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Controlled Release Fertilizers: Trends and Technologies

Sunil K. Jain

Dr.Sunil K. Jain

Nowadays scientists are engaged in the
development of various multi- and nano particulate delivery systems of
fertilizers with an objective to control their release and increase the
production of various medicinal plants.

Polymer coated controlled
release fertilizers look promising for widespread use in agriculture because
they can be designed to release nutrients in a more controlled manner by
manipulating properties of polymer coating. Application of controlled release
fertilizers (CRFs) at the time of planting offers a means to improve the
establishment of forest tree seedlings. As compared to conventional fertilizer,
the gradual release of nutrient release from CRFs may better coincide with
plant needs, minimize leaching and improve fertilizer use efficiency. Many
different CRFs types are available and products differ in both the technologies
by which nutrients are contained and the environment stimulus for a nutrient
release. The coming years represent a critical time in this field as commercial
applications are being explored. In near future, CRFs with their ability to
provide efficient and desirable controlled release will revolutionize
agriculture industry. The advantages of CRFs are numerous which make further
study in this field extremely
important

Introduction

Measured nutrient uptake by plants from
fertilizers can be achieved through modification of fertilizer products -
either chemically to reduce their solubility or physically, for example, by
coating encapsulation. Controlled release fertilizers
(CRFs) have been used for many years, beginning with the sulfur coated
ureas. The early sulfur coated materials did not always give a uniform
response, because either the coating sometimes might have cracked or the
coating would have been of uneven thickness, allowing the fertilizer granules
to break down at different times. However, newer generation of CRFs have resin
coats that better control the release of the fertilizer. CRFs could match the
fertilizer application more closely with the amount actually needed for plant
growth. Coated CRFs use a polymer or sulfur coating to encapsulate
water-soluble nutrients. The coating thickness and media temperature primarily
control the rate of nutrient release. The uncoated nitrogen reaction products
are relatively insoluble in water and nutrient release is generally controlled
by water availability and/or microbial decomposition

1

.



Objectives of controlled release
fertilizers

CRFs are broadly defined as products that
release the nutrients to the soil for plant uptake at a pre-determined time and
rate

2

. They have some objectives like,

  1. Increased production of crops
  2. Increased nutrient efficiency and
    quality
  3. Reduction of plant toxicity and stress
  4. Substantial reduction in pollution of
    soil, water reservoirs and atmosphere
  5. Reduction of fertilizer application
    costs.

Modes of nutrient release by slow- and controlled-release
fertilizers

The terms slow-release fertilizer (SRF) and
CRF are often used synonymously, but technically they distinguish different
fertilizer materials. SRF is used to define organic fertilizer materials like
hoof and horn and urea formaldehyde, as well as chemical fertilizers of low
solubility like dicalcium phosphate and magnesium-ammonium phosphate. On the
other hand, the term CRF is used for inorganic fertilizers that have been
coated by materials like acrylic resins, polyethylene, waxes and sulfur that
reduce their immediate solubility and availability to plants. A new type of
CRF, which enables control over both release rate and release pattern, was
developed

3

. A dry mixture of soluble fertilizer and thickener is
contained in a non-permeable envelope, having at least one small opening. Two
main processes govern the release of nutrients from the device: (a) water
penetrates through the opening(s) into the dry mixture forming a distinct and
sharp wetting front. The process starts with a ‘burst' of water into the
device; (b) nutrients leave the device through the opening either by diffusion
alone or by diffusive and convective flows. Several factors, which affect both
the water penetration and the nutrients release, were investigated: fertilizer
type (solubility, density), thickener type (Na-polyacrylamide and
Na-carboxymethylcellulose), thickener concentration in the dry mixture, size of
the device, and opening diameter. It was found that the rate of wetting and the
magnitude of the ‘burst' effect increase with fertilizer solubility.

In general, temperature and sometimes moisture
are the main factors affecting nutrient release of CRF, whereas SRF are more
affected by the particle size of fertilizer, moisture, and microorganisms
present in the medium

4

. Investigations have indicated that release
rates from fertilizer held at 100

o

F could be up to 60% higher than
fertilizer at 80

o

F.

3

The temperature not only affected
nutrient diffusion across the coating of CRF, it also exerted a major influence
on microbial activity, and thus on the release of nutrients from SRF. Careful
attention must be paid during and right after periods of temperature
fluctuation, particularly following high temperatures. Conversely, SRF and CRF
may be inadequate sources of nutrients in situations with low ambient and soil
temperatures.

