Learning objectives
- Draw and label the nitrogen cycle
- Name the bacteria responsible for each step
- Explain why legumes improve soil fertility
1. The four processes and their bacteria
Nitrogen fixation converts N2 into ammonia — biologically by Rhizobium in legume root nodules and free-living Azotobacter and Clostridium, physically by lightning, and industrially by the Haber process. Ammonification is the release of ammonia when decomposers break down dead matter and urea.
Nitrification is a two-step oxidation: Nitrosomonas converts ammonia to nitrite, then Nitrobacter converts nitrite to nitrate — the form roots absorb. Denitrification by Pseudomonas in waterlogged, airless soil reverses this, returning nitrogen gas to the atmosphere and wasting fertiliser.
- Rhizobium — symbiotic fixation in legume nodules
- Nitrosomonas — ammonia to nitrite
- Nitrobacter — nitrite to nitrate
- Pseudomonas — denitrification, a fertility loss
2. Legumes and the farmer
Rhizobium lives in nodules on the roots of cowpea, groundnut, soybean, beans and clover, receiving sugars and returning fixed nitrogen. A well-nodulated legume can add 40-150 kg of nitrogen per hectare per season, which is why legumes are placed in every rotation and why nodules must be pink inside — pink indicates active leghaemoglobin and real fixation.
3. Deficiency, excess and diagnosis
Nitrogen deficiency gives uniform yellowing (chlorosis) of the older, lower leaves first, because nitrogen is mobile and is moved to new growth. Excess nitrogen produces dark, soft, leafy growth that lodges and is far more susceptible to blights, rusts and aphids — a nutrition problem that shows up first as a disease problem.
Key definitions
- Nitrogen fixation
- Conversion of atmospheric nitrogen gas into nitrogen compounds usable by plants.
- Nitrification
- Bacterial oxidation of ammonia to nitrite and then to nitrate in the soil.
- Symbiosis
- A close association between two organisms in which both partners benefit.
Exam tips
- Name the specific bacterium for each step — generic answers such as 'soil bacteria' score no marks.
- In diagrams, draw arrows in one consistent direction and label each arrow with the process, not just the substance.
Practice exam questions & model answers
Q1.Explain how legumes improve soil fertility.
Legume roots form nodules containing symbiotic Rhizobium bacteria that fix atmospheric nitrogen into ammonia, which is converted to nitrate. When the nodules and crop residues decay, this nitrogen is released into the soil, raising fertility for the following crop.
Q2.State two farm practices that reduce nitrogen loss from soil.
Improve drainage and avoid waterlogging to limit denitrification, and split-apply nitrogen fertiliser (or use cover crops and mulching) to reduce leaching of nitrate below the root zone.
Q3.Describe the symptoms of nitrogen deficiency in a maize crop.
Older, lower leaves turn pale green then yellow, starting as a V-shaped yellowing from the leaf tip along the midrib; plants are stunted with thin stems, and cobs are small and poorly filled.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.