What makes dead leaves and wood rot? Decomposers do. These living things break down dead leaves, wood and other dead things. Fungi and bacteria are decomposers, and they get to work when a plant or another living thing dies. Without them, dead leaves and wood would pile up on the ground. They also turn dead things back into food for new plants.

Rot happens in steps, and the size of the bits matters too. Bugs and worms chew dead plants into small bits first. Then tiny fungi and bacteria finish the job. Earthworms, snails and beetles help break down dead plants too. Cutting scraps into small pieces makes them rot faster, because microbes have more surface to work on.

Fungi are not plants. A fungus has no green chlorophyll, so it cannot make its own food from sunlight. Here is how fungi break down wood. They grow thin threads called hyphae into wood and leaves. Together the threads are called mycelium. They make chemicals called enzymes. The enzymes break down wood, and then the fungus takes in the nutrients. Some mushrooms grow on dead logs and stumps. They are decomposer fungi.

The weather changes how fast things rot. Decomposers work best when it is warm and wet. In warm, wet places like tropical rainforests, dead things decay very fast. Cold goes the other way. It slows down the tiny living things that spoil food, which is why we keep food in the fridge. Cold slows mould but does not stop it.

Compost is what decomposers make from old leaves and food scraps. It is dark, crumbly and smells like fresh soil. You can compost fruit and vegetable scraps and dry leaves. Cheese and greasy food should not go in a home compost pile, because they can attract animals. A good pile feels like a wrung-out sponge: damp, but not dripping.

The middle of a compost pile gets warm because the decomposers in it are busy. In a well-kept pile, compost is ready in about three to five months. Left alone, a pile can take a year or more. Turning a pile with a fork lets in air and speeds up decomposition. A pile that smells like rotten eggs is too wet or short of air. Adding dry leaves and turning it helps.

Healthy compost smells earthy, like fresh soil. It helps soil hold water and feeds plants as it releases nutrients slowly. Earthworms leave behind nutrient-rich castings that plants can use. Whenever things rot, nutrients go back into the soil, and living plants use them to grow. So rot is not the end of a leaf. It is food for what grows next.

Words to know

• decomposers: break things down
• enzymes: special chemicals
• hyphae: fungus threads
• microbes: tiny living things
• nutrients: food for plants

The short version

Things rot because decomposers break them down. Fungi and bacteria get to work when a plant dies, and bugs and worms chew dead plants into small bits first. Warm, wet places speed up rot, while cold slows it. Rot puts nutrients back in the soil for new plants.

Quick facts

• Fungi make enzymes that break down wood, and then the fungus takes in the nutrients.
• In a well-kept pile, compost is ready in about three to five months. Left alone, a pile can take a year or more.
• Cutting scraps into small pieces makes them rot faster, because microbes have more surface to work on.
• In warm, wet places like tropical rainforests, dead things decay very fast.