Water-soluble fertilizers may be coated or
encapsulated in membranes to slow the release of nutrients. For example,
Osmocote

®

, a CRF 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. Another type of coated fertilizer is
sulfur-coated urea (SCU), which is manufactured by coating hot urea with molten
sulfur and sealing with polyethylene oil or a microcrystalline wax

5

.
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. Microorganisms, and chemical and mechanical
action break down SCU. 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

6

. 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.
Encapsulated fertilizers are known to be very effective and efficient sources
of slow release nutrients for the long-term feeding of plants. The nutrients
are released at slow, controlled rates through the fertilizer's coating
resulting in a sustained feeding of the plants. As a result, one application of
an encapsulated fertilizer can provide the necessary nutrients for a plant that
would ordinarily take multiple applications of soluble fertilizers. In
addition, encapsulated fertilizers can also be more efficient and cause less
environmental concerns than soluble fertilizers due to their slow release of
nutrients. Since the nutrients are released at a slow sustained rate rather
than a sudden surge, more of the nutrients are absorbed by the plant and thus
are not washed away or leached through the soil, where they can enter the
ground water.



Advantages of CRFs

  1. Fertilizer burn is not a problem with
    CRFs even at high rates of application
  2. Fertilizers are released at a slower
    rate throughout the season; plants are able to take up most of the
    fertilizers without waste by leaching
  3. Less frequent application is required
  4. Uniform particle size allows easier
    and precise mechanical distribution
  5. Flexibility
    of release periods from 40 to 360 days at 25º C
  6. Reduced
    capital and labor outlay in horticultural crop production
  7. Reduced nutrient loss via
    leaching and run-off
  8. Reduced seed or seedling
    damage from high local concentrations of salts
  9. Reduced leaf
    burn from heavy rates of surface-applied fertilizers
  10. Improved
    storage and handling properties of fertilizer materials
  11. Product
    differentiation resulting in improved market potential.

Despite the advantages of slow and
controlled release fertilizers, only about 0.15% of the total fertilizers
consumption is such products. This is mainly due to the very high costs of CRFs
and lack of proper legislation in most parts of the world to restrict the use
of soluble fertilizers

7

.



Marketed controlled release formulations
with their technology

Many different types of CRF are currently
marketed for use with forest tree seedlings. CRFs primarily vary in terms of
their nutrient formulations, estimated product longevities, and mechanisms of
nutrient release. The ultimate goal of CRF manufacturers has been to develop a
product that delivers nutrients at a rate matching the plant demand, thus
improving crop yield and minimizing the loss of nutrients due to leaching

1,8,9

.

1. Urea-formaldehyde reaction products
.

These are also known as Nitroform,
Ureaform, Methylene Urea, Blue Chip, Nutralene or Methex, represent one of the
oldest controlled-release nitrogen technologies, having been first produced in
1936 and commercialized in 1955.

2. Sulfur-coated fertilizers.

The Tennessee
Valley Authority developed SCU technology in the 1960s and 1970s

10

. Sulfur was chosen
as the principal coating material because of its low cost and its value as a
secondary nutrient

9

. SCU is often marketed for use in the turf grass
industry.

3. Polymer-coated fertilizers (PCFs).

These represent the most
technically advanced state of the art in terms of controlling product longevity
and providing nutrient efficiency. PCFs were manufactured as early as 1970 in Japan. Polymer coatings can be categorized as either thermoset resins or thermoplastic
resins. Because of the relatively high cost of the coatings on most
polymer-coated products, their use has been restricted mostly to high-value
applications. Many different PCFs are marketed for use in continuous production
of forest tree seedlings. Perhaps the most common three marketed products types
are Nutricote

®

, Osmocote

®

, and Polyon

®

.

11

3.1. Osmocote.

Production of
Osmocote involves the coating of a soluble fertilizer core with a thermoset
copolymer of dicyclopentadiene and a glycerol ester (linseed oil) dissolved in
an aliphatic hydrocarbon solvent. The Osmocote market has been mainly limited
to commercial ornamental horticulture production, such as nurseries and
greenhouses, citrus production, and strawberry production. Osmocote has been
shown to release nitrogen at a more rapid initial rate than comparable polymer
coated CRFs

12

.


3.2. Nutricote
.

Nutricote employs
thermoplastic resins such as polyolefins, polyvinylidene chloride, and
copolymers as coating materials. The coatings are dissolved in fast-drying
chlorinated hydrocarbon solvents and are applied to a variety of substrates
including urea, diammonium phosphate, potassium sulfate, potassium chloride,
and ammonium nitrate. Because the thermoplastic polymers used are highly
impermeable to water, release-controlling agents such as ethylene-vinyl acetate
and surfactants are added to the coating to obtain the desired diffusion
characteristics. Coating thicknesses are essentially the same for all products
with the release pattern being controlled by the level of release-controlling
agent. Blending talc and resin into the coating can also alter release rates.


3.3. Reactive layer coating

.

A relatively new
coating technology known as reactive layer coating (RLC) polymerizes two
reactive monomers as they are applied to the fertilizer substrate in a
continuous coating drum. This

in situ

reactive layer polymerization
forms an ultra-thin membrane coating, which controls nutrient release by
osmotic diffusion. A number of these products are being marketed under the
trade name Polyon. These include coated basic fertilizer materials, in various
particle sizes i.e., urea, potassium nitrate, potassium sulfate, potassium
chloride, ammonium sulfate, ammonium phosphate and iron sulfate. Coating
weights on urea vary from 1.5 to 15%, depending on the release duration
desired.

4. Multicote products
.

In the production
of multicote products, fertilizer granules are heated in a rotating pan and
treated with fatty acid and metal hydroxide, such as stearic acid and calcium
hydroxide. The two react to form a coating of the metal salt of a fatty acid,
such as calcium stearate. Multiple layers of fatty acid salt are reacted

in
sit

u, followed by the application of a paraffin topcoat. Coating weights
are relatively large compared to other technologies, but this problem is offset
by the comparatively low cost of the coating materials. Substrates coated
include potassium nitrate, urea, and triple superphosphate. The various coated
components are blended together into different grades, which are marketed under
the Multicote name.

5. Polymer/Sulfur-coated fertilizers
.

Polymer/sulfur
coated fertilizers (PSCF) are hybrid products that utilize a primary coating of
sulfur and a secondary polymer coat. These fertilizers were developed to
deliver controlled- release performance approaching polymer-coated fertilizers,
but at a much reduced cost. Sulfur is employed as the primary coating because
of its low cost. Low levels of a polymer surface-coat are used to control
nutrient release rate. Unlike the soft wax sealants used to cover imperfections
in the sulfur coatings of SCUs, the polymers in this case are chosen to provide
a continuous membrane through which water and nutrients must diffuse. The water
permeability characteristic of the polymer controls the rate of water diffusion
into the particle. The combination of the two coatings permits a positive
cost/benefit value over products with singular coatings of sulfur or polymer.
Tomaszewska and Jarosiewicz

13

studied the possibility of using of
polysulfone as a coating for water-soluble NPK granular fertilizer. The
coatings were formed from the polymer solutions by the phase inversion
technique. As the polysulfone coatings are not biodegradable, the starch was
added to facilitate the destruction of the polymeric coatings in soil. It was
observed that the addition of modifying agent (starch) to the polymer solution
influenced the release rate of NPK from encapsulated granules.

One type of slow release encapsulated
fertilizer currently in wide use for both nursery and turf applications is
sulfur-coated fertilizer. Coating weights ranging from 15% to 35% by weight are
applied to granular fertilizer in coating drums. There are two modes of release
of the sulfur coated fertilizer's / nutrients

14

. The first is by
diffusion through cracks and / or other imperfections in the sulfur coating.
This allows the nutrients to be released rather quickly, and is the predominate
mode of release for short-term fertilizer products. The second mode of nutrient
release is through coating breakdown. This allows for the longer residual
feeding of plants and this is the primary mode of release for nursery-type
sulfur coated fertilizers. The major advantage of the sulfur coated fertilizers
is their relatively low cost. This is due to low cost of raw material as well
as inexpensive manufacturing costs associated with the coating drum process.

A second type of encapsulated slow release
fertilizer utilizes solvent applied polymer coatings. For this type of
encapsulated fertilizer, the polymer is first dissolved in an organic solvent
and then sprayed onto the fertilizer base in either a coating drum or a fluid
bed. As the solvent evaporates, a very uniform, continuous polymer film is left
behind, forming the fertilizer's barrier coating.

Solvent and latex applied polymer
encapsulated fertilizers both offer similar important benefits over sulfur
coated products concerning consistency of release rates and the ability to
provide extended fertilizer residuals. These benefits are chiefly due to the
uniform, continuous, and rather defect-free film coating which surrounds the
fertilizer core in each product type. These polymer coatings are also very
tough and durable and generally are not prone to significant mechanical
breakdown. In addition, the coatings are biologically inert and, thus, are not
susceptible to breakdown resulting from microbial activity in the soil or other
potting media. As a result, the majority of the nutrient release is by
diffusion through the polymer coat, rather than release through imperfections
and flaws or as a result of particle breakdown. This allows for a much more
uniform and consistent nutrient release rate and, if the barrier properties of
the polymer are sufficient, a longer residual nursery-type encapsulated
fertilizer than sulfur coated products. Some of the marketed products of CRFs
are listed in Table 1.

Summary

Application of CRFs as a one-time
application appears effective. CRFs are generally 3 to 4 times more expensive
than current fertilizer materials, but the ability to apply all of the
fertilizer in the fule and to eliminate spring sidedress applications helps
defray part of that expense. Also, being able to apply 65% or less of the
current application rate would help reduce costs. Controlled release
technologies an invaluable scientific tool for improving performance and safety
of chemicals, which involve materials such as barriers surrounding active
materials to deliver the latter at the optimum time and rate needed. The
technical objective of this science is to find and judiciously use barriers,
usually specially designed polymers.  The use of CRFs
starts to evolve as a promising direction offering an excellent means to
improve management of nutrient application and by these reducing significantly
environmental threats while maintaining high crop yields of good quality.

