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.

SUMMARYA laboratory investigation was made of the ammonification and nitrification in soil of pure urea with and without pure biuret and pure biuret alone. The soils ranged from acid (pH 5·1) to calcareous (pH 7·9). The work was done at two temperatures 5 and 25°C. The time between adding urea to the soil and the first analysis was from 1 h to 1 week. The soil moisture contents ranged from the wilting percentage to field capacity. In all work involving urea, ammonium sulphate was used as a standard to compare rates of nitrification.Biuret had no detectable effect on the rates of ammonification and nitrification of urea. Biuret by itself nitrified in all the soils at both temperatures being slowest at 5 °C in the acid soil and fastest in the calcareous at 25°C.Neither pH nor temperature appreciably affected the ammonification of urea but both had a marked effect on nitrification. The lower the pH and the lower the temperature the slower the nitrification. Ammonium from urea nitrified faster than ammonium sulphate and on the acid soil only ammonium from the urea nitrified.The moisture content of the soil only affected ammonification of urea slightly but had a marked effect on the nitrification, the drier the soil the slower the nitrification. The same observation applied to ammonium sulphate.

1. Pot culture experiments are described comparing the recovery of nitrogen, and sulphur by the difference and ‘labelled atom’ methods and the recovery of carbon by the latter method. Italian ryegrass was used as the test crop with ammonium sulphate, calcium nitrate and urea as the fertilizers.2. The recovery of fertilizer nitrogen was less using the ‘labelled atom’ technique than the conventional difference method.3. The recovery of sulphur from ammonium sulphate ‘labelled’ with 35S was measured. It was considerably less than the nitrogen, but not inconsiderable especially as a basal dressing of ‘unlabelled’ K2SO4 was applied.4. Using urea ‘labelled’ with 13C there was no evidence that any of the carbon was in the ryegrass four weeks after application.

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The effects of four five-course and six four-course rotations on some physical and chemical properties of a sandy loam overlying London Clay are reported.Periods under ley increased the water stability of the air-dry soil aggregates, the resistance of moist soil aggregates to simulated raindrops, the rate of drainage under 60 cm. of water tension as measured in the laboratory, decreased the draw-bar pull during ploughing and the force required to crush air-dry aggregates. These effects increased with the number of years under grass.After 2 years of arable cultivation the effect of a 3-year ley on water-stable aggregation was still apparent whereas the effect of a 2-year ley had largely disappeared, and those of a 1-year ley had completely disappeared.A 2-year ley in a five-course ley-arable rotation was sufficient to maintain the total soil nitrogen at about 0·16%. With one year of ley in a four-course rotation a slow decline in nitrogen was found when the initial level was about 0·21%.The rate of CO2 formation in soils of the arable break was proportional to the number of years they had been under ley; the longer the period the greater the volume of CO2 produced.Evidence is presented that fertilizer nitrogen is used most efficiently by cereals growing in soil in good physical condition. This is usually best following a period under grass.