Basic 9 · Science · Earth Science
The Nitrogen Cycle
In this topic you will explain the process of the nitrogen cycle as a repeated pattern in nature, and describe the importance of the nitrogen cycle to the environment.
By the end of these notes you should be able to:
- Identify the stages of the nitrogen cycle
- Explain the role of bacteria
- Describe the importance of the cycle
What you already know
- Carbon cycle
- Proteins in food
Copy everything that has a blue line on its left into your notebook. The other parts are for reading.
Lesson 1
- Strand
- Cycles
- Sub-strand
- Earth Science
- Indicator
- B9.2.1.1.1 – Explain the process of the nitrogen cycle as a repeated pattern in nature
Introduction
The air around you is mostly nitrogen, but you cannot use a single breath of it to build your body.
1. Why Nitrogen Matters
Nitrogen makes up about 78% of the air. All living things need nitrogen to make proteins and DNA. However, nitrogen gas is very unreactive, and plants and animals cannot use it directly. It must first be changed into compounds such as nitrates. The nitrogen cycle describes how nitrogen moves between air, soil and living things.
2. Stages
| Stage | What happens | Agent |
|---|---|---|
| fixation | nitrogen gas changes into ammonia or nitrates | bacteria, lightning, factories |
| nitrification | ammonia changes into nitrites, then nitrates | nitrifying bacteria |
| assimilation | plants absorb nitrates and make proteins | plant roots |
| consumption | animals eat plants | food chains |
| decomposition | dead matter is turned into ammonia | decomposers |
| denitrification | nitrates change back to nitrogen gas | denitrifying bacteria |
3. Following the Cycle
- Nitrogen in the air is fixed by bacteria or lightning.
- Soil bacteria convert it to nitrates.
- Plants take up nitrates.
- Animals eat the plants.
- Waste and dead matter return nitrogen to the soil.
- Other bacteria release nitrogen gas back into the air.
Most of this work is done by bacteria that you cannot see. Without them, plants would run out of usable nitrogen, and life would slowly stop.
4. Legumes
Beans, groundnuts, cowpeas and soya beans are legumes. Their roots have small swellings, called nodules, containing Rhizobium bacteria. These bacteria fix nitrogen from the air, and in exchange the plant gives them food. This partnership is an example of symbiosis. When legumes die, they enrich the soil. Farmers therefore rotate legumes with crops such as maize. Pull up a groundnut plant, and you may see the nodules for yourself.
5. Lightning
Lightning provides enormous heat that makes nitrogen and oxygen in the air react. The nitrogen oxides formed dissolve in rain and reach the soil. Thus thunderstorms add a little natural fertiliser to the ground, although bacteria do much more of this work.
New words
- nitrogen fixation
- Changing nitrogen gas into compounds.
- nitrate
- A nitrogen compound that plants can use.
- nodule
- A swelling on a legume root.
- denitrification
- Nitrates changing to nitrogen gas.
Summary
- Nitrogen forms 78% of the air but must be fixed.
- Bacteria drive most stages of the cycle.
- Legumes host nitrogen-fixing bacteria.
Class activity: Nodule Search
- Dig up a legume plant.
- Look for root nodules.
- Draw what you see.
You need: Cowpea plant with root nodules, Soil sample, Printed nitrogen-cycle diagram
Exercises
Answer in your exercise book first, then check your answers.
1. What percentage of air is nitrogen?
- A. 78%
- B. 21%
- C. 50%
- D. 1%
Show answer
Answer: 78%Air.2. Which plant has nodules?
- A. groundnut
- B. maize
- C. rice
- D. cassava
Show answer
Answer: groundnutLegume.3. Plants can absorb nitrates.
- A. True
- B. False
Show answer
Answer: TrueAssimilation.4. Changing nitrogen gas into compounds is nitrogen ______.
Show answer
Answer: fixationTerm.5. State three stages of the nitrogen cycle.
Show answer
Answer: Fixation, nitrification and denitrification.Any three.6. Why do farmers rotate maize with beans?
Show answer
Answer: Beans enrich the soil with nitrogen for the maize.Application.
Lesson 2
- Strand
- Cycles
- Sub-strand
- Earth Science
- Indicator
- B9.2.1.1.1 – Explain the process of the nitrogen cycle as a repeated pattern in nature
Introduction
Nitrogen is never used up. Like water and carbon, it just keeps going round.
1. Cycles in Nature
| Cycle | What moves | Main store |
|---|---|---|
| water cycle | water | oceans |
| carbon cycle | carbon | rocks, oceans |
| nitrogen cycle | nitrogen | atmosphere |
Nature has a limited supply of matter. It is reused again and again. The nitrogen in your muscles may once have been in a cow, a groundnut or a dinosaur. This is the meaning of a repeated pattern in nature: matter cycles through living and non-living parts of the environment.
2. Living and Non-Living Parts
In the nitrogen cycle, the non-living parts are the air, soil and water. The living parts are bacteria, plants and animals. Each depends on the others. If the bacteria were lost, the whole cycle would stop. Similarly, if soil is poisoned by chemicals, the bacteria may die, reducing soil fertility. Good farming looks after the living soil.
3. Sample Cycle
Tracing nitrogen
Trace a nitrogen atom from the air to a child and back to the air.
- Bacteria in groundnut roots fix it from the air.
- The groundnut plant turns it into protein.
- A child eats the groundnuts.
- Waste is broken down by decomposers into ammonia.
- Bacteria change this to nitrates, and others to nitrogen gas.
