Application of anhydrous ammonia or urea during the fallow period for winter cereals on the Darling Downs, Queensland .I. Effect of time on soil mineral N at sowing

Abstract

Nine field experiments were conducted in 1978, 1981 and 1982 to evaluate applications of anhydrous ammonia (AA) or urea applied during the fallow period (January-May) for winter cereal crops. Following fertilizer application, soil was sampled using a stratified soil coring procedure to determine the rate of transformation of applied N to nitrate (nitrification), the quantity of N remaining in mineral forms (NH4+NO3 and NO2), and the movement of applied N into the subsoil. Nitrification of applied N was usually quite rapid in moist soil, particularly with early application (January, February or March when mean soil temperature was >20°C. Very similar rates of nitrification (0.6-4.7 kg N ha-1 day-1) were found for AA and urea applications in May 1982. Extreme drying of soil following N application reduced nitrification to a very low rate in May 1982 (0.6 kg N ha-1 day-1) and to an undetectable level in January 1981. In moist soil in February 1978, AA applied at 56 kg N ha-1 was nitrified completely after 11 days and the 112 kg N ha-1 rate was estimated to have nitrified completely in about 12 days. Also, AA applied to moist soil in May 1978 was estimated to have nitrified completely in about 28 and 42 days for 56 and 112 kg N ha-1 rates, respectively. Low recovery of early applied N as soil mineral N in June 1981 was associated with very heavy rain received during the latter part of the fallow period (March-May). Soil erosion on sloping sites and on a level site was a likely cause for the very low recovery (<47% that of a May application) of January-applied N, and some movement of mineral N below 0.2 m was also evident. Low recovery in fertilized soil (0.2 m) at the level sites was due to a large proportion of mineral N moving into the subsoil (below 0.9 m at one site). Also, prolonged periods of waterlogging during April probably promoted some loss of N due to denitrification, thus resulting in reduction in soil mineral N levels. Low recoveries of early applied N in mineral forms at the end of relatively drier fallows in 1978 and 1982 were also associated with soil saturating rainfall during the latter part of the fallow period. Where wheat crops responded to applied N, January or February applications were less effective than May applications to increase yield and N content of grain.