🐢 Slow-Release Nitrogen Fertilizers

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Slow release N fertilizers

neem cake blended and coal tar coated urea- chemically modified forms-urea

formaldehyde, IBDU, CDU

Slow-release fertilizers are excellent alternatives to soluble fertilizers. Because

nutrients are released at a slower rate throughout the season, plants are able to take up

most of the nutrients without waste by leaching. A slow-release fertilizer is more

convenient, since less frequent application is required. Fertilizer burn is not a problem

with slow-release fertilizers even at high rates of application; however, it is still important

to follow application recommendations. Slow-release fertilizers may be more expensive

than soluble types, but their benefits outweigh their disadvantages.

Slow-release fertilizers are generally categorized into one of several groups based on

the process by which the nutrients are released. Application rates vary with the different

types and brands, with recommendations listed on the fertilizer label.

Pelletized:

One type of slow-release fertilizer consists of relatively insoluble nutrients in

pelletized form. As the pellet size is increased, the time it takes for the fertilizer to

breakdown by microbial action is also increased. An example of this type is MagAmp,

a 7-40-0 fertilizer that is available in a coarse grade lasting two years and a medium

grade lasting one year. MagAmp is used commercially for container plants, but is

appropriate for use on turf, tree seedlings, ornamentals, vegetables, and flower

borders.

Chemically Altered:

A fertilizer may be chemically altered to render a portion of it water insoluble. For

instance, urea is chemically modified to make Ureaform (ureaformaldehyde) -- a fertilizer

that is 38 percent nitrogen, 70 percent of which is water-insoluble. This percentage is

often listed on fertilizer labels as the Percent W.I.N., or the percent of water-insoluble

nitrogen. This form of nitrogen is released gradually by microbial activity in the soil.

Because microbial activity is greatly affected by soil temperature, pH, aeration, and

texture, these variables can affect the release of nitrogen from Ureaform. For example,

there will be less fertilizer breakdown in acid soils with poor aeration -- an environment

unfavorable to soil microorganisms. Ureaform is used for turfgrass; landscaping;

ornamental, horticulture, and greenhouse crops.

IBDU (isobutylidene diurea) is similar to Ureaform, but contains 32 percent nitrogen, 90

percent of which is insoluble. However, IBDU is less dependent on microbial activity than

Ureaform. Nitrogen is released when soil moisture is adequate. Breakdown is increased

in acid soils. IBDU is used most widely as a lawn fertilizer.

Coated fertilisers

Controlled- or slow-release fertilizers are broadly divided into uncoated and

coated products. Uncoated products rely on inherent physical characteristics, such as

low solubility, for their slow release. Coated products mostly consist of quick-release N

sources surrounded by a barrier that prevents the N from releasing rapidly into the

environment. Different mechanisms, but similar (though not identical) end results.

The terms “controlled-release” and “slow-release” can mean different things to

different people, but for purposes of this discussion, the two terms are synonymous.

Except for a few slow-release K sources, almost all slow-release fertilizers are N

sources. They represent a relatively small segment of the total fertilizer industry (3 to 4

percent), but their use is growing faster than soluble (quick-release) materials. This is

primarily because they reduce the overall environmental impact of N fertilizers, as now

mandated in BMPs.

Coated:

Water-soluble fertilizers may be coated or encapsulated in membranes to slow the

release of nutrients. For example, Osmocote, a controlled-release fertilizer is composed

of a semipermeable membrane surrounding water-soluble nitrogen and other nutrients.

Water passes through the membrane, eventually causing enough internal pressure to

disrupt the membrane and release the enclosed nutrients. Because the thickness of the

coating varies from one pellet, or prill, to another, nutrients are released at different

times from separate prills. Release rate of these fertilizers is dependent on temperature,

moisture, and thickness of the coating. Osmocote is recommended for turf, floriculture,

nursery stock, and high-value row crops.