Answer: It returns to the air, and the cycle starts again
4. Farming and the Cycle
| Practice | Link to the cycle |
|---|---|
| crop rotation | legumes add nitrogen |
| intercropping | maize grows with cowpeas |
| compost | decomposers return nitrogen |
| fallow | soil recovers |
| manure | adds nitrogen compounds |
Each harvest removes nitrogen from the soil. If nothing is returned, the soil becomes poor, and yields fall. Traditional farmers solved this using fallow land and mixed crops. Today, with less land, many farmers use fertilisers. Using compost and manure returns nitrogen to the soil and makes use of waste.
5. Human Effects
Burning fuels adds nitrogen oxides to the air, causing smog and acid rain. Too much fertiliser washes into rivers. Sewage adds nitrogen as well. These changes can damage the cycle. A healthy cycle depends upon careful use of fertilisers and on protecting rivers and soils.
New words
- crop rotation
- Growing different crops in turn.
- intercropping
- Growing crops together.
- compost
- Decayed plant and animal matter.
- fertility
- The ability of soil to support growth.
Summary
- Nitrogen cycles through air, soil and living things.
- Farming practices can restore nitrogen.
- Human activity can disturb the cycle.
Class activity: Cycle Chain
- Make a paper chain with one stage on each link.
- Join it into a circle.
- Explain each stage.
You need: Cowpea plant with root nodules, Soil sample, Printed nitrogen-cycle diagram
Exercises
Answer in your exercise book first, then check your answers.
1. Crop rotation means…
- A. growing different crops in turn
- B. burning crops
- C. selling crops
- D. watering crops
Show answer
Answer: growing different crops in turnTerm.2. Which adds nitrogen to soil?
- A. compost
- B. plastic
- C. sand
- D. glass
Show answer
Answer: compostPractice.3. Matter cycles in nature.
- A. True
- B. False
Show answer
Answer: TruePattern.4. Growing crops together is ______.
Show answer
Answer: intercroppingTerm.5. Name two living parts of the nitrogen cycle.
Show answer
Answer: Bacteria and plants.Any two.6. Why do harvests reduce soil fertility?
Show answer
Answer: They remove nitrogen, and it must be replaced.Application.
Lesson 3
- Strand
- Cycles
- Sub-strand
- Earth Science
- Indicator
- B9.2.1.1.2 – Describe the importance of the nitrogen cycle to the environment
Introduction
Without the nitrogen cycle, there would be no crops, no meat and no life.
1. Importance
| Benefit | Explanation |
|---|---|
| makes proteins possible | all life needs nitrogen |
| supports crops | nitrates feed plants |
| recycles waste | decomposers release nutrients |
| keeps the air balanced | denitrification returns nitrogen gas |
| reduces fertiliser needs | natural fixation is free |
2. Food Security
The world's food supply depends upon nitrogen. In 1909, German scientists discovered how to fix nitrogen in factories. This is the Haber process, and it makes ammonia for fertilisers. Much of the food that the world eats is now grown with such fertilisers. In Ghana, the Planting for Food and Jobs programme has helped farmers to get fertiliser. But fertilisers are costly, so natural methods remain important.
3. Problems of Excess Nitrogen
Too much of a good thing causes harm. When fertiliser washes into rivers and lakes, it feeds algae. The algae multiply fast, forming a green layer. When they die, bacteria feed on them and use up the oxygen, and fish die. This is called eutrophication. Too many nitrates in drinking water are also a health risk for babies.
4. Using Fertiliser Wisely
- Test the soil.
- Apply the correct amount.
- Apply at the right time.
- Do not apply before heavy rain.
- Combine with compost.
- Keep fertiliser away from streams.
5. Careers and Research
Soil scientists, agronomists and extension officers help farmers to make wise use of nitrogen. Ghana's Soil Research Institute (CSIR) studies the country's soils. In the future, scientists hope to develop crops that can fix their own nitrogen, as legumes do. Understanding this cycle could help to feed the world with less pollution. It is one of the many ways in which science can improve the lives of farmers.
New words
- Haber process
- The industrial production of ammonia.
- eutrophication
- Water harmed by excess nutrients.
- algae
- Simple plants living in water.
- fertiliser
- A substance that feeds plants.
Summary
- The nitrogen cycle makes life and farming possible.
- Fertilisers increase food supply.
- Excess nitrogen harms rivers.
Class activity: Fertiliser Debate
- Debate chemical fertiliser and compost.
- Give evidence.
- Agree on advice for farmers.
You need: Uprooted cowpea or groundnut plants with root nodules, Hand lens, Local soil samples
Exercises
Answer in your exercise book first, then check your answers.
1. Ammonia is made industrially by the…
- A. Haber process
- B. water cycle
- C. photosynthesis
- D. fermentation
Show answer
Answer: Haber processTerm.2. Excess fertiliser in rivers causes…
- A. eutrophication
- B. evaporation
- C. erosion
- D. condensation
Show answer
Answer: eutrophicationTerm.3. Nitrogen is needed to make proteins.
- A. True
- B. False
Show answer
Answer: TrueLife.4. A substance that feeds plants is a ______.
Show answer
Answer: fertiliserTerm.5. State three benefits of the nitrogen cycle.
Show answer
Answer: It supports crops, recycles waste and makes proteins possible.Any three.6. How can farmers reduce the harm from fertiliser?
Show answer
Answer: Apply the correct amount at the right time and avoid heavy rain.Advice.
Homework
- Ask a farmer how they feed their soil, and write down what you learn.