Unit 5: Chemical fertilizers and soil amendments

1. Classification of Fertilizers

Fertilizers are industrially manufactured chemicals containing plant nutrients. They are classified based on the nature of nutrients, chemical form, and physical properties.

A. Based on Nutrient Content

1. Straight Fertilizers: Supply only one major primary nutrient (e.g., Urea supplies N, MOP supplies K).
2. Complex Fertilizers: Supply at least two or more primary nutrients in chemical combination (e.g., DAP supplies N and P).
3. Mixed Fertilizers: Physical mixtures of straight fertilizers.

B. Based on Physical Form

1. Solid: Granules (DAP), Prills (Urea), Crystals (Ammonium Sulphate), Powder (SSP).
2. Liquid: Liquid ammonia, Urea ammonium nitrate solutions.

2. Major Nitrogenous Fertilizers

Nitrogenous fertilizers are classified based on the chemical form of nitrogen present.

A. Ammoniacal Fertilizers ()

Nitrogen is present as the ammonium ion. They are resistant to leaching but can be lost via volatilization if applied to the surface of alkaline soils.

• Ammonium Sulphate []:
  • Composition: 20.6% N, 24% S.
  • Properties: White crystalline salt; highly soluble; leaves an acidic residue in soil (acid-forming). Best for submerged rice soils.
• Ammonium Chloride []:
  • Composition: 25% N.
  • Properties: More acid-forming than ammonium sulphate. Not recommended for chloride-sensitive crops (tobacco, tomato).

B. Nitrate Fertilizers ()

Nitrogen is present as the nitrate ion. Highly mobile in soil, prone to leaching, suitable for top dressing.

• Sodium Nitrate []:
  • Composition: 16% N.
  • Properties: Basic in nature; continued use degrades soil structure (deflocculation due to Na).

C. Nitrate-Ammoniacal Fertilizers

Contains N in both forms.

• Calcium Ammonium Nitrate (CAN):
  • Composition: 25% N (12.5% Ammoniacal + 12.5% Nitrate).
  • Properties: Known as "Kisan Khad"; neutral in reaction (neither acid nor alkaline forming); highly safe for handling; slightly hygroscopic.
• Ammonium Nitrate []:
  • Composition: 33-34% N.
  • Properties: Highly hygroscopic and explosive; usually handled with care or mixed with limestone.

D. Amide Fertilizers

Nitrogen is present in amide form (), which must be converted to ammoniacal and then nitrate form by soil microorganisms to be available to most plants.

• Urea []:
  • Composition: 46% N.
  • Properties: White prills; highly concentrated; hygroscopic. It produces acidity in soil upon long-term use.
  • Impurity: Contains Biuret. According to FCO, biuret content must not exceed 1.5% for general crops and 0.5% for foliar spray.

3. Major Phosphatic Fertilizers

Phosphorus availability is pH-dependent. These fertilizers are classified by solubility.

A. Water Soluble Phosphates

Available immediately to plants; suitable for neutral to alkaline soils.

• Single Super Phosphate (SSP):
  • Composition: 16% , 19% Ca, 12% S.
  • Properties: Greyish powder/granules. Excellent source of S and Ca along with P.
• Triple Super Phosphate (TSP):
  • Composition: 46-48% .
  • Properties: Concentrated, no Gypsum content (unlike SSP).

B. Citrate Soluble Phosphates

Insoluble in water but soluble in weak acids (citric acid); suitable for acidic soils and long-duration crops.

• Dicalcium Phosphate (DCP):
  • Composition: 34-39% .
• Basic Slag: By-product of the steel industry; useful for acidic soil reclamation.

C. Insoluble Phosphates

Soluble only in strong mineral acids; suitable for strongly acidic soils and plantation crops.

• Rock Phosphate:
  • Composition: 20-30% .
  • Properties: Raw material for manufacturing other P-fertilizers.

4. Major Potassic Fertilizers

Potassium does not form gases or toxic compounds easily, making these fertilizers relatively stable.

A. Chloride Containing

• Muriate of Potash (MOP) / Potassium Chloride []:
  • Composition: 60% .
  • Properties: Crystalline, often reddish (due to iron oxide impurities) or white. Cheapest source of K.
  • Usage: Suitable for most crops except chloride-sensitive ones (Tobacco, Potato, Grapes) as Cl affects burning quality and starch accumulation.

B. Non-Chloride Containing

• Sulphate of Potash (SOP) / Potassium Sulphate []:
  • Composition: 50% , 17.5% S.
  • Properties: White crystals. Preferred for quality crops and chloride-sensitive plants. Expensive.

5. Complex, Customised, and Water-Soluble Fertilizers

A. Complex Fertilizers

Fertilizers containing at least two primary nutrients () chemically combined.

• Diammonium Phosphate (DAP):
  • Composition: 18% N, 46% .
  • Properties: Hard granules; slightly acidic. Best basal fertilizer for legumes and cereals.
• Nitrophosphates (Suphala): Various grades like 20:20:0 or 15:15:15.