Another type of coated fertilizer is sulfur-coated urea (SCU), which is manufactured by

coating hot urea with molten sulfur and sealing with a polyethylene oil or a

microcrystalline wax. Nitrogen is released when the sealant is broken or by diffusion

through pores in the coating. Thus, the rate of release is dependent on the thickness of

the coating or the sealant weight. SCU is broken down by microorganisms, and chemical

and mechanical action. The nitrogen in SCU is released more readily in warm

temperatures and dry soils. SCU appears to be more effective when applied to the soil

surface, rather than mixed into the soil. Any method of application that crushes the

granules will increase the release rate to some extent.

SCU is best used where multiple fertilizer applications are normally necessary, such as

on sandy soils or in areas of high rainfall or irrigation. SCU is used on grass forages, turf,

ornamentals, and strawberries.

Nutricote is characterized by coating nitrate compound fertilizers with a special resin.

The duration of nutrient release is controlled by the porosity of the resin coating. A more

porous coating results in quicker release. This technology ensures consistency and

precision of nutrient release from Nutricote controlled release fertilizers.

When Nutricote is applied to the soil, the water in the soil enters the granule through

micropores which dissolves the nutrient elements. The nutrient elements will then be

released steadily through the same pores. Most Nutricote granules are 3 to 4 mm in

diameter and the nutrient content are NPK: 14-14-14 and NPK: 20-7-10.

Nutricote’s release rate is influenced by soil temperature, the higher the soil

temperature, the greater the release rate. Absorption of nutrients and water by plants is

generally increased with increasing temperature and plant growth will become more

vigorous as a result. Nutrient supply through Nutricote nicely matches the physiology of

plant response to temperature.

The release rate of Nutricote is not significantly influenced by soil moisture levels nor by

soil type or pH. Nutricote does not depend upon microbiological decomposition for its

action.

2.Polymer-coated fertilizers

Polymer-coated fertilizers (PCF) represent the most technically advanced state of

the art in terms of controlling product longevity and nutrient efficiency. Most PCFs

release nutrients by diffusion through a semipermeable polymer membrane, and the

release rate can be controlled by varying the composition and thickness of the coating.

The type of fertilizer substrate also may influence the rate of N release.

Meister products : Meister products are produced by using thermoplastic resins as

coating materials. The coatings are applied to a variety of substrates including urea,

diammonium phosphate, potassium sulfate, potassium chloride and ammonium nitrate.

Release-controlling agents such as ethylene-vinyl acetate and surfactants are added to

the coating to obtain the desired diffusion characteristics, while coating thicknesses

remain similar for most products. Release rates can also be altered by blending talc

resin into the coating.

Reactive Layer Coating: A relatively new coating technology known as reactive layer

coating (RLC) combines two reactive monomers as they are simultaneously applied to

the fertilizer substrate. These reactions create an ultra-thin membrane coating, which

controls nutrient release by osmotic diffusion. RLC products include coated basic

fertilizer materials such as urea, potassium nitrate, potassium sulfate, potassium

chloride, ammonium sulfate, ammonium phosphate and iron sulfate, in various particle

sizes. Coating weights on urea vary from 1.5 to 15 percent, depending on the release

duration desired.

Multicote products: In the production of multicote products, fertilizer granules are

heated in a rotating pan and treated with materials that create multiple layers of a fatty

acid salt. This is followed by the application of a paraffin topcoat. Coating weights are

relatively large compared to other technologies, but this is offset by the comparatively

low cost of the coating materials. Substrates include potassium nitrate, urea and triple

superphosphate. The various coated components are blended together into different

grades.

Coated N Fertilizers;

1.Ureaformaldehyde reaction products

Ureaformaldehyde (UF) reaction products represent one of the oldest

controlled-release N technologies, having been first produced in 1936 and

commercialized in 1955. Urea and formaldehyde are reacted together to various extents

to produce polymer-chain molecules of varying lengths. The more these products are

reacted, the longer the chains tend to be. Chain length, in turn, affects release

characteristics.

Ureaform is the oldest class of UF reaction products. It is sparingly soluble, and

contains at least 35 percent total N, with at least 60 percent of the total N as cold-water

insoluble N (CWIN). Ureaform is composed largely of longer-chained molecules of UF

polymers. The unreacted (and, therefore, quick-release) urea N content in UF is usually

less than 15 percent of the total N.

