The soil beneath our feet is densely filled with tiny forms of life that cannot be seen with the naked eye. Let us understand through observation the mysterious process by which fallen leaves disappear.
Basic Knowledge
Small organisms in soil(visible to the naked eye〜with a magnifying glass):
・Pill bugs・sow bugs(arthropods, crustaceans): Eat fallen leaves.
・Earthworms(annelids): “Tillers of the soil” that eat soil, improve aeration, and enrich it.
・Springtails・mites: Break fallen leaves into small pieces.
・Spiders・centipedes・millipedes: Consumers that eat other small animals.
These soil animals act as consumers and crush fallen leaves into small pieces.
Microorganisms(require a microscope):
・Fungi(molds・mushrooms): Spread hyphae over the surface of fallen leaves and decompose them.
・Bacteria(bacteria): The ultimate decomposers. There are several billion in 1g of soil.
Example experiment: Investigating the functions of microorganisms
① Prepare beaker A containing soil with fallen leaves and beaker B containing soil that has been heated to kill microorganisms.
② Add a starch solution to both.
③ Add iodine solution several days later.
Result: A does not change color(starch has been decomposed). B turns blue-violet(starch remains).
→ Evidence that microorganisms decomposed organic matter.
Soil animals（Small animals that live in soil, such as pill bugs and earthworms）
Microorganisms（Tiny organisms that require a microscope, such as fungi and bacteria）
Iodine solution（A reagent that turns blue-violet when it reacts with starch）
Control experiment（An experimental method that compares conditions by changing only one factor）
Leaf mold（Soil formed by the decomposition of fallen leaves; rich in nutrients）
Compost（Fertilizer made by allowing microorganisms to decompose organic matter）
Further Study (Background・Significance)
The process by which one fallen leaf returns to the soil proceeds as a relay: first, soil animals such as pill bugs and earthworms physically crush it(and it becomes even smaller as it passes through their digestive tracts); next, fungi decompose cellulose; finally, bacteria break it down into even smaller organic and inorganic substances.
Why forest soil is rich: Fallen leaves accumulate over hundreds of years, and microorganisms decompose them into leaf mold, allowing plants to grow. When forests are destroyed, the soil ecosystem is also damaged, and recovery takes a long time.
Uses of familiar microorganisms:
・Natto bacteria: Ferment soybeans to make natto
・Koji mold: Rice koji→sake・miso・soy sauce
・Lactic acid bacteria: Yogurt・cheese・pickles
・Yeast: Bread・beer・wine
・Blue mold: Production of penicillin(an antibiotic)
・Escherichia coli: Host for genetic recombination experiments
The decomposing action of organisms is also used in agricultural compost, food-waste processing, and purification at sewage treatment plants. This is a modern theme related to SDG 12(Responsible Consumption and Production).
- Soil animals=pill bugs・earthworms・springtails, etc.
- Microorganisms=fungi(molds)・bacteria
- Order of decomposition: soil animals crush→fungi→bacteria
- Control experiment: Compare with heated soil
- Iodine solution+starch=blue-violet color
- Familiar fermentation: natto・miso・yogurt・bread・alcoholic beverages
- Turn food waste into a resource with compost
Points to Note (Easily Confused)
① Soil animals(consumers) and microorganisms(decomposers) have different roles. ② Molds・mushrooms are not plants but fungi(they do not photosynthesize and obtain nutrients through decomposition). ③ In a control experiment, the key is to change “only one” condition. ④ Fermentation and decay: when useful to humans it is fermentation, and when harmful it is decay—the function of the microorganisms themselves is the same.
Practice
- Name one small animal in soil that physically crushes fallen leaves and dead organisms.
- In a control experiment investigating the function of microorganisms, why is one soil sample heated?
- Name the microorganism that functions when natto is made.