At a Glance
Four facts to hold on to.
Real impacts
Soil acidification and nitrate leaching.
False claim
More nitrogen-fixing microbes.
Why
Fixers thrive only when nitrogen is scarce.
Downstream
Algal blooms and eutrophication.
In this lesson
What Excess Nitrogen Fertilizer Does
Not more microbes, and acid soil
- Excess nitrogen does not increase nitrogen-fixing microbes.
- It suppresses them. Plants stop relying on them when soil nitrogen is plentiful.
- Nitrification turns ammonium into nitrate and releases hydrogen ions.
- Over time this makes the soil more acidic.
Nitrate leaches away
- Nitrate dissolves easily in water.
- It does not bind well to soil particles.
- So extra nitrate leaches down into groundwater.
- Only two of the three classic claims are real: acidification and leaching.
- Source: US EPA on agriculture and nutrient pollution.
True or False: Excess nitrogen fertilizer increases the number of nitrogen-fixing microorganisms in the soil.
Why Nitrogen-Fixers Decline
Rhizobium needs scarcity
- Plants partner with Rhizobium to solve a shortage of usable nitrogen.
- The bacteria fix nitrogen. The plant feeds them sugars.
- Cheap fertilizer ends the shortage. The plant stops paying for the partnership.
- So the bacteria decline.
A downstream health risk
- Nitrate in drinking water is a recognised health concern.
- Infants are the main worry.
- Source: WHO drinking-water guidance.
True or False: Nitrogen-fixing bacteria like Rhizobium thrive best when soil nitrogen is scarce, not abundant.
Previous Year Questions
UPSC asked about nitrogen twice.
- UPSC 2019Agricultural soils release nitrogen oxides. Cattle release ammonia. The poultry industry releases reactive nitrogen compounds. All three are correct. View the question.
- UPSC 2015Only soil acidification and nitrate leaching are genuine impacts. The microbe-proliferation claim is false. View the question.
True or False: UPSC’s 2015 question confirmed that excess nitrogen fertilizer genuinely increases nitrogen-fixing microorganisms.
The Bigger Picture
Eutrophication, and India’s subsidy problem
- Nitrate reaching rivers and lakes can trigger algal blooms.
- This is eutrophication, a related effect of runoff.
- Cheap, subsidised urea has long encouraged over-application in India.
- Nitrogen leaves farms in other forms too. Soils release nitrogen oxides, and cattle urine breaks down into ammonia.
- Urea from urine meets the enzyme urease in dung and releases ammonia gas.
- Read about policy in the Green Revolution and crop policy.
Exam traps
- Excess nitrogen suppresses fixers. It does not feed them.
- Nitrate leaches. It binds poorly to soil.
- Nitrification acidifies soil.
- All three sources in UPSC 2019 were correct.
Think it through: why does excess nitrogen suppress nitrogen-fixing microbes instead of feeding them?
Rhizobium exists because usable nitrogen is scarce. The plant pays the bacteria with sugars for nitrogen. Once fertilizer floods the soil, the plant can absorb nitrogen directly. It scales back the partnership. The bacteria decline with it. The trap works because intuition says more nitrogen should help them.
True or False: Nitrate reaching rivers and lakes can trigger algal blooms, a downstream consequence of nitrogen runoff.
True or False: India’s heavily subsidised urea pricing has discouraged farmers from over-applying nitrogen fertilizer.
Quick Q&A
Does excess N feed fixers?
No. It suppresses them.
What acidifies the soil?
Nitrification.
Why does nitrate leach?
It dissolves easily and binds poorly.
What do cattle release?
Ammonia.
What does nitrate runoff cause?
Algal blooms.
🎲 Practice: Excess Nitrogenous Fertilizer
7 verified questions from this topic.
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