Think about a leaf that has fallen from a tree and died. What happens to it next? The answer is a crew of living things called decomposers. Decomposers break down dead leaves, wood and other dead things. Their work matters more than it looks. Without decomposers, dead leaves and wood would pile up on the ground. Let's follow the crew and see how they get the job done.

The crew has more than one kind of worker. Earthworms, snails, millipedes and beetles help break down dead plants. The first stage belongs to bugs and worms, which chew the dead plants into small bits. The second stage belongs to tiny fungi and bacteria, which finish what is left. Think of a relay race, where the bigger chewers run the first part and the tiniest workers carry the job over the finish line.

Scientists have a name for the fungi and bacteria that feed on dead things. They call them saprotrophs. It is a mouthful, but the idea is simple. A dead leaf is the food, and the saprotrophs are the ones eating. Now when you read that word, you know what it means. Try saying it slowly, one chunk at a time. It helps to remember that fungi and bacteria are tiny, and that each one is feeding on something that has already died.

Here is something the crew cannot do without: air. Composting needs oxygen. Decomposition with oxygen is called aerobic. That is why people who make compost give their piles a turn. Turning a compost pile with a fork lets in air and speeds up decomposition. A pile is not just a heap to leave alone. Air is part of how the job gets done.

Decomposers give something back to the air. As they break down dead plants, they release carbon dioxide. Plants then use that carbon dioxide to make food. Follow the trail and you can see a loop. A plant dies, decomposers go to work, and the carbon dioxide they release helps living plants make food.

Compost is rot that people help along, and it has a recipe. Compost has two kinds of ingredients. Carbon-rich "browns," like dry leaves, are one kind. Nitrogen-rich "greens," like food scraps, are the other. The microbes in compost need a balance of carbon and nitrogen. Carbon gives them energy, and nitrogen gives them what they need for growth.

How can you tell if a compost pile is healthy? Use your hands and your nose. Squeeze a handful and it should feel damp, like a sponge that has been wrung out, with no drips. Healthy compost has an earthy smell, like fresh soil. If the smell is more like rotten eggs, the pile is too wet or short of air, and a few dry leaves and a turn with a fork can help.

Size matters, too. Small pieces rot faster than big ones, because there is more surface for microbes to work on. Picture one big block and the same amount cut into many small pieces. The small pieces give the workers more outside to reach, so the job goes faster. That is why people who make compost cut their scraps small.

Can a heap of old leaves and scraps get hot? A compost pile can. A pile that gets to 130 to 135 degrees F kills most weed seeds and germs. Those two numbers, 130 and 135 degrees F, are the heat at which most weed seeds and germs are killed.

Here is why the whole crew matters. Suppose nothing ever broke down dead plants and animals. The nitrogen in them would stay locked inside. Decomposers set it free for new plants. Every leaf that rots is a small act of unlocking. What was locked inside the dead leaf becomes something new plants can use.

Words to know

• decomposers: dead-thing breakers
• saprotrophs: dead-thing feeders
• aerobic: with oxygen
• microbes: tiny living things