🚶Nitrogenous Fertilizers: Forms, Properties, and Slow-Release Technologies

Why Nitrogen Fertilizers Matter in Agriculture

A paddy farmer in Uttar Pradesh applies urea to his crop but finds that much of the nitrogen is lost — some evaporates as ammonia gas, some leaches below the root zone, and some escapes as N₂ gas through denitrification. Only 30-40% of applied nitrogen actually reaches the plant. Understanding which form of nitrogen to use, when to apply it, and how to reduce losses is the difference between a profitable harvest and a costly failure.

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Sources of Nitrogen in Nature

Maximum consumption of N and P is in the state of Uttar Pradesh.

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Basic Classification of Fertilizers

Before studying nitrogen fertilizers specifically, understand the general fertilizer categories:

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Classification of Nitrogen Fertilizers

Nitrogen fertilizers are classified by the chemical form of nitrogen present. Each form behaves differently in soil.

Quick Comparison: Forms of Nitrogen in Fertilizers

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Nitrate Fertilizers

Nitrate fertilizers contain nitrogen as NO₃^-. The nitrate ion is negatively charged, so it is not held by negatively charged soil colloids — making it highly mobile and prone to leaching.

Key properties:

• Readily available to plants (no conversion needed)
• Suitable for top dressing and dry soils
• Basic residual nature
• Losses: Leaching in wet soils, denitrification in waterlogged soils

Nitrate Fertilizer Examples

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Ammoniacal Fertilizers

Ammoniacal fertilizers contain nitrogen as NH₄⁺. The ammonium ion is positively charged, so it is adsorbed on clay colloids — reducing leaching.

Key properties:

• Less leaching than nitrate (held by soil colloids)
• Best for waterlogged soils (NH₄⁺ is stable under anaerobic conditions)
• Acidic residual nature
• Losses: Volatilization (especially on alkaline soils), denitrification after nitrification
• Except rice and potato, crops absorb N mainly as nitrate — so NH₄⁺ must first be converted

a. Ammonium Sulphate ((NH₄)₂SO₄)

• Contains 20.6% N and 24.5% Sulphur (highest S content among N-fertilizers) ^{UPPSC 2021}
• Best N-fertilizer for paddy and waterlogged soils (NH₄⁺ stays stable under anaerobic conditions)
• Also suited for tea, jute, groundnut, sugarcane
• White crystalline salt, hygroscopic, very soluble — quick-acting fertilizer
• Continuous use depletes soil calcium and increases acidity (needs periodic liming)
• Equivalent acidity: 110 meq/100g

Think of ammonium sulphate as the “rice farmer’s friend” — ideal for waterlogged paddy fields where nitrate fertilizers would be lost through denitrification.

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b. Ammonium Chloride (NH₄Cl)

• Contains 25.5% N (some sources: 26%)
• Extensively used for paddy in Japan
• Not recommended for: tomatoes, tobacco (chloride harms quality)
• Cereals are unaffected — Cl^- is mostly retained by straw, not translocated to grain
• For potato and sweet potato, use K₂SO₄ or KNO₃ instead (avoid high chloride)
• Equivalent acidity: 128 meq/100g

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c. Anhydrous Ammonia (NH₃)

• Contains 82% N — the highest nitrogen content among all fertilizers
• Liquid under pressure; used as fertigation (applied through irrigation water)
• Equivalent acidity: 148 meq/100g (highest among all N-fertilizers)

d. Aqueous Ammonia

• Contains 80% N; used as fertigation
• Most concentrated liquid nitrogen form

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Ammonium + Nitrate Fertilizers

These provide both forms of nitrogen: NO₃^- for immediate uptake and NH₄⁺ for sustained supply.

(i) Ammonium Nitrate (NH₄NO₃)

• Contains 33% N (16.5% as NH₄⁺ + 16.5% as NO₃^-)
• Most hygroscopic among all fertilizers (absorbs moisture rapidly from air)
• Explosive — requires careful storage and strict regulations ^{IBPS AFO 2012}
• Equivalent acidity: 60 meq/100g

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(ii) Calcium Ammonium Nitrate (CAN) — “Kishan Khad”

• Contains 26% N (half ammoniacal form) ^{IBPS AFO 2019}
• Neutral in reaction — does not change soil pH (suitable for all soil types)
• Most suitable for vegetables
• Made from ammonium nitrate + ground limestone (safer than pure ammonium nitrate, also supplies calcium)
• Standards: moisture < 1% by weight, calcium nitrate < 0.5% by weight

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(iii) Ammonium Sulphate Nitrate

• Contains 25.6% N (NH₄⁺: 19.5%, NO₃^-: 6.6%)
• Provides both immediate and sustained nitrogen supply

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Amide Fertilizers

Amide fertilizers are organic in form. Nitrogen must undergo a two-step conversion in soil before plants can use it:

Amide -> (NH₄)₂CO₃ (ammoniacal form) -> NO₃^- (nitrate form)

