Nitrogen requirements of irrigated wheat on the Darling Downs

Abstract

Eighteen fertilizer trials, each with five levels of nitrogen (N) and three levels of phosphorus (PI, were conducted on black earth soils of the Darling Downs to establish optimal economic rates of N fertilizer in commercial, irrigated wheat crops. The optimal economic rate of N with a fertilizer: wheat price ratio (kg N: kg grain) of 5:l, the yield response of 100 kg/ha of applied N, the yield without fertilizer, and the yield with fertilizer not limiting were calculated from derived yield response relations at each site. A multi-variate regression procedure was used to determine which soil or crop management factors significantly influenced the rate of N needed to optimize wheat yield. Delay in planting after June 1 and the level of residual mineral N in the soil at planting had strong negative effects on the response to fertilizer and the optimal rate of fertilizer required. The results indicate that yields of irrigated wheat may be below the economic optimum because of sub-optimal applications of N. Other soil and management factors such as available soil P and number of irrigations also affected grain yield. At 1 3 sites low protein wheat (< 1 1.4¦1~) was produced with all but the highest two rates of N fertilizer and at two sites even the highest rate produced low protein wheat. The effect of N fertilizer applied at planting on grain protein concentration was changed by the yield response to the fertilizer application. Grain protein concentration was curvilinearly related (R2 = 0.81) to relative grain yield (yield as a proportion of the maximum yield); grain protein was at its minimum at a relative yield of 0.5. Although heavy rates of N fertilizer at planting increased grain protein concentration on a few sites, usually these applications led to an inefficient use of N fertilizer; apparent incorporation of fertilizer N into grain decreased with increasing rate of fertilizer.