Field experiments comparing the use of urea to other fertilizers were performed. The fertilizer was placed with the seed in all but two experiments.
Urea containing 2.5% biuret reduced stands of small grain by 30% when the equivalent of 20 pounds nitrogen per acre was applied with the seed. Ammonium nitrate at 80 pounds nitrogen per acre caused a reduction in stand of 10%. Stands were depressed by 25 and 60% when 40 and 80 pounds nitrogen, respectively, as reagent urea were used per acre.
Urea, containing 10% biuret, applied broadcast at 160 pounds nitrogen per acre, caused no damage to germination of either corn or barley.
Urea was intermediate between ammonium carbonate (most damaging) and ammonium nitrate in a greenhouse germination trial.
When fertilizer was mixed with the soil in the laboratory, considerable ammonia was evolved from the soil atmosphere by aeration when either urea, ammonium hydroxide or ammonium carbonate was used. Only traces of ammonia were removed from soils treated with either ammonium nitrate, ammonium sulfate or biuret. Ammonia gas formed from urea, and the contaminant biuret in urea, are therefore both instrumental in causing commercial urea to be toxic to germinating seeds.