Slicko, citrus is more sensitive than most crops to biuret, an impurity present in urea fertilizer created by excessive heating of chemical reactants during urea manufacture.  Toxicity can result from soil or foliar spray applications of urea applications with elevated biuret concentration. If urea is used as a nitrogen source for citrus, it should contain not more than 0.25 - 0.50% biuret by weight.  The leaf symptoms of biuret toxicity appear as an irregular butter yellow chlorosis that starts at the leaf tip.  Young leaves are more vulnerable than old ones, and affected leaves do not regreen.  On very young, expanding leaves, urea with a biuret concentration in excess of 1.0% may cause distortion as well as typical yellowing.  Succeeding growth flushes appear normal, provided applications of High biuret urea is not repeated.
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