Biuret is an impurity in urea fertilizer caused by a side reaction at higher temperature during processing. It has been identified as injurious to citrus foliage and to certain germinating seeds.
The earliest urea plant designs produced urea with up to five percent biuret. From 1955 to present many urea plants were designed to produce low-biuret product with not more than three-tenths percent biuret. The lower biuret concentration was found to be safe for these special applications which amount to about two percent of total urea fertilizer consumption.
New plant designs are successful in producing urea prills with eight-tenths percent biuret and urea granules with one percent biuret.
These plants can handle 98 percent of the urea market with simpler and more cost-effective urea finishing designs that produce higher quality products with better storage and handling characteristics.

1987 Oomen James Chemical Eng The Biuret Myth.pdf