Definition of Fertilizer

A fertilizer is any natural or synthetic substance that supplies essential nutrients to plants to enhance their growth, improve soil fertility, and increase agricultural productivity. Fertilizers contain primary macronutrients like nitrogen (N), phosphorus (P), and potassium (K), as well as secondary nutrients (calcium, magnesium, sulfur) and micronutrients (iron, zinc, copper, etc.).

Definitions by Various Organizations:

• FAO (Food and Agriculture Organization): “Any material, organic or inorganic, natural or synthetic, that is added to the soil or plant to supply nutrients essential for plant growth.”
• Fertilizer Control Order (FCO), India: “Any substance used directly or in combination with others to enhance soil fertility and crop productivity by providing essential nutrients to plants.”
• ISO (International Organization for Standardization): “A substance that contains one or more recognized plant nutrients and is used for its nutrient content to promote plant growth.”

Types of Fertilizers:

• Organic Fertilizers: Compost, manure, biofertilizers, green manure
• Inorganic (Chemical) Fertilizers: Urea, DAP, NPK fertilizers
• Biofertilizers: Rhizobium, Azotobacter, Mycorrhiza

Classification of Nitrogenous Fertilizers

Nitrogenous fertilizers supply nitrogen (N), which is essential for plant growth, chlorophyll formation, and protein synthesis. These fertilizers are classified based on the form in which nitrogen is present.

• Nitrate Fertilizers These fertilizers contain nitrogen in the form of nitrate (NO₃⁻), which is readily available to plants. They are highly soluble but prone to leaching in heavy rainfall.

Examples:

• Sodium Nitrate (NaNO₃) – 16% N
• Calcium Nitrate [Ca(NO₃)₂] – 15.5% N and 19.5% Ca
• Potassium Nitrate (KNO₃) – 13% N and 45% K₂O

Advantages:
Immediate availability to plants
Suitable for acidic soils
Promotes rapid vegetative growth

Disadvantages:
Highly leachable in sandy soils
Can increase soil alkalinity

• Ammoniacal Fertilizers

These fertilizers contain nitrogen in the ammonium (NH₄⁺) form. Ammonium nitrogen is less prone to leaching and is gradually converted to nitrate by soil microbes.

Examples:

• Ammonium Sulphate [(NH₄)₂SO₄] – 20.6% N and 24% S
• Ammonium Chloride (NH₄Cl) – 26% N
• Monoammonium Phosphate (MAP) [NH₄H₂PO₄] – 11-13% N and 48-62% P₂O₅
• Diammonium Phosphate (DAP) [(NH₄)₂HPO₄] – 18% N and 46% P₂O₅

Advantages:
Less leaching compared to nitrate fertilizers
Suitable for submerged crops like rice
Supplies ammonium, which is slowly converted to nitrate

Disadvantages:
Acidifies the soil, requiring liming in acidic soils

• Nitrate-Ammoniacal Fertilizers

These fertilizers contain nitrogen in both nitrate (NO₃⁻) and ammonium (NH₄⁺) forms, making them efficient sources of nitrogen for plants.

Examples:

• Ammonium Nitrate (NH₄NO₃) – 33-35% N
• Calcium Ammonium Nitrate (CAN) – 26% N and 10% Ca
• Ammonium Sulphate Nitrate [(NH₄)₂SO₄·NH₄NO₃] – 26% N and 12.1% S

Advantages:
Provides both quick-acting and slow-release nitrogen
Reduces leaching loss
Suitable for a wide range of crops

Disadvantages:
Ammonium nitrate is explosive and requires careful handling

• Amide Fertilizers

These fertilizers contain nitrogen in the amide (-CONH₂) form, which is gradually converted into ammonium and nitrate in the soil.

Examples:

• Urea [CO(NH₂)₂] – 46% N (most widely used nitrogen fertilizer)
• Calcium Cyanamide (CaCN₂) – 20.6% N

Advantages:
High nitrogen content (most concentrated N fertilizer)
Easily transportable and storable
Cost-effective

Disadvantages:
 Hygroscopic (absorbs moisture) and can lead to nitrogen loss through volatilization
Requires incorporation into soil to reduce ammonia loss

Comparison of Different Nitrogenous Fertilizers

Classification of Phosphatic Fertilizers

Phosphatic fertilizers are classified based on their solubility in water, neutral ammonium citrate, or citric acid. These classifications determine their availability to plants and suitability for different soil types.

