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¶ Introduction
Monoammonium phosphate (MAP) is a widely used source of phosphorus and nitrogen. It is made of two constituents common in the fertilizer industry and has the highest phosphorus content of any common solid fertilizer. It is also known as “ammonium phosphate monobasic” or as “ammonium dihydrogen phosphate.”
¶ Table 1. |
|
| Analysis | 10% to 12% N |
| 48% to 61% P2O5 | |
| Chemical formula | NH4H2PO4 |
| Water solubility, 68°F | 370 g/L |
| pH (1% solution) | 4.0 – 4.5 |
| Bulk density, lbs./cu.ft. | 60 - 62 |
¶ Production
The process for manufacturing monoammonium phosphate is relatively simple. In a common method, a one-to-one ratio of ammonia (NH3) and phosphoric acid (H3PO4) is reacted and the resulting slurry of monoammonium phosphate is solidified in a granulator. The second method is to introduce the two starting materials in a pipe-cross reactor where the reaction generates heat to evaporate water and solidify monoammonium phosphate.
Variations of these methods are also in use for monoammonium phosphate production. An advantage of producing monoammonium phosphate is that lower quality phosphoric acid can be used compared with other phosphorus fertilizers that often require a higher purity grade of acid.
The P2O5 equivalent content of monoammonium phosphate varies from 48 to 61%, depending on the amount of impurity in the acid. The most common fertilizer composition is 11-52-0.
¶ Agricultural Use
Monoammonium phosphate has been an important granular fertilizer for many years. It is water soluble and dissolves rapidly in soil if adequate moisture is present. Upon dissolution, the two basic components of the fertilizer separate again to release NH4+and H2PO4-.
Both of these nutrients are important to sustain healthy plant growth. The pH of the solution surrounding the granule is moderately acidic, making monoammonium phosphate an especially desirable fertilizer in neutral and high pH soils. Agronomic studies show that there is no significant difference in phosphorus nutrition from various commercial phosphorus fertilizers under most conditions.
Granular monoammonium phosphate is applied in concentrated bands beneath the soil surface in proximity of growing roots or in surface bands. It is also commonly applied by spreading across the field and mixing into the surface soil with tillage.
In powdered form, it is an important component of suspension fertilizers. When monoammonium phosphate is made with especially pure H3PO4, it readily dissolves into a clear solution that can be used as a foliar spray or added to irrigation water. The P2O5 equivalent content of high-purity monoammonium phosphate is usually 61%.
¶ Figure 1. Monoammonium phosphate (MAP)
¶ Management Practices
There are no special precautions associated with the use of monoammonium phosphate. The slight acidity associated with this fertilizer reduces the potential for ammonia loss to the air. monoammonium phosphate can be placed in close proximity to germinating seeds without concern for ammonia damage.
When monoammonium phosphate is used as a foliar spray or added to irrigation water, it should not be mixed with calcium or magnesium fertilizers.
Monoammonium phosphate has good storage and handling properties. Some of the chemical impurities (such as iron and aluminum) naturally serve as a conditioner to prevent caking.
Highly pure monoammonium phosphate may have a conditioner added or may require special handling to prevent clumping and caking. As with all P fertilizers, appropriate management practices should be used to minimize any nutrient loss to surface or drainage water.A high purity source of monoammonium phosphate is used as a feed ingredient for animals. The NH4+ is synthesized into protein and the H2PO4- is used in a variety of metabolic functions in animals.
¶ Non-Agricultural Uses
Monoammonium phosphate is used in dry chemical fire extinguishers commonly found in offices, schools, and homes. The extinguisher spray disperses finely powdered monoammonium phosphate, which coats the fuel and rapidly smothers the flame.
¶ References
IPNI. Nutrient Source Specifics No. 9: Monoammonium Phosphate. International Plant Nutrition Institute, Norcross Georgia. 1 pg.