Phosphate Fertilizers & Slow Release Fertilizers

1. Water Solubility of Phosphate Fertilizers

Definition:

• Water solubility refers to the proportion of phosphate in a fertilizer that dissolves in water and becomes immediately available to plants.
• Important for predicting P availability and fertilizer efficiency.

Common Phosphate Fertilizers & Solubility

Key Points:

• Highly soluble fertilizers (TSP, MAP, DAP): Quick P supply, good for high-demand crops.
• Low solubility fertilizers (Rock phosphate, SSP partly): Slow P release, suitable for acid soils or long-term use.
• Water solubility affects fertilizer efficiency and choice for fertigation or foliar application.

2. Slow Release Fertilizers (SRF)

Definition: Fertilizers designed to release nutrients gradually over time, matching crop demand and reducing nutrient losses (leaching, volatilization, fixation).

Types of Slow Release Fertilizers

• Coated Fertilizers; Nutrients coated with sulfur, polymers, or resins. Example: Sulfur-coated urea (SCU), Polymer-coated urea.
• Inorganic Slow Release e.g., Calcium ammonium nitrate slow-release formulations. Release controlled by soil moisture and microbial activity.
• Organic-based / Biofertilizers; Fertilizers combined with organic materials or nitrification inhibitors.
• Nitrification/urease inhibitors; Slow conversion of NH₄⁺ → NO₃⁻ → reduces leaching and volatilization. Example: NBPT (N-(n-butyl) thiophosphoric triamide) for urea.

Advantages of Slow Release Fertilizers

• Matches nutrient release with crop demand.
• Reduces nutrient losses (leaching, volatilization).
• Improves fertilizer use efficiency.
• Reduces environmental pollution.
• Decreases frequency of fertilizer application, saving labor.

Disadvantages

• Higher cost than conventional fertilizers.
• Uneven nutrient release if coating is defective.
• Limited availability in some regions.

Common Examples

Key Exam Points

• Water solubility of P fertilizers: TSP > DAP > SSP > Rock phosphate.
• Slow release fertilizers: Improve nutrient efficiency, reduce losses, match crop demand.
• Coated fertilizers (sulfur, polymer) are most common SRFs.
• Urease & nitrification inhibitors also act as slow-release agents.
• SRFs are important for precision farming, fertigation, and environmental sustainability.

What are Nitrification Inhibitors?

• Definition: Substances that delay the microbial oxidation of ammonium (NH₄⁺) to nitrate (NO₃⁻) by inhibiting the activity of Nitrosomonas bacteria.
• This keeps nitrogen in the ammonium form (less leachable, less denitrifiable) for a longer time.
• Result: Better synchronization of N release with crop demand → improved yield & fertilizer efficiency.

Why Needed in Crop Production?

• Nitrate (NO₃⁻) is highly mobile → lost by leaching and denitrification.
• NH₄⁺ is less mobile → retained on soil colloids.
• Inhibitors keep more N in NH₄⁺ form, reducing:
  • Nitrate leaching (common in sandy soils).
  • Denitrification losses (in waterlogged soils, e.g., rice).
  • Nitrous oxide (N₂O) emissions (a greenhouse gas).

Common Nitrification Inhibitors

Crops & Situations Where Useful

• Rice (paddy): Reduces denitrification & N₂O emission.
• Maize, Wheat, Sorghum, Sugarcane: Increases N recovery, yield.
• Vegetables & Potato: Prevents nitrate leaching in sandy soils.
• Pasture crops: Reduces nitrate pollution in groundwater.

Benefits in Crop Production

• Increase N-use efficiency (up to 10–15%).
• Reduce fertilizer requirement (saves cost).
• Reduce groundwater pollution (nitrate leaching).
• Reduce GHG emissions (N₂O) → climate-smart agriculture.
• Sustains crop yields under high rainfall/waterlogged conditions.

Key Exam Points (ASRB NET)

• Nitrification is carried out by Nitrosomonas → NO₂⁻ and Nitrobacter → NO₃⁻.
• Inhibitors act mainly on Nitrosomonas.
• Widely used inhibitors: Nitrapyrin, DCD, Neem oil/cake, NBPT (urease inhibitor but related).
• Neem-coated urea is mandated in India → eco-friendly & effective.
• Major advantage: Reduced N losses → higher NUE, crop productivity, and lower pollution.