SummaryPlant damage and reduced yields were associated with delayed ammonification or nitrification of added urea‐nitrogen. In a neutral sandy loam, the adverse effects of urea application were associated with the accumulation of toxic levels of first ammonia and later nitrite. Addition of hydrogen‐ion as phosphoric or sulphuric acid alleviated the damage thereby supporting the conclusion that ammonia and nitrite accumulation were the causes of phytotoxicity. In an acid sand, toxicity was associated first with the presence of large quantities of urea remaining in the soil though the evidence was insufficient to show in what way the urea was toxic. There was then a second phase of toxicity associated with the presence of considerable quantities of ammonium‐nitrogen under conditions where toxic levels of ammonia were likely to have occurred. The later phase (nitrite toxicity) did not occur. Phytotoxicity arising from urea addition to the soil could be due apparently to the persistence of urea as such in the soil and also to the accumulation of significant quantities of free ammonia and nitrite. The occurrence of one or more of these phases of urea toxicity depended on differences in soil condi