Decomposition
Decomposers break detritus into CO₂, water and nutrients through fragmentation, leaching, catabolism, humification and mineralisation; warmth, moisture and oxygen speed it up.
What is decomposition?
Earthworms are called the farmer's friend because they break down dead matter and loosen the soil. Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. This process is called decomposition. Its raw material is detritus: dead plant remains such as leaves, bark and flowers, and dead animal remains, including faecal matter.
The steps of decomposition
Fragmentation, leaching and catabolism go on **at the same time** on the detritus. Humification and mineralisation take place in the soil.
- 1Fragmentation: detritivores such as earthworms break detritus into smaller particles.
- 2Leaching: water-soluble inorganic nutrients move down into the soil and get precipitated as unavailable salts.
- 3Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
- 4★Humification: a dark, amorphous substance called humus builds up. It is highly resistant to microbial action and decomposes extremely slowly. Being colloidal, it acts as a reservoir of nutrients.
- 5★Mineralisation: some microbes degrade humus further and release inorganic nutrients into the soil.
★ key step
Decomposition at a glance
Leaching runs alongside: soluble nutrients wash down the soil and form unavailable salts.
Factors that control the rate of decomposition
| Factor | Speeds it up | Slows it down |
|---|---|---|
| Chemical nature of detritus | Rich in nitrogen and water-soluble substances like sugars | Rich in lignin (wood) and chitin (insect shells) |
| Temperature | Warm conditions | Low temperature |
| Soil moisture | Moist soil | Very dry soil |
| Oxygen | Aerobic conditions: decomposition largely needs oxygen | Anaerobiosis (no oxygen), so organic matter builds up |
In vermicomposting, earthworms turn kitchen and farm waste into rich compost within weeks; many Indian farmers and city homes now do this. Fallen leaves rot fast in the warm, wet monsoon, but a dead tree trunk, rich in lignin, takes years.
2 marks: “What is humus? Why is it important for the soil?” A dark, amorphous, colloidal substance formed by humification. It resists microbial action and decays very slowly, so it acts as a reservoir of nutrients for the soil.
2 marks: “Why does decomposition proceed slowly in cold regions and in waterlogged soil?” Low temperature and lack of oxygen (anaerobiosis) slow down the soil microbes, so organic matter builds up.
5 marks: “Describe the steps of decomposition. Name the factors that control its rate.” Define decomposition and detritus; fragmentation by detritivores; leaching; catabolism by enzymes; humification forming humus; mineralisation. Factors: chemical nature (lignin and chitin slow it, nitrogen and sugars speed it), temperature, moisture and oxygen.
Farmers Love Compost Heaps Most: Fragmentation, Leaching, Catabolism, Humification, Mineralisation.