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From many years, the main goal of applying fertilizers is to provide nutrients to plants to increase or sustain crop yields. Thus, to improve fertilizer use efficiency in terms of nutrient uptake and better crop yields is important to fertilizer producers and users. However, any fertilizer, whether in the natural, organic, or inorganic form, can harm the environment if misused. Recently, fertilizers usage has been labelled by environmentalists as one source of polluting soil, water, and air. The main environmental impacts associated with fertilizer use have been linked to 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. To ensure that proper use of fertilizer is beneficial to both crop production and the environment, researchers and fertilizer producers have tried to find ways to achieve the newly defined goal of fertilizer usage that may improve nutrient use efficiency of applied fertilizers and minimize environmental impacts.
Urea is widely used as a fertilizer because of its high nitrogen content (about 46.6 per cent). Having high water solubility nitrogen (from urea) may be leached from the soil before plants have an opportunity to assimilate it. It is estimated that 75 per cent of the nitrogen may be lost in areas with high, intermittent rainfalls. Such losses result not only in increased costs, but also they contribute to the contamination of local waters. Although urea losses can be minimized by the repeated application of smaller fertilizer quantities, the costs associated with repeated spreading are often high. There is a fundamental flaw in how we apply N fertilizer – we dont apply N as the crop needs it. In some cases, applying all N at pre-plant does not result in optimal use of nitrogen. It is subjected to environmental losses like volatilization, denitrification, leaching and runoff.
In such case, controlled release fertilizers (CRF) reduces risk of environmental losses. These can be divided into 3 categories based on their coating and nutrient composition. (1) Uncoated, nitrogen-based fertilizers. This oldest class of CRF consists of chemically-bound urea and the release rate is determined by particle size, available water, and microbial decomposition. Urea form and isobutylidene-diurea (IBDU) are examples of uncoated, nitrogen- based fertilizers. (2) Coated, nitrogen-based fertilizers – Sulphur-coated urea was one of the first CRF and nitrogen release is controlled by the thickness of the sulphur coating. 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 and most technically sophisticated fertilizers being used in horticultural plant production, and consist of a core of soluble nutrients surrounded by a polymer coating.
Each polymer-coated fertilizer particle is known as a prill and nutrient release is precisely controlled by the chemical composition and thickness of the polymer coating. Polymer coatings that gradually release nutrients over extended periods; release rates can be as short as 3 months or as long as 18 months and have special mechanism to release nutrients. Water moves in through coating to dissolve urea and N diffuses out through porous polymer membrane.
Particle coating is becoming increasingly important in fertilizer, pharmaceutical and food industries. The demand for coating granular fertilizers with minerals is increasing. Urea is coated with Phospho-gypsum, neem oil, polymeric suspensions and micronutrients like sulphur, zinc etc. Coating of Phospho-gypsum on urea fertilizer is an important application of coating process. Coating of urea particles is done to increase nitrogen use efficiency of urea. Nitrogen losses due to leaching, surface volatilization, and denitrification can be minimized by controlling the dissolution rate. Coating also helps to impart strength, increase in bulk density and lowering of caking tendency, thus improving the handling properties.
Polymer Coated Fertilizers (PCFs) have largely been used for containerized plants and high-value field crops in high-leaching environments. One 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. Unique 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.
Polymer-coated controlled release fertilizers offer several advantages to plants such as easy to adjust fertilization type and rate for different crops, better fertilizer use efficiency, less fertilizer pollution in wastewater, no rinsing required after fertilization, 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 per cent, 10 per cent and 15 per cent. Coatings were done with both Phospho-gypsum, neem oil, LAB and water (wet coating) and Phospho-gypsum, neem oil and LAB (dry coating). Wet method is better than the dry method. Advantages of Phospho-gypsum coating is when Phospho-gypsum slurry applied on urea, forms fine coating and protects the loss of nitrogen by denitrification ensuring regulated continuous availability of nitrogen for a longer period, as per the requirement of crops.
From all above discussion we can say that whatever the coating is, we should use fertilizers either Polymer or Phospho-gypsum both are beneficial for crops. Coated 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 coated fertilizer is more convenient, since less frequent application is required. Fertilizer burn is not a problem with coated fertilizers even at high rates of application; however, it is still important to follow application recommendations. Coated fertilizers may 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 may provide more uniform plant nutrition, better growth, and improved plant performance. The maximum benefit from coated fertilizer can only be achieved 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.