Introduction to Fertilizers

Fertilizers are substances (natural or synthetic, organic or inorganic) that are added to the soil to supply essential plant nutrients and enhance crop productivity. They help improve soil fertility and provide vital nutrients like nitrogen (N), phosphorus (P), and potassium (K) in available forms.

Note: Fertilizers differ from manures, which are organic matter applied in bulk to enrich the soil. Fertilizers are concentrated nutrient sources.

Historical Background

• Justus von Liebig (Germany) and John Bennet Lawes (England) pioneered the use of chemical fertilizers.
• Around 1840, they independently developed the superphosphate by treating phosphate rock with sulfuric acid (H₂SO₄).
• In 1843, Lawes established the first fertilizer factory in Deptford, England, to manufacture superphosphate commercially.

What Are Soil Amendments?

Soil amendments (also called ameliorants or conditioners) are substances used to improve soil structure or correct pH, but not primarily for nutrient supply.
Examples include:

• Lime (to neutralize acidity)
• Gypsum (to improve structure in sodic soils)

Classification of Fertilizers

1. Based on Nutrient Content

2. Based on Nutrient Percentage

• Low-analysis fertilizers: Less than 25% primary nutrients; Example: SSP (16% P₂O₅)
• High-analysis fertilizers: More than 25% nutrients; Examples: Urea (46% N), DAP (18-46-0)

Key Fertilizer Concepts

Fertilizer Grade

• Indicates the guaranteed analysis of N, P₂O₅, and K₂O in a fertilizer.
• Example: 6:24:24 → 6% N, 24% P₂O₅, 24% K₂O

Fertilizer Ratio

• Shows the relative proportion of nutrients.
• 6:24:24 → 1:4:4 (Divide each by 6)

Materials Used in Mixed Fertilizers

Precautions While Mixing Fertilizers

1. Avoid mixing hygroscopic fertilizers (absorb moisture and cake).
   • Order of hygroscopicity:
     Ammonium nitrate > Urea > Ammonium sulphate > ASN > CAN
2. Do not mix NH₄⁺ fertilizers with lime or basic slag – results in NH₃ gas loss.
3. Superphosphate + free lime = Insoluble phosphates (ineffective).
4. Chloride-based fertilizers can corrode gunny bags and seed drills.

Acidity and Basicity of Fertilizers

• Equivalent Acidity: Amount of lime (CaCO₃) required to neutralize acid left by a fertilizer. Example: 100 kg Ammonium sulphate requires 110 kg CaCO₃
• Equivalent Basicity; Amount of base (CaCO₃ equivalent) added by a basic fertilizer (e.g., calcium nitrate).

Types of Nitrogenous Fertilizers

Important Nitrogenous Fertilizers

1. Urea

• 46% N, widely used in India.
• Acidic, coated to reduce moisture absorption.
• Contains <1.5% biuret (biuret >2% is toxic).
• Conversion to usable form: 4–7 days.
• Apply 3–4 days before sowing.

2. Ammonium Sulphate

• 20.6% N + 24% S
• Reduces calcium levels in soil → increases acidity.
• Common in rice and jute fields.

3. Calcium Ammonium Nitrate (CAN)

• 26% N, neutral pH.
• Contains both nitrate and ammoniacal N.
• Branded as “Kisan Khad”.

4. Ammonium Chloride

• 26% N. Used in paddy (Japan).
• Avoid in tobacco, tomato, potato (Cl-sensitive crops).

5. DAP (Diammonium Phosphate)

• 18% N + 46% P₂O₅.
• Highly concentrated phosphorus source.

6. Cyanamide (Calcium Cyanamide)

• 20.6% N.
• Toxic intermediates—apply 7 days before sowing.
• Good for acidic soils due to lime content.

💧 Aqueous Ammonia

• 80% N.
• Used in fertigation (application with irrigation water).

Biuret in Urea – Why It’s Important

• Biuret forms when urea is manufactured at high temperatures.
• Toxic above 2% → causes leaf burn and germination issues.
• Fertilizer Control Order limits biuret to <1.5% in commercial urea.

Quick Revision 

• First fertilizer factory: Lawes (1843, Deptford, UK).
• Most used N fertilizer in India: Urea.
• Hygroscopic fertilizers: Ammonium nitrate > Urea > Ammonium sulphate.
• Fertilizer grade vs. ratio: Grade = absolute %; Ratio = relative proportion.
• DAP: 18-46-0 → High analysis & complete P fertilizer.
• CAN: Neutral pH, ideal for vegetables.
• Urea spray concentration: 2% common; 6% max.