1. Water-Soluble Phosphatic Fertilizers These fertilizers dissolve completely in water, making phosphorus readily available for plant uptake. They are highly efficient and widely used in agriculture.

Examples:

• Single Super Phosphate (SSP) – 16% P₂O₅
• Triple Super Phosphate (TSP) – 46% P₂O₅
• Monoammonium Phosphate (MAP) – 48-55% P₂O₅
• Diammonium Phosphate (DAP) – 46% P₂O₅
• Ammonium Polyphosphate (APP) – 34-62% P₂O₅

🔹 Best suited for: All soil types, especially where quick phosphorus availability is needed.

2. Citrate-Soluble Phosphatic Fertilizers These fertilizers are insoluble in water but dissolve in neutral ammonium citrate solution, making phosphorus available gradually to plants.

Examples:

• Basic Slag (Thomas Slag) – 14-18% P₂O₅
• Dicalcium Phosphate (DCP) – 34-39% P₂O₅
• Bone Meal – 20-25% P₂O₅

🔹 Best suited for: Acidic soils, as they release phosphorus slowly.

3. Insoluble Phosphatic Fertilizers

These fertilizers are not soluble in water or citrate solution but slowly become available through microbial activity and soil acidification.

Examples:

• Rock Phosphate – 20-40% P₂O₅
• Colloidal Phosphate – 18-22% P₂O₅

🔹 Best suited for: Strongly acidic soils where microbial activity can break them down for slow phosphorus release.

4. Organic Phosphatic Fertilizers

These are naturally derived phosphorus sources from plant and animal remains, providing slow but long-lasting phosphorus availability.

Examples:

• Bone Meal – 20-25% P₂O₅
• Fish Guano – 15-20% P₂O₅
• Compost & Manure – Low P₂O₅ content but improves soil fertility.

🔹 Best suited for: Organic farming and sustainable agriculture.

Summary Table of Phosphatic Fertilizers

Conclusion

• Water-soluble fertilizers provide immediate phosphorus availability.
• Citrate-soluble fertilizers release phosphorus gradually, suitable for acidic soils.
• Insoluble fertilizers need microbial activity to break down and are best for highly acidic soils.
• Organic fertilizers improve soil health while releasing phosphorus slowly.

Classification of Potassium Fertilizers

Potassium is an essential nutrient for plants, playing a critical role in various physiological processes such as photosynthesis, water regulation, protein synthesis, and resistance to diseases. Potassium fertilizers are used to supplement soil with the potassium needed for plant growth. These fertilizers are classified based on their chemical composition, solubility, and production methods.

1. Based on the Chemical Form of Potassium

• Potassium Chloride (KCl) – Muriate of Potash (MOP)
  • Content: 60-62% K₂O (Potassium Oxide), which is equivalent to about 50-53% elemental potassium (K).
  • Description: It is the most commonly used potassium fertilizer. It is highly soluble and provides a quick release of potassium. However, it contains chloride, which can be harmful to some crops.
  • Uses: Widely used for most crops except those sensitive to chloride, such as tobacco, potatoes, and certain fruits.

• Potassium Sulfate (K₂SO₄)
  • Content: 50-53% K₂O (equivalent to 41-45% K) and 18-24% Sulfur (S).
  • Description: It is a chloride-free potassium fertilizer. It is useful in cases where high chloride content might harm plants. Additionally, it provides sulfur, which is an essential nutrient for plants.
  • Uses: Used for crops sensitive to chloride, like fruits, vegetables, and tobacco, and also in crops requiring sulfur, such as brassicas and legumes.

• Potassium Nitrate (KNO₃)
  • Content: 44% K₂O (equivalent to 37% K) and 13% Nitrogen (N).
  • Description: It is highly soluble and provides both potassium and nitrogen. It is often used in fertigation and hydroponic systems because of its solubility.
  • Uses: Suitable for crops that require both potassium and nitrogen, such as tomatoes, peppers, and other high-nitrogen-demanding crops.

• Potassium Magnesium Sulfate (K₂SO₄·2MgSO₄)
  • Content: 22-25% K₂O (equivalent to 18-20% K), 11-13% Magnesium (Mg), and 18-22% Sulfur (S).
  • Description: This fertilizer provides both potassium and magnesium, which are essential for chlorophyll production and enzyme activation in plants. It is ideal for crops that require both nutrients.
  • Uses: Commonly used for crops such as vegetables, potatoes, tomatoes, and fruit trees.

