The biuret formed at following conditions in Urea process. 1. High Temperature. 2. High Concentrations 3. High residence time

Urea is suitable for all crops and soils. Being non-polart, this is an ideal Fertilizer, but urea having more than 1.2% 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.
The biuret formed at following conditions in Urea process.
1.       High Temperature.
2.       High Concentrations
3.       High residence time
Formation of biuret takes place when urea is heated to its melting point it starts decomposition with evolution of ammonia presumably,urea first isomerizes which dissociates into isocynic acid and ammonia.
•          CO(NH₂)₂    =  NH₄CNO       +              NH₃.
•          (UREA)           (AMM.CYNATE)
•          The isocynic acid reacts with urea to form biuret.
•          NHCO  +  CO(NH₂)₂    =    NH₂CONHCONH₂
•          In the presence of excess ammonia biuret is formed at substantially lower rate by direct reaction between urea molecules.
•          2CO(NH₂)₂ =  NH₂CONHCONH₂+NH_3.
•             (UREA)                    BIURET                      AMMONIA
•          BIURET + UREA    =   TRIURET  + AMMONIA