Summative Practice Chapter 8 – Question 9 (Form 5 Biology Textbook Exercise and Answer)

Question 9:
You are given a plot of land and are advised to plant the plants in Photograph 4.
Explain how you will use the plants to keep your plot of land fertile.

• Crop rotation is an agriculturally sustainable method of planting different types of crops on the same piece of land.

• Legume plants are planted in rotation as this type of plant can put nitrates back into the soil thereby keeping it fertile for other types of plants.

• High amounts of nitrogen gas in the atmosphere cannot be used directly by plants.

• Nitrogen-fixing bacteria that live in the root nodules of legumes like Rhizobium sp. and free-living nitrogen-fixing bacteria in the soil such as Azotobacter sp. fixes the nitrogen from the atmosphere and changes it to ammonium ions (NH4+) via ammonification.

• Ammonium ions are converted into nitrite ions via nitrification by the nitrifying bacteria Nitrosomonas sp.

• Nitrite ions will be converted to nitrate ions by the nitrifying bacteria Nitrobacter sp.

• The nitrates will then be absorbed by plant roots and used to synthesise proteins.

• The roots of legume plants will remain in the soil after the yield has been harvested.

• Leafy green crops which have high nitrogen needs will benefit from the high nitrogen contents of the soil that has been bound there by the legume plants.

• Tuber crops require low amounts of nitrogen. This crop can be planted next after the leafy green crops have been harvested.

• The cycle can be repeated over and over again.

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