Diammonium Phosphate (DAP)

Diammonium Phosphate (DAP)

Complete Guide to Chemical Formula, Composition, Properties, Preparation, Manufacturing, Agricultural Uses, DAP vs MAP, Advantages, Limitations, Safety and Environmental Effects

1. Introduction

Diammonium phosphate (DAP) is an important inorganic ammonium phosphate salt and a widely used high-analysis fertilizer. It supplies two primary plant nutrients: nitrogen (N) and phosphorus (P).

Nitrogen is essential for vegetative growth, chlorophyll formation, amino-acid and protein synthesis, whereas phosphorus plays important roles in energy transfer, root development, nucleic-acid metabolism, flowering and seed development.

Key Point: Commercial fertilizer-grade DAP is commonly represented by the fertilizer grade 18-46-0. This means approximately:

• 18% nitrogen (N)
• 46% phosphorus expressed as P₂O₅
• 0% potassium expressed as K₂O

2. Chemical Identity

(NH₄)₂HPO₄

Common Name Diammonium Phosphate

Abbreviation DAP

Chemical Formula (NH₄)₂HPO₄

Molar Mass Approximately 132.06 g mol⁻¹

Chemical Class Ammonium phosphate salt

Main Nutrients Nitrogen and phosphorus

3. Nutrient Composition

Fertilizer grades are conventionally expressed as N–P₂O₅–K₂O. Therefore, DAP is commonly labelled as 18-46-0.

Component	Typical Grade	Meaning
Nitrogen	18%	Elemental nitrogen content
Phosphate	46% P2O5	Phosphorus reported as P2O5 equivalent
Potash	0% K2O	DAP supplies essentially no potassium

Important Exam Conversion: 46% P₂O₅ corresponds to approximately 20.1% elemental phosphorus. Thus, the 46% figure should not be interpreted as 46% elemental phosphorus.

4. Chemical Structure and Ions

DAP contains two ammonium ions and one hydrogen phosphate ion.

• 2NH₄⁺ ions
• 1HPO₄²⁻ ion

Charge balance is:

2(+1) + (−2) = 0

Therefore, the compound is electrically neutral.

2NH₄⁺ + HPO₄²⁻ → (NH₄)₂HPO₄

5. Physical Properties

Property	Description
Appearance	Usually a white to off-white crystalline or granular solid.
Odour	Generally odourless under normal storage conditions.
Water Solubility	Highly soluble in water.
Moisture Behaviour	Can absorb moisture and may cake under humid storage conditions.
Thermal Behaviour	Under strong heating, DAP undergoes decomposition and can form other ammonium phosphate species with release of gaseous products.
Fertilizer Grade	Commonly marketed as granular 18-46-0 fertilizer.

Note: Thermal decomposition of DAP can involve several products depending on temperature and conditions. Therefore, a single decomposition equation should not be treated as universally valid for all heating conditions.

6. Chemical Properties

6.1 Dissolution in Water

DAP dissolves readily in water and dissociates into ammonium and hydrogen-phosphate species.

(NH₄)₂HPO₄ → 2NH₄⁺ + HPO₄²⁻

6.2 Reaction with Strong Bases

Because DAP contains ammonium ions, treatment with a sufficiently strong base can promote the release of ammonia:

NH₄⁺ + OH⁻ → NH₃ + H₂O

This is relevant to fertilizer handling and compatibility with strongly alkaline materials.

6.3 Thermal Decomposition

On strong heating, DAP undergoes thermal transformations and can form other ammonium phosphate species while releasing ammonia and water. The exact products depend on temperature and experimental conditions.

7. Preparation of DAP

DAP is manufactured by reacting phosphoric acid with ammonia. The overall stoichiometric reaction can be represented as:

H₃PO₄ + 2NH₃ → (NH₄)₂HPO₄

This is an acid-base neutralization process in which phosphoric acid reacts with two equivalents of ammonia.

Main Feedstocks:

• Phosphoric acid
• Anhydrous or aqueous ammonia, depending on process design

8. Industrial Manufacturing Process

Industrial production involves controlled ammoniation and granulation steps designed to obtain the desired nutrient composition and physical properties.

