Urea as a fertilizer Laboratory and pot-culture studies

A laboratory and pot-culture investigation to seek reasons for the variable response of grass and other crops to top-dressings of urea, and to the occasional phytotoxicity to seedlings of urea either mixed in the soil or drilled in contact with the seeds is described. The part played by biuret, the chief impurity in commercial urea, has been examined. It can be phytotoxic to germinating seedlings when in contact, but in the quantities likely to be applied in agriculture in top-dressings, e.g. 150 lb. of urea with up to 2·5% of biuret, its effect is negligible. Up to at least 5% of biuret can be present in urea without affecting its ammonification or nitrification in soil. It has been shown that loss of nitrogen to the atmosphere can account for the poorer responses of grassland to top-dressings of urea as compared with ‘Nitro-Chalk’ or ammonium sulphate. The phytotoxic behaviour of pure urea to germinating seeds seems to be due to rapid production of ammonia. This gas is also evolved from topdressings of urea. Both the phytotoxicity and the loss to the atmosphere can be reduced by mixing urea with acid salts to neutralize the ammonia.