Explanation:

Ans. 1.5% 

Urea is suitable for all crops and soils. Being non-polart, this is an ideal Fertilizer, but urea having more than 1.5% Biuret it is very harmful for certain crops like pineapple, Citrus, Tobacco and coffee which are sensitive to Biuret, urea with a lower Biuret content of 0.5 to 0.8% is preferable for application as a foliar spray. Chlorosis occurs on the leaf of the plants, when Biuret 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 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.