1

Phosphoric Acid

Phosphoric acid provides the phosphate component.

2

Ammoniation

Ammonia reacts with phosphoric acid under controlled conditions.

3

Slurry Formation

The neutralization process produces an ammonium-phosphate slurry or melt depending on the process.

4

Granulation

The material is converted into fertilizer granules of suitable size.

5

Drying

Excess moisture is removed to obtain a stable granular product.

6

Screening

Granules are separated according to size and unsuitable fractions may be recycled.

7

Cooling

The product is cooled before storage and handling.

8

Packaging

The finished fertilizer is packed or transferred to bulk storage.

9. Agricultural Importance

Role of Nitrogen

• Promotes vegetative growth.
• Supports chlorophyll formation.
• Participates in amino-acid and protein synthesis.
• Supports formation of important plant biomolecules.

Role of Phosphorus

• Supports early root development.
• Participates in ATP-mediated energy transfer.
• Plays an important role in nucleic-acid metabolism.
• Supports reproductive development.
• Contributes to flowering, seed and fruit development.

10. Application of DAP Fertilizer

DAP is commonly used as a basal or starter fertilizer when nitrogen and phosphorus are required. The appropriate rate depends on soil-test results, crop requirements and the overall fertilizer program.

Common Application Methods

1. Broadcast application: Fertilizer is distributed over the soil surface and may be incorporated when appropriate.
2. Band placement: Fertilizer is placed in bands near the crop root zone.
3. Basal application: DAP may be applied around planting to supply nutrients during early crop establishment.
4. Soil application: The amount should be selected according to soil nutrient status and crop demand.

Application Warning: Avoid excessive fertilizer rates. Concentrated fertilizer placed directly against seeds can cause injury because of high local salt concentration and ammonia-related effects.

11. Crops in Which DAP May Be Used

DAP may be used in a wide range of agricultural and horticultural systems when its nutrient ratio is appropriate.

Cereals Wheat, rice, maize and other cereal crops.

Pulses Various pulse and legume crops.

Oilseeds Mustard and other oilseed crops where appropriate.

Vegetables Used according to crop and soil nutrient requirements.

Commercial Crops Application depends on soil fertility and crop nutrient demand.

Horticultural Crops May be used as part of a balanced nutrient-management program.

12. Advantages of DAP

• High nutrient concentration: Provides significant amounts of nitrogen and phosphorus.
• Two essential nutrients: Supplies N and P in one fertilizer.
• Good water solubility: Allows rapid dissolution in moist soil.
• Convenient granular form: Easy to transport, store and apply.
• Important phosphorus source: Widely used where phosphorus fertilization is required.

13. Disadvantages and Limitations

• Contains essentially no potassium.
• May not provide the nutrient ratio required by every crop or soil.
• Excessive phosphorus application can contribute to phosphorus accumulation in soils.
• Repeated use without soil testing can produce nutrient imbalance.
• Direct contact with seeds at excessive application rates can cause seedling injury.
• Poor storage can cause moisture absorption and caking.
• Improper fertilizer management can contribute to nutrient losses and environmental pollution.

14. DAP vs MAP

DAP and MAP are both ammonium phosphate fertilizers, but they differ in chemical formula, nitrogen content and phosphate grade.

Feature	DAP	MAP
Full Name	Diammonium Phosphate	Monoammonium Phosphate
Formula	(NH4)2HPO4	NH4H2PO4
Typical Fertilizer Grade	18-46-0	11-52-0
Nitrogen	≈18%	≈11%
Phosphate	≈46% P2O5	≈52% P2O5
Potassium	0%	0%
General Character	Higher N relative to MAP	Higher P2O5 grade and lower N

Exam Trick: DAP = (NH₄)₂HPO₄ → 18-46-0
MAP = NH₄H₂PO₄ → 11-52-0

15. Effect of DAP on Soil

After dissolution, DAP supplies ammonium and phosphate species to the soil solution. Ammonium can subsequently undergo microbial nitrification, while phosphate interacts with soil minerals and organic matter.

