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Improved fertilizer nitrogen (N) use efficiency in irrigated maize (Zea mays L.) often results from delaying application of moderate N rates until the crop is rapidly growing. The objective of this experiment was to determine how N application time, N rate, and planting date influenced the pattern of N and fertilizer−derived N (FN) accumulation in irrigated maize grown on a deep, productive silty clay loam soil (Typic Argiudoll, fine, montmorillonitic, mesic). A factorial combination of early and late planting, N application at planting, or at the 4−, 8− or 16−leaf growth stage, and two N rates (90 and 180 kg N ha−1 as 15N−depleted ammonium sulfate) were applied to the same plots in 1979–1980. Above−ground plant samples were taken at the 8−leaf, l2−leaf, silking, soft dough, and physiological maturity stages of growth. Average net rates of N and FN accumulation were calculated using a temperature index as the divisor. Rates of N and FN accumulation per plant were greatest between the 12−leaf and silking growth stages in 1979, except when N application was made at the 16−leaf stage where maximum accumulation rates occurred during early grain fill. Effects of N application time on N accumulation rates were more pronounced in 1980 and followed patterns similar to those in 1979. Substantial amounts of residual FN from 1979 accumulated in above−ground tissue in 1980. Residual FN uptake was apparently greater, in comparison to newly applied FN, in the heavily−fertilized or late−planted crop. Time of maximum N accumulation was delayed as time of N application was delayed. Grain yield was reduced by early N deprivation only when active N uptake ceased during early grain fill. Excessive N application minimized the effect of application time on final grain FN content. Delayed planting reduced FN recovery at both levels of N fertilization. Relative recovery of FN in maize grain was maximized by applying the low N rate late in vegetative growth, irrespective of planting date.

The objective of this experiment was to evaluate the effects of irrigation sequence and of time and rate of nitrogen application on the dry matter production and fertilizer use efficiency of irrigated corn (Zea mays L.), grown on a representative fine‐textured soil of eastern Nebraska, and to monitor the movement of fertilizer N in this soil. During 1974–1976, 15N‐depleted ammonium sulfate was banded at rates of 112, 168, or 224 kg N/ha at planting or sidedressed at the eight‐leaf growth stage. No N was applied in 1977–1979. In 1974–1977, irrigation applications of 5, 7.5, or 10 cm of water were made at 2‐, 3‐, or 4‐week intervals, respectively, until a total of 30 cm had been applied. The plots were uniformly irrigated in 1978–1979.Grain yield and N uptake in 1974–1976 were not influenced by time of N application, and were highest with light, frequent irrigation. From 1977–1978, yield increased with previous N rate in response to residual mineral N in the soil. During both treatment and residual years, fertilizer N recovery in the grain was greater with sidedressing and increased with N rate. Heavier, infrequent irrigation decreased tracer N recovery in 1974–1976. Maximum fertilizer use efficiency was obtained with the low N rate, applied as a sidedressing, and with light, frequent irrigation.

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