soil, while others burrow six or seven feet deep.

Earthworms can be classified into three main groups based upon the soil areas where they prefer to eat and burrow.

• The worms you find in compost piles are in the Epigeic group. Epigeic earthworms live in plant litter near the surface of the soil and need a lot of decaying organic matter to feed on. These worms are relatively small and can tolerate living in conditions with many other worms. This makes them well suited for composting.
• Endogeic earthworms live in the upper-to mid-soil layers and feed on soil and decomposing organic matter. Endogeic earthworms don’t have permanent burrows.
• Anecic earthworms are deep burrowing and have permanent burrows that can be spotted by little piles of worm castings at the soil surface. These worms are large and are often referred to as “night crawlers.” Anecic earthworms feed on decaying vegetation that they pull into their burrows from the topsoil.

Earthworms are hermaphrodites, meaning they have both female and male reproductive parts. After mating, earthworms lay eggs inside a cocoon covering that hatches in about two or three weeks, but this can vary depending on species and weather conditions. Baby worms are called wormlets and look like tiny pieces of thread when newly hatched.

In addition to improving soil and decomposing waste, earthworms are important food for a variety of animals such as birds, snakes, amphibians, rodents, and other small animals.