Phosphorus Behaviour

Phosphate is relatively immobile in many soils. Depending on soil chemistry, phosphorus may become associated with calcium-containing phases in calcareous soils and with iron- and aluminum-containing phases in many acidic soils.

Nitrogen Behaviour

Ammonium can be oxidized by nitrifying microorganisms to nitrate. Because nitrate is relatively mobile in soil, it may be lost through leaching or runoff under suitable conditions.

Important: The behaviour of phosphorus and nitrogen after fertilizer application depends strongly on soil pH, moisture, temperature, microbial activity, crop uptake and fertilizer-management practices.

16. Environmental Impact

DAP is an important agricultural nutrient source, but inappropriate fertilizer management can cause environmental problems.

16.1 Eutrophication

Excess phosphorus entering rivers, lakes and other water bodies can contribute to eutrophication. Elevated nutrient availability may promote excessive algal and aquatic plant growth and degrade water quality.

16.2 Nitrogen Losses

Nitrogen from fertilizer can be lost through processes including nitrate leaching, runoff and gaseous losses. The importance of each pathway depends on soil, weather, irrigation, fertilizer placement and crop uptake.

16.3 Sustainable Management

• Use soil-test-based fertilizer recommendations.
• Match nutrient application with crop requirements.
• Avoid unnecessary phosphorus application.
• Use appropriate timing and placement.
• Minimize fertilizer runoff into surface waters.
• Maintain balanced N-P-K and micronutrient management where required.

17. Fertilizer Compatibility

DAP can be blended with some fertilizers, but compatibility depends on the specific materials, moisture conditions and application method.

Important: Do not indiscriminately mix fertilizers. Certain combinations may cause chemical reactions, moisture problems, caking or changes in nutrient availability. Follow manufacturer and agronomic recommendations when preparing fertilizer blends.

18. Storage

1. Store DAP in a dry and protected area.
2. Protect fertilizer from rain and direct water contact.
3. Minimize exposure to excessive humidity.
4. Keep bagged fertilizer on suitable pallets or raised surfaces.
5. Protect the product from contamination.
6. Follow the product-specific Safety Data Sheet (SDS).

19. Safety and Handling

DAP is generally handled as a fertilizer product. Appropriate precautions should be taken to minimize dust exposure and prevent accidental misuse.

• Use suitable protective equipment when handling dusty material.
• Avoid inhaling excessive fertilizer dust.
• Avoid unnecessary contact with eyes and skin.
• Wash hands after handling.
• Keep fertilizer away from food and drinking water.
• Follow the manufacturer's SDS for product-specific precautions.

Heating Warning: Strong heating or fire conditions can cause decomposition and release irritating or hazardous gases. Emergency response should follow the manufacturer's SDS and applicable fire-safety procedures.

20. Industrial and Other Uses

The principal use of fertilizer-grade DAP is agriculture. Ammonium phosphate chemistry also has applications in selected industrial, technical and fire-safety formulations.

• Manufacture of agricultural fertilizers.
• Preparation of fertilizer blends.
• Source of nitrogen and phosphorus in selected formulations.
• Ammonium-phosphate-based fire-retardant and fire-safety applications in appropriate formulations.
• Nutrient source in selected controlled biological or fermentation processes where the required grade is appropriate.

Qualification: Specific non-agricultural uses depend on product grade, purity and regulatory requirements. Fertilizer-grade DAP should not automatically be assumed suitable for technical or biological applications.

21. Economic Importance

DAP is an important phosphate fertilizer in international agricultural markets. Its economic significance is associated with its nutrient concentration, widespread agricultural use and established production and distribution systems.

DAP prices can be influenced by factors including phosphate-rock and phosphoric-acid availability, ammonia prices, energy costs, transportation, international supply and demand, exchange rates and agricultural demand.

22. DAP in Indian Agriculture

DAP is an important nitrogen-phosphorus fertilizer used in Indian agriculture. It is applied to a variety of crops where phosphorus and nitrogen are required.

