http://www.bio.tamu.edu/USERS/TONNA/CHEMSUM.HTM#BIURET
http://www.sciencenewsforkids.org/articles/20040225/ScienceFairZone.asp
Crop sensitivity to biuret levels in Urea depends upon factors such as plant species, soil properties, method and timing of fertiliser application.
A general guideline for the safe use of Urea applied to the soil would permit a maximum of at least 2% biuret. Many crops grown in New Zealand have a biuret tolerance level in the range of 4-5% in Urea.
At risk crops in New Zealand include citrus, new grass and possibly potatoes. It is likely effects may occur with levels >2.5% biuret. The risk to new grass appears greatest when Urea is sown with the seed, a practice which is not recommended.
The risk of effects of biuret to plants could be expected in contained growth systems such as indoor tomatoes and grapes, possibly hydroponic growing and pot plants. No information can be found of the effects from biuret in these situations.
Biuret is not retained in soil and leaches easily and is broken down by soil microorganisms relatively quickly.
Foliar application of Urea solutions containing biuret can cause injury symptoms on some plant species. The tolerance level for biuret concentration in Urea used for foliar spray is much lower than for soil application. At risk crops in New Zealand include avocado and citrus.
A safety guideline of 0.25% biuret in the product as applied for foliar application on many crops is widely accepted.
RECOMMENDATION
Unless the biuret level exceeds 2.5% there is unlikely to be any danger when Urea is applied to pastures.
Urea should not be applied with grass seed.
When Urea is applied as a foliar fertiliser the biuret level should not exceed 0.25% concentration.
http://www.ballance.co.nz/fsbiuret.html
Urea used for spraying should have a low biuret content (less than 0.4%), as higher levels cause leaf burn. Biuret is not a problem when urea is applied to the ground.
http://www.agric.nsw.gov.au/reader/citrus/h2311.htm 
