Why Do Farmers Add Fertiliser?
🌾 Why Do Farmers Add Fertiliser?
Walk past a farm in spring and you might notice a spreading machine driving back and forth, spraying the fields before planting. The farmer is not watering the crops — they are adding nitrogen. Nitrogen is the single most important limiting factor for plant growth. Without enough of it, plants cannot make proteins or DNA, growth stalls, and leaves turn pale yellow.
But here is the paradox: nitrogen makes up 78% of the air we breathe. There is nitrogen absolutely everywhere — yet plants cannot use a single molecule of it directly. Atmospheric nitrogen (N₂) is a pair of atoms locked together with one of the strongest chemical bonds in nature. Plants need nitrogen in the form of nitrates (NO₃⁻), dissolved in soil water. The gap between "nitrogen everywhere in the air" and "nitrogen available to plants in the soil" is bridged by some of the most important microorganisms on the planet. This is the nitrogen cycle.
Biology glossary
- What is ammonification?
- The process by which decomposers (bacteria and fungi) break down nitrogen-containing molecules (proteins and DNA) in dead organisms and waste products, releasing ammonia into the soil.
- What is nitrogen fixation?
- The conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) by nitrogen-fixing bacteria. This is the entry point of atmospheric nitrogen into the food chain.
The nitrogen cycle describes how nitrogen atoms move between the atmosphere, soil, living organisms, and back to the atmosphere. Unlike the carbon cycle, which is driven largely by photosynthesis and respiration, the nitrogen cycle depends almost entirely on bacteria performing chemical transformations that no other or
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🧠 Memory Aids
The four types of bacteria — "FNAD" (Fix, Nitrify, Ammonify, Denitrify):
- F — Fix: Nitrogen-fixing bacteria convert N₂ → ammonia
- N — Nitrify: Nitrifying bacteria convert ammonia → nitrites → nitrates
- A — Ammonify: Decomposers convert dead proteins → ammonia
- D — Denitrify: Denitrifying bacteria convert nitrates → N₂
Remember: "FNAD sounds like 'FED' — bacteria FEED the cycle."
Nitrifying vs Denitrifying — which one is "good" for farmers?
Nitrifying bacteria = NIce for farming (they make NItrates)
Denitrifying bacteria = DEstructive for farming (they DEstroy nitrates)
The conditions trick — aerobic vs anaerobic:
- Nitrifying bacteria need AIR (aerobic) → well-drained soil
- Denitrifying bacteria hate AIR (anaerobic) → waterlogged soil
Legume mnemonic — "Beans Put Nitrogen in Soil" (BPNS):
- Beans (and peas, clover)
- Partner with Rhizobium bacteria
- Nitrogen fixed in root nodules
- Soil enriched with nitrates after decomposition
What nitrogen is used for in organisms — "PAD-C":
- Proteins (amino acids)
- Amino acids (building blocks of proteins)
- DNA (nucleotides contain nitrogen)
- Chlorophyll (contains a nitrogen ring)
Now try it yourself
Quiz · Question 1 of 15
What percentage of the atmosphere is made up of nitrogen gas (N₂)?
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This topic in real past papers
Every real exam question we've found on the nitrogen cycle, with a full worked answer.