However, DAP should not be regarded as a universal fertilizer for every crop and soil. Fertilizer selection should ideally be based on soil-test results, crop requirements and balanced nutrient management.

Exam Point: DAP supplies N + P but essentially no potassium. Therefore, DAP is not a complete NPK fertilizer.

23. DAP Compared with Common Fertilizers

Fertilizer	Main Nutrients	Typical Grade	Main Feature
DAP	N + P	18-46-0	Concentrated source of nitrogen and phosphorus
MAP	N + P	11-52-0	Higher phosphate grade than DAP
Urea	N	46-0-0	High-nitrogen fertilizer
MOP	K	0-0-60	Potassium chloride fertilizer
NPK Fertilizer	N + P + K	Varies	Supplies all three primary macronutrients in specified proportions

24. Advantages vs Limitations

Advantages	Limitations
High nutrient concentration	Contains essentially no potassium
Supplies nitrogen and phosphorus together	May not match the nutrient ratio required by every crop
Highly water soluble	Excessive use can cause nutrient imbalance
Convenient granular form	Can cake under humid storage conditions
Widely used phosphorus source	Requires appropriate rate, timing and placement

25. Important Facts for Exams

1. DAP stands for Diammonium Phosphate.
2. Chemical formula: (NH₄)₂HPO₄.
3. Molar mass: 132.06 g mol⁻¹.
4. DAP is an ammonium phosphate salt.
5. It is manufactured by reacting phosphoric acid with ammonia.
6. Common fertilizer grade: 18-46-0.
7. DAP supplies nitrogen and phosphorus.
8. DAP contains essentially no potassium.
9. The 46% phosphate value is expressed as P₂O₅ equivalent.
10. 46% P₂O₅ corresponds to approximately 20.1% elemental P.
11. Phosphorus is important in ATP-mediated energy transfer, root development and reproductive growth.
12. Nitrogen is important for proteins, chlorophyll and vegetative growth.
13. DAP and MAP are both ammonium phosphate fertilizers but have different formulas and fertilizer grades.
14. Excess nutrient application can contribute to water pollution and eutrophication.

26. Quick Revision Table

Property	DAP
Full Name	Diammonium Phosphate
Formula	(NH4)2HPO4
Molar Mass	132.06 g mol−1
Fertilizer Grade	18-46-0
Nitrogen	18%
Phosphate	46% as P2O5
Potassium	0%
Main Feedstocks	Phosphoric acid + ammonia
Major Use	Agricultural fertilizer
Major Nutrients	Nitrogen + phosphorus
Potassium Source?	No

27. Conclusion

Diammonium phosphate (DAP) is an important inorganic ammonium phosphate fertilizer with the chemical formula (NH₄)₂HPO₄. Its approximate molar mass is 132.06 g mol⁻¹, and commercial fertilizer-grade DAP is commonly represented by the grade 18-46-0.

DAP provides two major plant nutrients, nitrogen and phosphorus. Nitrogen supports vegetative growth, protein synthesis and chlorophyll formation, while phosphorus contributes to root development, energy transfer, nucleic-acid metabolism and reproductive growth.

Although DAP is an effective source of nitrogen and phosphorus, it does not supply potassium and therefore should not automatically be considered a complete fertilizer. Excessive or poorly managed application can also contribute to nutrient imbalance, nutrient losses and eutrophication.

The most effective use of DAP is therefore based on soil testing, crop requirements, correct application rate, appropriate placement and balanced nutrient management.

28. Golden Exam Points

• DAP = (NH₄)₂HPO₄
• Molar mass = 132.06 g mol⁻¹
• DAP grade = 18-46-0
• DAP = Nitrogen + Phosphorus fertilizer
• No potassium
• Manufacture: H₃PO₄ + 2NH₃ → (NH₄)₂HPO₄
• MAP = NH₄H₂PO₄
• 46% P₂O₅ ≈ 20.1% elemental P
• Excess phosphorus → risk of eutrophication
• Soil-test-based fertilizer management is preferred