Diammonium Phosphate (DAP)

Diammonium phosphate
(DAP)

Diammonium phosphate (DAP) is the world’s most widely used phosphorus (P) fertilizer. It is made from two common constituents in the fertilizer industry and it is popular because of its relatively high nutrient content and its excellent physical properties.

Production

Ammonium phosphate fertilizers first became available in the 1960s and DAP rapidly became the most popular in this class of products. It is formulated in a controlled reaction of phosphoric acid with ammonia, where the hot slurry is then cooled, granulated, and sieved. DAP has excellent handling and storage properties. The standard grade of DAP is 18-46-0 and fertilizer products with a lower nutrient content may not be labeled as DAP.

The inputs required to produce one ton of DAP fertilizer are approximately 1.5 to 2 tons of phosphate rock, 0.4 tons of sulfur (S), to dissolve the rock, and 0.2 tons of ammonia. Changes in the supply or price of any of these inputs will impact DAP prices and availability. The high nutrient content of DAP is helpful in reducing handling, freight, and application costs. DAP is produced in many locations in the world and is a widely traded fertilizer commodity.

 Chemical Properties

Chemical formula:           (NH4)2HPO4

Composition:                   18% N  and 46% P2O5 (20% P)

Water solubility (20 ºC):  588 g/L

Solution pH:                    7.5 to 8

Agricultural Use

DAP fertilizer is an excellent source of P and nitrogen (N) for plant nutrition. It is highly soluble and thus dissolves quickly in soil to release plant-available phosphate and ammonium. A notable property of DAP is the alkaline pH that develops around the dissolving granule.

As ammonium is released from dissolving DAP granules, volatile ammonia can be harmful to seedlings and plant roots in immediate proximity. This potential damage is more common when the soil pH is greater than 7, a condition that commonly exists around the dissolving DAP granule. To prevent the possibility of seedling damage, care should be taken to avoid placing high concentrations of DAP near germinating seeds.

The ammonium present in DAP is an excellent N source and will be gradually converted to nitrate by soil bacteria, resulting in a subsequent drop in pH. Therefore, the rise in soil pH surrounding DAP granules is a temporary effect. This initial rise in soil pH neighboring DAP can influence the micro-site reactions of phosphate and soil organic matter.

DAP is made of
two molecules of ammonia
reacted with one molecule
of phosphate

Management Practices

There are differences in the initial chemical reaction between various commercial P fertilizers in soil, but these dissimilarities become minor over time (within weeks or months) and are minimal as far as plant nutrition is concerned. Most field compari­sons between DAP and monoammonium phosphate (MAP) show only minor or no differences in plant growth and yield due to P source with proper management.

Non Agricultural Uses

DAP is used in many applications as a fire retardant. For example, a mixture of DAP and other ingredients can be spread in advance of the fire to prevent a forest from burning. It then becomes a nutrient source after the danger of fire has passed. DAP is used in various industrial processes, such as metal finishing. It is commonly added to wine to sustain yeast fermenta­tion and to cheese to support cheese cultures.

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Compound Fertilizer: Mixing several nutrients in each granule

Compound fertilizers contain
several nutrients in each granule

Many soils require the addition of several essential nutrients to alleviate plant deficiencies. Farmers may have the option of selecting a combination of single-nutrient fertilizers or using a fertilizer that has several nutrients combined into each particle. These combination (compound or complex) fertilizers can offer advantages of convenience in the field, economic savings, and ease in meeting crop nutritional needs.

Production

Compound fertilizers are made using basic fertilizer materials, such as NH3, ammonium phosphate, urea, S, and K salts. There are many methods used for making these fertilizers, with the specific manufacturing processes determined by the available basic compo­nents and the desired nutrient content of the finished product. Here are four brief examples.

Compaction methods (agglomeration) involve binding small fertilizer particles together using compaction, a cementing agent, or a chemical bond. Various nutrient ratios can be combined using undersized particles that may not be suitable for other applications.

Accretion-based fertilizers are made by repeatedly adding a thin film of nutrient slurry which is continually dried, building up mul­tiple layers until the desired granule size is reached.

Pipe-cross reactors are used to chemically melt NH3, acids containing S or P, and other nutrients—such as K sources and micronutrients—into a solid fertilizer with the desired nutri­ent content.

The nitrophosphate process involves reacting phosphate rock with nitric acid to form a mixture of compounds containing N and P. If a K source is added during the process, a solid fertilizer with N, P, and K will result.

Three different techniques for making compound fertilizers
(l) Compaction, (c) accretion, (r) pipe-cross reactor

Agricultural Use

Compound fertilizer contains multiple nutrients in each individual granule. This differs from a blend of fertilizers mixed together to achieve a desired average nutrient composition. This difference allows compound fertilizer to be spread so that each granule delivers a mixture of nutrients as it dissolves in the soil and eliminates the potential for segregation of nutrient sources during transport or application. A uniform distribution of micronutrients throughout the rootzone can be achieved when included in compound fertilizers.

These fertilizers are especially effective for applying an initial nutrient dose in advance of planting. There are certain ratios of nutrients available from a fertilizer dealer for specific soil and crop conditions. This approach offers advantages of simplicity in making complex fertilizer decisions, but does not allow the flexibility to blend fertilizers to meet specific crop requirements. Turf managers and homeowners often find compound fertilizers desirable.

Management Practices

Compound fertilizers are sometimes more expensive than a physical combination or blend of the primary nutrient sources since they require additional processing. However, when a consideration is made of all the factors involved with nutrient handling and use, compound fertilizers may offer considerable advantages.

Nitrogen is the nutrient that most commonly needs to be carefully managed and reapplied during the growing season. It may not be feasible to supply sufficient N in advance of planting to meet the entire demand (using only compound fertilizer) without overapplying some of the other nutrients. It may be advisable to use a compound fertilizer early in the growing season and then later apply only N fertilizer as needed.

Compound fertilizers are usually produced regionally to meet local crop needs. There is a wide range of chemical and physical properties that can be adjusted to meet these needs. For example, a desire to minimize P in urban stormwater runoff has led some communities to restrict the addition of P to compound fertilizers sold for turf and ornamen­tal purposes. Soils of a region that are typically low in a specific nutrient may have this element boosted in the compound fertilizer. 

A pdf version of this post can be found at the IPNI website here: