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Controlled release fertilizers - uses and benefits

Since long, the main purpose of fertilizer application is to provide nutrients to plants in order to increase crop yields. Thus, improvement of fertilizer use efficiency of nutrient uptake and better crop yields is important for fertilizer producers and users. However, any fertilizer regardless of its form may harm the environment if misused. Environmentalists characterize fertilizer use as one of the source of polluting soil, water, and air. The adverse environmental impacts of fertilizer use are attributed towards nitrate leaching into ground water, emission of greenhouse gases (nitrous oxides), soils polluted with toxic heavy metals, and surface runoff of N and P nutrients causing aquatic eutrophication. Researchers and fertilizer producers have attempted to discover advanced techniques for fertilizer usage that can improve nutrient use efficiency and minimize environmental impacts. 
Urea is widely used fertilizer due to its high nitrogen content (about 46.6%). High water solubility nitrogen (from urea) can be leached from the soil before plants could assimilate it. Approximately, 75% of nitrogen can be lost in areas with high, intermittent rain falls. This factor not only results in additional cost but also contaminate local waters. Although, urea losses can be restricted by repetitive application of smaller fertilizer quantities, but costs of repetitive spreading are often high. A fundamental flaw exists in application method of N fertilizer that we adopt – we don’t apply N as the crop needs it. In rare cases, applying all N at pre-plant does not result in optimal use of nitrogen. It is subjected to environmental losses as volatilization, denitrification, leaching, and runoff.
In those cases, controlled release fertilizers (CRF) reduce risk of environmental losses. These can be divided into 3 categories based on their coating and nutrient composition. (1) Uncoated, nitrogen-based fertilizers are the oldest class of CRF that consist of chemically-bound urea and the release rate is determined by particle size, available water, and microbial decomposition e.g. Urea- form and isobutylidene-diurea (IBDU) (2) Coated, nitrogen-based fertilizers – Sulphur-coated urea is one of the first CRF. Thickness of sulphur coating controls the nitrogen discharge. Although still used in agriculture, sulphur-coated urea is rarely used in forest, conservation, and native plant nurseries. (3) Polymer-coated multi-nutrient fertilizers –Polymer-coated CRF (PCRF) are the newest, most technically sophisticated fertilizers being used in horticultural plant production. They consist of soluble nutrients core surrounded by a polymer coating.  
Each polymer-coated fertilizer particle is called as a “prill”. The nutrient release is controlled by the chemical composition and thickness of polymer coating. Polymer coatings gradually release nutrients over extended periods; release rates can be as short as 3 months or as long as 18 months. 
Particle coating has become very important in fertilizers, pharmaceutical, and food industries. The demand of coating granular fertilizers with minerals is increasing. Urea is coated with phospho-gypsum, neem oil, polymeric suspensions, and micronutrients like sulphur zinc etc. Phospho-gypsum coating on urea fertilizers is an important application because coating is done in order to enhance nitrogen use efficiency of urea, impart strength, increase the bulk density, and reducing caking tendency.
Polymer Coated Fertilizers (PCFs) are widely used for containerized plants and high-value field crops in high-leaching environments. A specialized application of PCF is the zero-leaching root zone of plants grown in space. To make PCF, fertilizer salts are aggregated into units called prills and polymer membranes are applied. Unconventional polymer coating materials and manufacturing processes have been developed by several companies for each PCF type. Almost 20 years ago Harvey M. Goertz in 1993 described the composition and manufacturing processes of the coatings. Osmocote Plus, Nutricote Total with minor nutrients, and Polyon Coated NPK Plus are all categorized as polymer-coated fertilizers. Polymer coatings can be categorized as either thermoset resins or thermoplastic resins. Osmocote is listed as a thermoset resin, Nutricote is listed as thermoplastic resin, and the category of Polyon is not clear. The fertilizer companies use the terms “resin” and “polymer” interchangeably.
The advantages of polymer-coated controlled release fertilizers include convenient to adjust fertilization type and rate for different crops, improved fertilizer use efficiency, reduced fertilizer pollution in wastewater, no need of post fertilization rinsing and nutrient availability at root initiation. 
Improvement in properties of urea obtained by fluidized bed coating of urea with phospho-gypsum using wet and dry methods. Emulsion of phospho-gypsum, neem (Azadirachta indica L.) oil, LAB (Linear Alkyl Benzene) and water was prepared in concentrations of 5%, 10% and 15%. Coatings were done with phospho-gypsum, neem oil, LAB, water (wet coating) and phospho-gypsum, neem oil and LAB (dry coating). Wet method is better than the dry method. Advantage of phospho-gypsum coating is unearthed when phospho-gypsum slurry is applied on urea and it forms fine coating, thus preventing nitrogen loss by denitrification.
It can be concluded that regardless of coating type, polymer or phospho-gypsum fertilizers should be used for their proven benefits. Coated fertilizers are excellent alternatives to soluble fertilizers because of slow release of nutrients amidst the season. In addition, fertilizer burn is also not a problem with coated fertilizers even at high rates of application. Plants can take up most of the nutrients without waste by leaching. Coated fertilizers are more suitable, as less application frequency is required. Though, it is still important to follow application recommendations. Coated fertilizers could be more expensive than soluble types, but their benefits outweigh their constraints like labour and application costs by eliminating the need for multiple fertilizer applications. Prolonged nutrient release can provide more uniform plant nutrition, better growth, and improved plant performance. The maximum benefit from coated fertilizer can only be attained when the duration of nutrient release is synchronized with the periods of plant nutrient uptake.
The authors are associated with Agro-biology Lab, Department of Agronomy, University of Agriculture, Faisalabad, Pakistan.
 

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