Granular Fertilizer

What is Granular Fertilizer? Which is the Best Granular Fertilizer?

In recent decades, sustainable and efficient agriculture has become a global priority for food and horticultural crop production. One of the key tools in this direction is the strategic use of granular fertilizers. Due to their uniform physical structure, controlled release of nutrients, and ease of storage and transportation, granular fertilizers have gained popularity across farms and orchards. The best granular fertilizer is determined based on soil testing and proper application methods.

Definition and Structure of Granular Fertilizer

Granular fertilizers are manufactured in uniform, compressed granules that contain macronutrients (nitrogen, phosphorus, and potassium), micronutrients, and in some formulas, additives like sulfur or humic acid. Compared to powdered fertilizers, granular types offer more uniform distribution and controlled nutrient release. Thanks to their density and specialized coatings, they can release nutrients gradually over several weeks (especially in slow-release forms), making them ideal for base fertilization in fields and orchards.

Variety of Formulations and Specialized Applications

Complete NPK granular fertilizers hold the largest market share. Depending on the nutrient ratios, they can be adjusted for different growth stages and soil types. For example, a 12-17-17formulation is a balanced mix suitable for field crops, vegetables, and greenhouses, especially effective during the reproductive stage. The 9-7-7 formula is ideal for rice paddies due to its mild nitrogen and phosphorus content, making it suitable for weak soils or salt-sensitive plants like permanent fruit trees.

Popular Granular NPK Fertilizers

In addition to 12-12-17and 9-7-7, other common types include 15-15-15 for general use, 20-20-20 for greenhouses and foliar application, 10-10-20 with high nitrogen for fast-growing vegetables, and 36-12-12 with high potassium to improve fruit quality.

Besides NPK types, diammonium phosphate (DAP) with a 0-46-18 formula is essential for fall crops (wheat, barley, rapeseed) at planting, as it supports root development and nutrient uptake. Sulfur-containing granular fertilizers such as ammonium sulfate, sulfur-coated urea (46% nitrogen, 5–10% sulfur), and bentonite sulfur (85–90% elemental sulfur with bentonite) are effective for alkaline soils and sulfur-deficient crops. They help improve soil pH and enhance micronutrient absorption like iron and zinc.

Lastly, granular micronutrient fertilizers—containing iron, zinc, manganese, and copper—are recommended for micronutrient-deficient soils or sensitive crops, especially during flowering and fruiting stages.

The Importance of Granule Size in Granular Fertilizers

Granule size, measured in millimeters (mm) or mesh, significantly affects:

• Nutrient absorption speed by soil and roots

• Uniform distribution by hand or spreader

• Contact surface with soil and solubility rate

• Resistance to early breakdown or leaching

1. Fine Granules (1–2 mm)

• Greater contact surface, faster solubility

• Suitable for fast-growing crops, light soils, or drip irrigation

• Common in vegetable gardens, melons, greenhouses, or quick-dissolving applications

• Note: Wind or hand-spreading may result in uneven distribution

2. Standard Granules (2–4 mm)

• Most common type; balances solubility and persistence

• Ideal for wheat, barley, corn, rice, pistachio, and citrus orchards

• Compatible with most fertilizer spreaders

3. Coarse Granules (4–6 mm)

• Very slow release, highly resistant to leaching and evaporation

• Suitable for hot, dry climates or sandy soils

• Often used in orchards, tree pits, or long irrigation cycles

• Note: May be too slow for drip systems or humid regions

Compatibility of Granule Size with Spreaders

Different fertilizer spreaders are optimized for specific granule sizes. Very fine granules may clog or get blown away by wind, while oversized granules reduce uniformity. Rotary or disc spreaders work best with 2–4 mm granules (mesh 5–10).

Common Granule Sizes in the Iranian Market

• Domestic and imported granular fertilizers mostly range from 2–5 mm.

• Some brands (e.g., Turkish or European exports) print granule size on the bag.

• Bentonite sulfur fertilizers have larger granules (up to 6 mm) that swell upon water contact.

• Crystalline fertilizers may look granular but are highly soluble crystals, different from standard granules.

• Granular poultry manure is a popular organic form, with slower release compared to liquid or pelletized forms.

Granule size plays a crucial role in the success of nutrient management. Fine granules dissolve quickly but may leach. Coarse granules last longer but need consistent irrigation and more time to release nutrients. Selection should be based on crop type, soil texture, irrigation method, and available tools.

Application Methods and Timing

Granular fertilizers can be used:

• As base fertilizer before planting

• In seed rows or bands

• In tree pits (Chalkood)

• Top-dressed during growth stages followed by irrigation

To maximize efficiency, apply in split doses matched to the plant’s growth stage (e.g., early season, pre-flowering, fruiting). After top-dressing, a gentle irrigation helps dissolve and absorb the nutrients.

Mechanized spreaders require consistent granule size and dust-free material to avoid technical problems.

Choosing the Best Granular Fertilizer

1. Soil Testing: Analyze pH, EC, and levels of NPK and micronutrients to identify deficiencies.

2. Plant Needs and Growth Stage: Nitrogen is key in vegetative stages, phosphorus for root/flower development, and potassium for fruiting.

3. Soil Type: Sandy soils benefit from slow-release and moisture-retentive fertilizers, while clay soils handle faster-soluble types.

4. Climate: In hot, dry areas, slow-release fertilizers prevent nitrogen loss. In rainy areas, avoid leachable nitrogen forms.

5. Physical Quality and Certification: Uniform granules, proper coatings, and standard certifications indicate high-quality products.

6. Cost-Efficiency: Factor in both price and absorption efficiency. Sometimes a pricier fertilizer offers better value long-term.

Comparing Granular, Liquid, Powdered, and Crystalline Fertilizers

Granular fertilizers are great for base and long-term feeding. However, other types may be better for urgent or specific situations.

• Liquid Fertilizers: Absorbed quickly via leaves or roots; ideal for urgent deficiencies. Common in drip irrigation and foliar sprays. More expensive and harder to store.

• Water-Soluble Powders: Fill the gap between granular and liquid types; allow precise dosing. Suitable for quick corrections or ornamental plants. Require full dissolution and may cause root burn at high doses.

• Crystalline Fertilizers: Highly pure and fast-dissolving (like Crystalon); ideal for greenhouse cultivation and precise fertigation. More expensive and sensitive to humidity.

Comparison Table: Fertilizer Forms