Different variants of polymer-coated urea have been designed to synchronize N release and crop N uptake with minimum side effects. These have already been tested for achieving high fertilizer N use efficiency along with high grain yield of rice in a large number of studies from all over the world (Sato et al., 1993; Shoji and Kanno, 1994; Singh et al., 1995; Blaise and Prasad, 1996; Fashola et al., 2002; Carreres et al., 2003; Wakimoto, 2004; Acquaye and Inubushi, 2004; Kondo et al., 2005; Singh et al., 2007; Tang et al., 2007; Yan et al., 2008; Kiran et al., 2010; Patil et al., 2010; Zhang et al., 2012; Ye et al., 2013; Wang et al., 2015). Three derivatives of polymer-coated urea with 6, 8, and 12 % coating (w/w) at an identical N application rate were evaluated by Wang et al. (2015) during two rice-growing seasons. While the 6 % polymer coated urea could improve 15N recovery and reduce 15N loss, and increase grain yield slightly due to an initial 15N burst occurring at high field temperatures after basal fertilization; 8 or 12 % coated urea better met plant N demand from transplanting to heading, greatly enhanced 15N recovery, and decreased 15N loss and NH3 volatilization. But, unlike a significant increase of yield for 12 % coated urea, 8 % coated urea did not increase yield due to 15N release and excessive 15N uptake by plants at ripening. In India, Patil et al. (2010) found that even by applying a 50:50 mixture of polymer coated urea and ordinary urea, total N dose can be reduced by 50% without any reduction in grain yield of rice. In China, a new variant of polymer-coated urea has been developed (Zhang et al., 2006) that uses coating of thermoplastic resin, which is made from recycled plastic films initially used for greenhouses on local vegetable farms. This material is about 70% cheaper than the new polymer coating material (Yang et al., 2012a). Derivatives of urea coated with cheap polymer are being produced on large scale in China using a fluidized bed boating process. Yang et al. (2012b, 2013) could record 23 to 104% increase in recovery efficiency by applying single dose of new polymer-coated urea fertilizer with varying N release longevities as compared to when conventional urea was applied at the same rate.
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