• Potassium Thiosulfate (K₂S₂O₃)
  • Content: 40% K₂O (equivalent to 33% K) and 17% Sulfur (S).
  • Description: This water-soluble potassium fertilizer is primarily used in fertigation systems and is known for its sulfur content as well, making it useful for crops that require sulfur.
  • Uses: Used in fertigation and hydroponic systems, as well as for crops needing both potassium and sulfur.

2. Based on Potassium Content

• High Potassium Fertilizers
  • These fertilizers contain a high percentage of potassium and are usually used for crops with high potassium demands. Example: Potassium Chloride (KCl), Potassium Sulfate (K₂SO₄).
  • Potassium content: Typically over 50%.

• Medium Potassium Fertilizers
  • These fertilizers provide a moderate amount of potassium, suitable for most crops under standard agricultural practices.
  • Potassium content: Between 30-50%.

• Low Potassium Fertilizers
  • These are fertilizers that contain relatively lower amounts of potassium and are often used in specialized or controlled environments like fertigation or hydroponics.
  • Potassium content: Below 30%.

3. Based on the Form of Application

• Granular Potassium Fertilizers
  • Content: Varies, but includes potassium chloride (60-62% K₂O), potassium sulfate (50-53% K₂O), etc.
  • Description: Solid form, commonly broadcasted or applied directly to soil. It is effective for surface application and slow-release of potassium over time.
  • Uses: Used in large-scale farming operations, typically for field crops.

• Water-Soluble Potassium Fertilizers
  • Content: Potassium nitrate (44% K₂O), potassium sulfate (50-53% K₂O), etc.
  • Description: Fertilizers in this form dissolve easily in water, making them ideal for drip irrigation, fertigation, and hydroponic systems.
  • Uses: Used in controlled irrigation systems for crops grown in greenhouses or fields with irrigation.

• Liquid Potassium Fertilizers
  • Content: Varies by type, but generally includes potassium nitrate, potassium sulfate, or potassium thiosulfate.
  • Description: Potassium is dissolved in water, making it suitable for foliar application or fertigation. It provides immediate potassium to plants.
  • Uses: Used in foliar feeding and fertigation, particularly for crops like vegetables and fruits grown in controlled systems.

4. Based on the Source of Potassium

• Natural Potash
  • Content: Varies, but natural deposits of potassium salts like sylvite (KCl), carnallite (KCl·MgCl₂·6H₂O), and langbeinite (K₂SO₄·2MgSO₄) are common sources.
  • Description: Derived from potassium-bearing minerals, these are mined and processed into fertilizers.
  • Uses: Commonly used in large-scale agricultural practices worldwide, particularly where local sources of potash are available.

• Synthetic Potassium Fertilizers
  • Content: Potassium chloride (60-62% K₂O), potassium nitrate (44% K₂O), potassium sulfate (50-53% K₂O), etc.
  • Description: Produced through industrial processes, often from potash extracted from natural sources or synthesized from chemical reactions.
  • Uses: These fertilizers are manufactured and widely available in different formulations, commonly used in all types of farming.

Classification of Micronutrients

Micronutrients are essential elements that plants need in small amounts, but they are crucial for various physiological processes. They can be classified based on their content (percentage in fertilizers), their function in plants, and their role in soil. Here’s a more detailed classification with the content of each micronutrient:

1. Classification Based on the Chemical Form
2. Micronutrient Fertilizers (With Content)

These are the fertilizers specifically designed to supply micronutrients to plants. They can be applied directly to the soil or used in foliar sprays.

Iron (Fe) Fertilizers:

• Forms: Iron sulfate (FeSO₄), Iron chelates (EDTA, DTPA)
• Content: Iron sulfate typically contains 30-33% Fe, and iron chelates can contain 6-10% Fe.
• Uses: Used to correct iron deficiencies in plants, especially in alkaline soils.

Manganese (Mn) Fertilizers:

• Forms: Manganese sulfate (MnSO₄), Manganese oxide (MnO₂)
• Content: Manganese sulfate contains about 27-30% Mn.
• Uses: Corrects manganese deficiencies in plants, particularly in acidic soils.