Urea (NH₂CONH₂) — India’s Most Important Fertilizer

• Most commonly used fertilizer in India (high N, low cost per unit N, government subsidy)
• Cheapest source of nitrogen
• Contains 46% N — highest N among solid fertilizers
• Hygroscopic — produced in granular/pellet form coated with inert material
• Acidic in residual effect; continuous use reduces soil pH

Quality standards:

• Maximum moisture: 1% by weight
• Biuret content: < 1.5% (biuret forms above 150 degree C during manufacturing and is toxic to plants above 2%)
• For foliar spray, biuret must not exceed 0.25% (higher levels cause leaf burn)
• General foliar spray concentration: 2% (range 2-6%)

Soil conversion: Urea is hydrolysed by the enzyme urease to form ammonium carbamate, which then converts to nitrate. The entire process takes 4-7 days. Apply urea 3-4 days before sowing to allow conversion.

• Equivalent acidity: 80 meq/100g

Neem Coated Urea (NCU)

• Urea coated with neem oil — acts as a natural nitrification inhibitor
• Azadirachtin content: 150 PPM
• Slow-release properties increase nitrogen use efficiency (NUE)
• Only 30-40% of urea-N is utilized by plants; neem coating reduces losses

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Calcium Cyanamide (CaCN₂)

• Contains 21% N
• Basic in reaction (one of the few basic N-fertilizers)
• Intermediary decomposition products injure seedlings — apply at least a week before sowing
• Produces lime during decomposition — valuable for acid soils
• Neither imported nor manufactured in India

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Equivalent Acidity and Basicity of Fertilizers

Equivalent acidity indicates how much lime (CaCO₃) is needed to neutralize the acidifying effect of a fertilizer. Higher values mean greater soil acidification.

Acidic Fertilizers (ranked by acidity)

Basic Fertilizers (ranked by basicity)

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Complete Nitrogen Content Summary

Nitrogenous Fertilizer - Quick Revision Summary

Fertilizer	N (%)	Residual Nature	Key Feature
Anhydrous ammonia	82	Acidic	Highest N content, liquid
Aqueous ammonia	80	Acidic	Most concentrated liquid
Urea	46	Acidic	Most commonly used
Ammonium nitrate	33	Acidic	Most hygroscopic, explosive
CAN	26	Neutral	Best for vegetables (Kishan Khad)
Ammonium chloride	25.5	Acidic	Used in Japan for paddy
Ammonium sulphate	20.6	Acidic	Best for paddy/waterlogged, highest S
Calcium cyanamide	21	Basic	Not used in India
Sodium nitrate	16	Basic	Pioneer N-fertilizer
Calcium nitrate	15.5	Basic	Supplies Ca
Potassium nitrate	13	Basic	Also supplies K

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Nitrification Inhibitors and Slow-Release Fertilizers

The Problem

Nitrogen is the most mobile major nutrient. Losses occur through:

1. Leaching — NO₃^- washes below root zone
2. Volatilization — NH₃ escapes as gas (especially from alkaline soils)
3. Denitrification — bacteria convert NO₃^- to N₂ gas in waterlogged soils

Traditional solution: split application at critical growth stages. But this increases labour cost. Better solution: use nitrification inhibitors or slow-release fertilizers.

Advantages of Slow-Release Technology

1. Slow, uniform nutrient release improves uptake
2. Labour saving — no need for split application
3. Higher efficiency — reduces fertilizer dose needed
4. Low pollution — minimal leaching into water bodies
5. Flexible timing — can apply as basal or top dressing

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Nitrification Inhibitors

These slow down the conversion of NH₄⁺ to NO₃^- by inhibiting nitrifying bacteria (Nitrosomonas and Nitrobacter). This keeps nitrogen in the less mobile ammonium form for longer.

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Slow-Release Fertilizers ^RRB-SO-2020

Three approaches to reduce nitrogen solubility and slow its release:

(A) Inherently Less Soluble Compounds

(B) Coated Fertilizers

(C) Super Granules / Modified Form

• Big granules of urea (1-4 g each) placed in the reduced zone of paddy soil
• Deep placement minimizes losses substantially
• Forms: super granules, briquettes, or urea-mud balls
• Especially effective for paddy cultivation

Other products:

• GROMOR: Trade name for urea-ammonium phosphate, grade 29:29:0
• Ammophos-B: Ammonium phosphate sulphate, grade 20:20:0

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Controlled-Release Fertilizers Using Zeolites

Zeolites are porous minerals with high cation-exchange capacity that can control nutrient release in soil. Their porous structure acts like a natural slow-release mechanism — holding nutrients and releasing them gradually as plants need them.

Benefits:

• Reduce nitrogen leaching (especially in sandy soils)
• Free soluble nutrients already locked in soil
• Improve soil fertility and water retention
• Common minerals — potentially useful for large-scale agriculture

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Exam Tips and Mnemonics

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Summary Table

Summary Cheat Sheet