B. Customised Fertilizers

These are crop-specific and area-specific fertilizers containing a combination of macro and micronutrients.

• Objective: To achieve Balanced Fertilization based on the "4R Stewardship" (Right Source, Right Rate, Right Time, Right Place).
• Manufacture: Produced through steam granulation technology.

C. Water Soluble Fertilizers (WSF)

100% water-soluble formulations designed for Fertigation (drip irrigation) and Foliar application. They have very low salt index and impurities.

• 19:19:19 (N:P:K): Vegetative growth booster.
• 12:61:0 (Mono Ammonium Phosphate): Root development.
• 0:52:34 (Mono Potassium Phosphate - MKP): Flowering and fruiting.
• 13:0:45 (Potassium Nitrate): Fruit finishing and maturation.

6. Secondary and Micronutrient Fertilizers

A. Secondary Nutrients (Ca, Mg, S)

• Gypsum (): 29.2% Ca, 18.6% S. Used for S deficiency and reclaiming alkali soils.
• Magnesium Sulphate (Epsom Salt): 9.6% Mg. Water-soluble.
• Elemental Sulphur: 85-100% S. Requires oxidation by Thiobacillus bacteria to become available ().

B. Micronutrients

Applied in small quantities. Can be inorganic salts or Chelates. Chelates (e.g., Fe-EDTA) prevent the nutrient from fixing in the soil, keeping it available to plants.

7. Nano-Fertilizers

Nano-fertilizers utilize nanotechnology to deliver nutrients. Particles are sized between 1 to 100 nm.

• Mechanism: High surface area-to-volume ratio allows for deep penetration through stomata and plant tissues.
• Advantages:
  • High Nutrient Use Efficiency (NUE): Reduces leaching and volatilization losses.
  • Reduced Dosage: Small quantities replace bulky conventional fertilizers (e.g., 500ml Nano Urea 45kg Bag Urea).
  • Environmental Safety: Less runoff pollution.
• Example: IFFCO Nano Urea (Liquid). It contains 4% N by weight (40,000 ppm).

8. Soil Amendments

Materials added to soil to improve its physical or chemical condition (reclamation), rather than primarily for nutrient supply.

A. Amendments for Acidic Soils (Liming)

Acid soils have high , , and toxicity.

• Materials:
  • Calcite / Limestone (): Standard material.
  • Dolomite (): Supplies both Ca and Mg.
  • Quick Lime (): Fast acting, caustic.
  • Slaked Lime (): Fast acting.
• Mechanism: replaces and on the soil exchange complex; carbonate/hydroxide neutralizes acidity.

B. Amendments for Alkali (Sodic) Soils

Alkali soils have high Exchangeable Sodium Percentage (ESP > 15) and high pH (> 8.5).

• Materials:
  • Gypsum (): Most common. replaces on clay particles. The displaced forms which is leached out.
  • Iron Pyrites (): Oxidizes to form sulphuric acid, which dissolves native calcium carbonate to release Ca.
  • Phosphogypsum: By-product of phosphoric acid industry.

9. Fertilizer Storage

Improper storage leads to caking (lumping), nutrient loss, and bag deterioration.

Key Factors Affecting Storage

1. Hygroscopicity: The tendency to absorb moisture from air (Critical Relative Humidity - CRH). Ammonium Nitrate and Urea have low CRH (absorb moisture easily).
2. Chemical Reactivity: Some fertilizers react if mixed (e.g., Urea + Superphosphate releases water).

Storage Guidelines

• Dunnage: Bags should not rest on the floor. Use wooden pallets or bamboo mats.
• Stacking: Height should not exceed 20 bags to prevent caking due to pressure.
• Spacing: Keep stacks away from walls (min 2 feet) for ventilation.
• Separation: Never store fertilizers with seeds, pesticides, or food grains.
• First-In, First-Out (FIFO): Use older stock first.

10. Fertilizer Control Order (FCO), 1985

The FCO is a legal order issued by the Government of India under the Essential Commodities Act, 1955.

Objectives

1. To ensure the availability of fertilizers at fair prices.
2. To ensure the quality of fertilizers distributed to farmers.
3. To regulate the trade and distribution.

Key Provisions

• Specifications: It lays down the standard specifications (physical and chemical properties) for all fertilizers sold in India. If a fertilizer does not meet these (e.g., lower N content than specified), it is deemed "Sub-standard".
• Registration: Mandatory registration of manufacturers and dealers.
• Packaging and Labeling: Bags must clearly indicate the name of the fertilizer, % of nutrients, gross/net weight, and manufacturer name.
• Adulteration: Strictly prohibits the sale of adulterated, misbranded, or spurious fertilizers.
• Scope: Originally for chemical fertilizers, now amended to include Bio-fertilizers, Organic Fertilizers, and Non-Edible De-Oiled Cake fertilizers.