Zinc (Zn) Fertilizers:

• Forms: Zinc sulfate (ZnSO₄), Zinc oxide (ZnO), Zinc chelates
• Content: Zinc sulfate contains about 36-37% Zn.
• Uses: Used to treat zinc deficiencies, especially in soils with high pH.

Copper (Cu) Fertilizers:

• Forms: Copper sulfate (CuSO₄), Copper oxide (CuO), Copper chelates
• Content: Copper sulfate contains around 25-27% Cu.
• Uses: Used to address copper deficiencies, common in high organic matter soils.

Boron (B) Fertilizers:

• Forms: Borax (Na₂B₄O₇·10H₂O), Boric acid (H₃BO₃)
• Content: Borax contains around 11% B, while boric acid contains about 17.5% B.
• Uses: Corrects boron deficiencies in plants, especially for crops like peanuts, cabbage, and broccoli.

Molybdenum (Mo) Fertilizers:

• Forms: Ammonium molybdate (NH₄MoO₄), Sodium molybdate (Na₂MoO₄)
• Content: Ammonium molybdate contains about 54% Mo.
• Uses: Used to treat molybdenum deficiencies, important for nitrogen fixation in legumes.

Chlorine (Cl) Fertilizers:

• Forms: Potassium chloride (KCl), Sodium chloride (NaCl)
• Content: Potassium chloride contains about 47% KCl, which is a significant source of chlorine.
• Uses: Corrects chlorine deficiencies, although it’s not commonly required for most crops.

Nickel (Ni) Fertilizers:

• Forms: Nickel sulfate (NiSO₄)
• Content: Nickel sulfate typically contains around 20-22% Ni.
• Uses: Used to address nickel deficiencies, important for urease enzyme activity.

Silicon (Si) Fertilizers (Beneficial Micronutrient):

• Forms: Silica (SiO₂), Calcium silicate (CaSiO₃)
• Content: Silica typically contains around 40-60% Si.
• Uses: Silicon is not always considered essential for all plants but can improve disease resistance and stress tolerance, especially in rice and other cereal crops.

2. Classification Based on Function in Plants

• Structural Micronutrients

These are involved in the formation of structural components such as cell walls, membranes, and overall plant tissue integrity.

• Boron (B): Involved in cell wall formation and cell membrane integrity.  Content: Borax (11% B), Boric acid (17.5% B)
• Silicon (Si): Strengthens plant tissues and increases resistance to environmental stresses.  Content: Silica (40-60% Si)

Catalytic Micronutrients

These micronutrients are important as cofactors for various enzymes involved in biochemical reactions.

• Iron (Fe): A component of cytochromes in electron transport chains and enzymes in photosynthesis and respiration. Content: Iron sulfate (30-33% Fe), Chelated iron (6-10% Fe)
• Manganese (Mn): Involved in the water-splitting reaction in photosynthesis and activation of enzymes involved in metabolic processes. Content: Manganese sulfate (27-30% Mn)
• Copper (Cu): A component of enzymes involved in photosynthesis and respiration. Content: Copper sulfate (25-27% Cu)
• Zinc (Zn): Essential for enzyme functions, protein synthesis, and hormone production. Content: Zinc sulfate (36-37% Zn)
• Molybdenum (Mo): Involved in nitrogen fixation and nitrate reduction. Content: Ammonium molybdate (54% Mo)

Metabolic Micronutrients These micronutrients regulate various metabolic pathways and cellular functions.

• Chlorine (Cl): Important in osmotic regulation, photosynthesis, and maintaining cellular function. Content: Potassium chloride (47% KCl)
• Nickel (Ni): A component of the enzyme urease involved in nitrogen metabolism. Content: Nickel sulfate (20-22% Ni)

3. Classification Based on Soil Availability

• Micronutrients More Likely to Be Deficient in Soils

These micronutrients are more commonly deficient, especially in certain types of soils (e.g., sandy, acidic, or alkaline soils).

• Iron (Fe)
• Zinc (Zn)
• Manganese (Mn)
• Copper (Cu)

Micronutrients Less Likely to Be Deficient These micronutrients are less likely to be deficient but may still be required in certain situations or for specific crops.

• Boron (B)
• Molybdenum (Mo)
• Chlorine (Cl)
• Nickel (Ni)

Summary Table of Micronutrients with Content