Decomposition and composting

Ecosystems and material cycles · Indicator species and decomposition · note 3 of 4

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Decomposition and compostingSpec 9.18B

In short

Composting uses decomposers to break down plant material and waste into compost, which returns nutrients to the soil. Decomposition is fastest when the heap is warm, because decomposer enzymes work faster up to an optimum; moist, because decomposers need water; and well aerated by turning, so decomposers can respire aerobically.

Composting uses decomposers to break down plant material and waste into compost, which returns nutrients to the soil. Here the aim is to make decomposition fast, so the conditions are the opposite of food preservation.

Temperature: warm, because the enzymes of decomposers work faster as temperature rises, up to an optimum. Very high temperatures denature enzymes.
Water content: moist, because decomposers need water. Too dry slows decomposition, and waterlogged material has little air.
Oxygen availability: well aerated, for example by turning the heap, so that decomposers can respire aerobically. With little oxygen decomposition is much slower.

Many gardeners turn the compost heap regularly and keep it moist but not soaked.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

What are indicator species?

Higher tier Indicator species are organisms whose presence or absence gives evidence of the level of pollution in an area. In water, bloodworms and sludgeworms suggest pollution, while freshwater shrimps and stonefly nymphs suggest clean water with plenty of oxygen. For air, the species of lichen present and blackspot fungus on roses show the level of sulfur dioxide.

How does food preservation slow decay?

Food preservation slows decay by making conditions poor for microorganisms. Fridges and freezers keep food cold, so enzymes in the microorganisms work more slowly. Drying, or adding salt or sugar, removes water, which microorganisms need to grow. Vacuum packing and sealed cans keep out oxygen, which most decomposers need for aerobic respiration.

What conditions make compost decompose fastest?

Compost decomposes fastest when it is warm, moist and well aerated. Warmth speeds up the decomposers' enzymes up to an optimum, although very high temperatures denature them. Decomposers need water, but waterlogged material has little air. Turning the heap provides oxygen, so the decomposers can respire aerobically and decomposition is faster.

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