Urea of foliar spray should not contain biuret more than

The Urea of foliar spray should not contain a biuret of more than 1.5%.
Fertilizer-grade urea contains the impurity biuret which is toxic to the plant, especially when applied as a leaf dressing.
Biuret of more than 1.5% can damage the root of the plant and also useful bacteria and chock the capillary of the root cell, Nitrogen carrying capacity will be reduced.