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.

A pdf version of this is available from the IPNI website here:

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: 

Ten Laws of Sustainable Soil Management (Lal)

Dr. Rattan Lal developed 10 laws for sustainably meeting the demands of the growing world population.  They are worthy of some discussion and thought by soil scientists.

(1) Soil degradation and poverty: The biophysical process

of soil degradation is driven by economic, social, and political

forces.

(2) Stewardship and desperateness: The stewardship concept

is relevant only when the basic necessities are met. Desperate

people do not care about the stewardship.

(3) The soil bank: The nutrient and C pools in soil bank can

only be maintained if all outputs are balanced by the inputs.

(4) The law of marginality: Marginal soils cultivated with

marginal inputs produce marginal yields and support marginal

living.

(5) The organic dilemma: Plants cannot differentiate the

nutrients supplied through organic or inorganic sources. It is

a question of logistics and availability.

(6) Soil as a source or sink of greenhouse gases: Agricultural

soils can be a major sink for CO2 and CH4, depending

on land use and management.

(7) Extractive farming and the environment: Extractive

farming and mining soil fertility adversely impact soil quality,

perpetuate hunger and poverty, exacerbate CO2 emissions,

and reduce ecosystem services.

(8) Synergism between soil management and improved

germplasm: The yield potential of improved germplasm can

be realized only if grown under optimal soils and agronomic

conditions.

(9) Agriculture as a solution to environmental issues:

Rather than a problem, agriculture must always be integral to

any solution towards environmental development. Humans

will always depend on agriculture, and it must be the engine

of economic development.

(10) Modern innovations: Yesterday’s technology cannot

resolve today’s problems.

The entire article can be downloaded here:

Lal. 2010. Managing sols for a warming earth in a food-insecure and energy-starved world. J. Plant Nutr. Soil Sci. 173:4-15.

Don’t Forget to Keep Your Alfalfa in Top Shape with Phosphorus!

Many factors are involved in producing a top-quality alfalfa crop. 

Healthy alfalfa growth requires adequate mineral nutrition

Although many factors (like weather) cannot be controlled, many other critical components need to be carefully managed.  As the demand for high-quality hay increases, a closer look at the role of proper nutrition is needed

There is no substitute for maintaining an adequate plant nutrient supply for production of high-yielding and high-quality alfalfa.  Alfalfa production removes large amounts of nutrients from the soil that must eventually be replaced to remain sustainable (a

bout 15 pounds of P₂O₅ removed in each ton of hay).  Since phosphorus (P) has many essential roles in alfalfa, both yield and quality are reduced when this nutrient is limited.

 

ATP uses P for energy storage

Most P in the plant is rapidly converted into organic compounds involved in a variety of essential reactions. For example, P in alfalfa is essential for formation nucleic acids, phospholipids and ATP- associated with things like photosynthesis, protein formation and nitrogen fixation.

Rhizobia nodules

Alfalfa field in Washington

In addition to direct plant growth benefits, P fertilization has also been shown to increase nitrogen fixation, nodule number and nodule size.  There are frequent reports that P or K nutrition have been found to improve disease tolerance or resistance.

Soils vary in their ability to supply P and nutrient deficiency symptoms in alfalfa are hard to detect before the deficiency becomes quite severe.  Therefore, soil testing is best way of predicting the potentially available nutrient supply.  It is generally best that P be applied prior to establishing the crop, since an adequate supply of P is critical for rapid stand development and a strong root system.  For established stands, surface applications are a good way to meet plant needs.

Harvesting alfalfa hay

Many sources of fertilizer P are successfully used for alfalfa production- including both solid and liquid forms.  A number of comparisons have demonstrated that most P fertilizer sources are equivalent, when properly used.  The selection of a specific P fertilizer is generally based on local availability, ease of application, and the cost per unit of nutrient.

Phosphorus fertilization is an essential component of alfalfa production.  High-yielding alfalfa removes large amounts of P which must be replaced when the soil P supply can not meet the plant demand.  Soil and tissue tests are useful for determining the appropriate amount of P to apply.  Failure to monitor and replace the nutrients removed in harvested hay will lead to losses of yield, plant stand, and profit. 

A productive milk cow turning a meal of alfalfa into ice cream

Sulphate of Potash for Quality... scan

Potassium fertilizer is commonly added to improve the yield and quality of plants growing in soils that are lacking an adequate supply of this essential nutrient. Most fertilizer K comes from ancient salt deposits located throughout the world.

The word “potash” is a general term that most frequently refers to potassium chloride (KCl), but it also applies to all other K-containing fertilizers, such as potassium sulfate (K2SO4), commonly referred to as sulfate of potash (or SOP).

I came across a nice pamphlet describing some of the advantages of using potassium sulfate as a source of potassium fertilizer.  It was written several years ago by the Potash Export Company in Vienna (Kali Export Gesellschaft).

Here is a link to the pdf of the booklet:

Sulfur, an overlooked nutrient…Are you keeping track?

Sulfur deficiency symptoms in corn

Since S deficiencies are increasing in many areas, the use of this nutrient is becoming more common. The most common forms of S used in fertilizer are elemental S and SO₄. Thiosulfate forms of S are also commonly available in many regions. A review of how S behaves in the soil is useful to get top crop performance.

Sulfur plays two important roles in agriculture…as an essential nutrient required for proteins and enzymes…and as a soil amendment for improving alkaline soils.

Many crops require between 10 to 25 lb/A of S each year. While this is not as much as some other nutrients, the frequency of crop S deficiency has been steadily increasing since many fertilizers do not routinely contain S and deposition of air-borne S has decreased. 

Although S exists in many different chemical forms in nature, plants primarily absorb it in the SO₄ form. The SO₄ molecule carries a negative charge, so it moves freely with soil moisture. As a result, SO₄ concentrations are sometimes greater with increasing depth in the soil below the rootzone. There are several excellent sources of plant-available SO₄ that will provide immediate crop nutrition. These include materials such as potassium-magnesium sulfate, ammonium sulfate, or potassium sulfate.

Elemental S is totally unavailable for plant uptake since it can not be directly taken up by roots. How­ever, when elemental S is added to soil, it gradually becomes converted (oxidized) to the plant-available SO₄ form. 

Large particles of sulfur will be slow to convert to sulfate

The transformation of elemental S to SO₄ is controlled by many factors. Since this conversion is done by soil microbes, several environmental and physical conditions govern how quickly this change takes place. In general, S oxidation takes place most rapidly in warm and moist soils. But field application should take place some time before the plants have a need for SO₄.

The physical properties of elemental S are also important. Small-sized particles have the most surface area and the most rapid reaction. However, fine particles of S can be difficult to apply. Fertilizer manufacturers have developed useful techniques where very fine S particles are clumped together with expandable clay to form a pellet which disintegrates in the soil.

Dr. Tim Hartz examines sulfur pastilles

Elemental S is highly acidifying after it is oxidized in the soil. It is commonly used to treat high-pH soils or to amend calcareous soils loaded with harmful concentrations of sodium. The specific S application rates should be calculated with the aid of a crop adviser.

Thiosulfate has also become a popular source of S nutrition for crops. Thiosulfate generally converts to SO₄ within a few weeks in typical summer growing conditions. Thiosulfate has also been shown to have beneficial effects on N transformations and may offer some unique benefits for plant metabolism.

Thiosulfate fertilizer

There is no reason to risk yield loss from S deficiencies. When the need for S is suspected, there are many excellent materials that are available to meet crop needs.

Sulfur burners are sometimes used to treat irrigation water with high concentrations of bicarbonate