SummaryThe causes and mechanisms of phytotoxicity that can bring about poor or adverse crop responses to applications of solid urea to soil are reviewed. Compounds which may cause phytotoxicity are urea, transformation products of urea formed during manufacture (biuret), and transformation products produced in the soil or in plants (cyanate, carbamate, ammonia, and nitrite). Of these, ammonia and nitrite are considered to be the most likely causes of phytotoxicity when urea of low biuret content is used.