Methylene ureas are a class of sparingly soluble products that evolved during

the 1960s and 1970s. These products predominantly contain intermediate-chain-length

polymers. The total N content of these polymers is 39 to 40 percent, with between 25

and 60 percent of the N present as CWIN. The unreacted urea N content generally is in

the range of 15 to 30 percent of the total N.

UF solutions are clear water solutions. They contain only very-low-molecular

weight, water-soluble UF reaction products, plus unreacted urea. Various combinations

of the UF solutions are produced. They contain a maximum of 55 percent unreacted

urea with the remainder as one or more of methylolureas, methylolurea ethers, MDU,

DMTU or triazone.

Isobutylidene diurea (IBDU) : Unlike the reaction of urea and formaldehyde,

which forms a distribution of different UF polymer chain lengths, the reaction of urea with

isobutyraldehyde forms a single type of molecule. Although similar in chemical structure

to methylene diurea (MDU), its physical properties are quite different. IBDU is a

white crystalline solid available in fine (0.5 to 1.0 mm), coarse (.7 to 2.5 mm) and chunk

(2.0 to 3.0 mm) particle sizes. The product contains a minimum of 30 percent N with 90

percent of the N in water-insoluble form. The typical commercialized product contains

about 31 percent N.

Crotonylidene Diurea (CDU) : This slow acting nitrogen compound is formed by

reaction with crotonaldehyde or acetaldehyde. Powdered CDU containing 30 percent N

has been directly used as a fertilizer. The microbial decomposition of the chemically

bound CDU is temperature dependant

Agronomic properties and nutrient release mechanisms of UF materials

The conversion of UF reaction products to plant-available N is a multi-step

process, involving dissolution first, and then microbial decomposition. Once in the soil

solution, UF reaction products are converted to plant-available N through either microbial

decomposition or hydrolysis. Microbial decomposition is the primary mechanism of N

release. Environmental factors such as soil temperature, moisture, pH and aeration

affect microbial activity and, therefore, the rate of N release.

The rate of N release from UF reaction products is directly affected by polymer chain

length. The longer the methylene urea polymer, the longer it takes for the N to become

available. For ureaform and methylene urea products, the rate of mineralization is

reflected by the CWIN content and its Activity Index. The higher the AI value, the more

rapidly the N will become available. It is questionable if the very long methylene urea

polymers (HWIN) are effectively used by the plant.

Agronomic properties and nutrient release mechanisms of IBDU : Nitrogen from

IBDU becomes available to plants through hydrolysis. In the presence of water, the

compound will hydrolyze (break down) to urea and isobutyraldehyde. The rate of

hydrolysis is accelerated by low pH and high temperature. Unlike UF polymers that rely

on soil microbial populations to make the N available, IBDU is primarily dependent on

water as the critical element in N availability. Its low water solubility controls the transport

of the product into the soil solution.

Agronomic properties and nutrient release mechanisms of SCU: The mechanism of

N release from SCU is by water penetration through micropores and imperfections (i.e.,

cracks) or incomplete sulfur coverage in the coating. This is followed by a rapid release

of the dissolved urea from the core of the particle. When wax sealants are used, a dual

release mechanism is created. Microbes in the soil environment must attack the sealant

to reveal the imperfections in the sulfur coating. Because microbial activity varies with

temperature, the release properties of the wax-sealed SCUs are also temperature

dependent.during the cool-season growth period.

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Summary Cheat Sheet

Topic	Key exam point
Main concept	Nitrogen fertilizers that release N gradually rather than all at once
Examples	Neem-coated urea, sulphur-coated urea, urea supergranules, and urea-formaldehyde products
Main purpose	Improve N use efficiency and reduce loss by leaching, volatilization, or denitrification
Coated fertilizers	Release rate is controlled by coating material around the fertilizer particle
UF products	Urea-formaldehyde reaction products are classic slow-release N sources
Advantage over conventional N	More sustained nutrient availability and fewer applications in some systems
Agronomic relevance	Useful where split application is difficult or N losses are high
Exam contrast	Slow-release fertilizers differ from controlled-release fertilizers mainly in precision of nutrient-release control
Key memory point	Neem coating is a major India-specific example often asked in exams
Trap	Do not treat every coated fertilizer automatically as the same category with identical release mechanism

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website