References

  1. Jacobs,
    D.F., Bose, R., Hasse, D.L. “Incorporating controlled release fertilizer
    technology into outplanting”, in National Proceedings: Forest and
    conservation nursery associations, L.E. Riley, R.K. Dumroese, T.D. Landis,
    Technical coordinators. Ogden, 2003, pp. 37-42.
  2. Shaviv, A.,

    Adv. Agro.,

    71

    :

    1-49, 2000.
  3. Shavit, U., Shaviv, A., Shalit, G.,
    Zaslazvsky, D.,

    J. Controlled Release,

    43:131-138,
    1997.
  4. Wybraniec, S., Schwartz, L., Wiesman,
    Z., Markus, A., Wolf, D.,

    J. Environ. Sci. Health.,

    37(3): 235-45,
    2002.
  5. Giordano, P.M., Mortvedt, J.J.,

    Agron.
    J.,

    62: 612-614,1970.
  6. Hashimoto, I., Mullins, R.C.,

    Soil
    Sci. Soc. Am. J.,

    43: 1165-1168, 1979.
  7. Sartain, J.B.,

    Proc. Int.
    Symp.Integr. Nutr. Mgmt. Syst. Banana Prod.,
    Costa Rica,1999.
  8. Hauck, R.D., “Slow release and bioinhibitor-amended
    nitrogen fertilizers”, in Fertilizer Technology and use, O.P. Engelstad, 3

    rd

    Ed. Soil Sci. Soc. Am. Madison, WI, 1985, pp. 293-322.
  9. Goertz, H.M., “Controlled release
    technology”, in Encyclopedia of chemical technology, Howe-Grant, M., 4

    th

    ed. New York (NY), John Wiley & Sons, Inc. 1993, pp. 254-274.
  10. Allen, S.E., Mays, D.A.,

    J. Agr.
    Food Chem.,

    19(5): 809-812, 1971.
  11. Jacobs, D.F.,

    USDA


    Forest


    Service Proceedings RMRS,

    P-35: 113-18, 2005.
  12. Huett, D.O., Gogel, B.J.,

    Commun.
    Soil Sci. Plant Anal.,

    31: 959-973, 2000.
  13. Tomaszewska
    , M., Jarosiewicz, A.,

    Desalination,

    163 (1-3): 247-252,
    2004.
  14. Hummel, N.W., Waddinton, D.V.,

    Hort.
    Sci.,

    21(5): 1155-1156, 1986.

Table 1. Different types of CRFs, and their
marketed products


Category     


Examples of marketed products

Coated Materials

  1. Sulfur coated
  2. Polymer coated

a) Polymer-resin

b)Polyurethane

c)Thermoplastic resin

Lesco

®

Osmocote

®

, Sierra

®

,
High N

®

Polyon

®

Nutricote

®

(Polyolefin)

 

Uncoated materials

  1. Ureaform
  2. IBDU

 

Nitroform

®

Woodace

®

About Authors

Sunil K. Jain 1 *, Manoj Kumar 1 , Nalini Anande 1 , G.P.Agrawal 2    

1 SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur (C.G.), 495 009, INDIA

2 Department of Pharmaceutical Sciences, Dr. H. S. Gour University, Sagar (M.P.) 470 003, INDIA

* Address Correspondence to: 

SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur-495 009, INDIA, E-mail: suniljain25in@yahoo.com

Sunil K. Jain  

*Dr. Sunil K. Jain is lecturer in Pharmaceutics at SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur (C.G.) 495 009, INDIA, with 5 years of teaching and research experience. He has supervised 1 M. Pharm. thesis and has published 13 research papers in International and 5 National journals and presented papers in various National and International conferences. He is a life member of IPA and APTI. His area of research includes novel floating drug delivery systems, colon targeted delivery systems and novel delivery of topical sun screening agents.

Email address: suniljain25in@yahoo.com

Dr. G.P. Agrawal is currently Professor of Pharmaceutics in the Department of Pharmaceutical Sciences, Dr. H.S. Gour University, Sagar, India. He has more than 31 years of teaching and research experience with more than 50 research publications in reputed pharmaceutical journals to his credit. He has guided Five PhD students and six students are working for their PhD program under his supervision. His areas of research interest include solid dispersions, hydrogels, and novel drug delivery systems.

Mr. Manoj Kumar is presently working as a Lecturer in Pharmaceutics at SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur (C.G.) 495 009, INDIA .

Ms. Nalini Anande is doing M. Pharm. from SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur (C.G.) 495 009, INDIA.

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