dressing N results in grain yields and fertilizer use efficiencies greater than that produced by applying preplant Fine tuning current best nitrogen management practices, such as N (Miller et al., 1975;Olson et al., 1986;Reeves and delayed N application to maize (Zea mays L.), is needed to improve fertilizer recommendations. This study was conducted to determine Touchton, 1986;Welch et al., 1971). Delaying N applicathe relationship between relative maize N deficiency and time of tion too long, however, may reduce yield and N fertilizer N application. Levels of N deficiency were established by applying recovery (Jung et al., 1972). It seems that the soil N different rates of N fertilizer. Additional N was applied to each level status would affect how late N application could be of N deficiency at eight growth stages ranging from early vegetative delayed without reducing yield. growth to late reproductive growth. Chlorophyll meter readings were Low soil N status early in the season caused the maxitaken before each N application as a measure of maize N deficiency. mum rate of N uptake to be delayed (Russelle et al., A N sufficiency index (SI) was calculated based on the relationship1983). Thus, it would seem that highly N deficient maize between N-deficient and non-N-deficient maize. Delaying N applicawould be able to respond to N applied late in the season. tion to the six-leaf stage resulted in nearly a 12% decrease fromThere is little data to show how soil N status early in the maximum grain yield when the SI was below 0.90, indicating N deficiency can be severe enough to prevent full recovery when N is side season affects maize response to delayed N application. dressed. The greater the N deficiency, the earlier N had to be appliedNitrogen applied to establish the LND plus N applied each J. Series Paper no. 12976. Received 26 July 1999. *Corresponding author (dwalters1@unl.edu).Abbreviations: CMR, chlorophyll meter reading; DAE, days after emergence; LND, level of nitrogen deficiency; SI, sufficiency index.

To improve management of winter wheat (Triticum aestivum L.), more information is needed on how grain yield is influenced by planting date, seeding rate, and applied P. A 3‐yr study was conducted to measure the effects of these variables on grain yield and yield components of wheat grown in low‐P soils. All soils were Crete silty clay loams (fine Montmorillonitic mesic Pachic Argiustoll) and had Bray & Kurtz no. 1 soil tests of less than 10 mg kg−1. A randomized complete block designed experiment with a split plot treatment arrangement using three planting dates (each in 1985, 1986, and 1988) as whole plots, and factorial combinations of three seeding rates and three P rates as split plots. Grain yield, spikes meter−2, kernels spike−1, and kernel weight data were collected. Relative grain yield was greatest when 400 growing degree days (GDD, 4.4 °C base temperature) accumulated between the planting date and 31 December. Increasing the seeding rate from 34 to 101 kg ha−1 resulted in yield increases of 0.39, 0.48, and 0.21 Mg ha−1 in 1986, 1987, and 1988, respectively. Increasing the P rate from 0 to 34 kg P ha−1 resulted in 0.67, 0.53, and 0.79 Mg ha−1 yield increase in 1986, 1987, and 1988, respectively. Planting date by P rate and seeding rate by P rate interactions in 1988 indicated that P reduced the negative influence of late planting and low seeding rate on grain yield. Path coefficient analysis indicated that under conditions resulting in low tiller numbers, kernel weight contributed most in yield determination, while under high tillering conditions the number of spikes meter−2 was the most important yield component. This study showed that wheat grain yields were optimized with planting dates that allowed 400 GDD accumulation before 31 December with a 101 kg ha−1 seeding rate when available soil P is sufficient.

Preliminary studies showed that winter wheat (Triticum aestivum L.) was losing relatively large amounts of N during the grain formation growth period. In order to further document this N loss, winter wheat plants were sampled at different times and at different locations before and after anthesis until maturity. The objective was to determine the nature and extent of dry matter and N losses that occur during the later stages of wheat development as influenced by N fertilization. While N is translocated rapidly from other plant parts to the grain after anthesis, total N losses ranged from 25 to 80 kg ha−1 in different experiments. These losses occurred during the grain filling period after anthesis. Dry matter and N losses were attributed primarily to the stems where 83 to 87% of dry matter losses occurred and 73 to 75% of the N. Dry matter and N losses from leaves and roots were relatively small. The N losses increased with increasing rates of N application. Plant N losses could account for much of the N losses found in soil N balance studies and certainly influence calculations involving fertilizer N efficiency. Grain protein could be doubled if plant N losses could be translocated instead into the grain.

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Research monitoring N uptake by various agricultural crops has shown total N accumulations in the plant to increase prior to the growth stages around heading to flowering with a subsequent decrease in total N occurring after flowering. Volatilization of N from the plant may account for much of this loss as well as account for some of the deficits exhibited in N balance studies. Experiments were conducted in a gas‐tight growth chamber to determine what role plants alone may play in the overall loss of N from the soil‐plant system. Winter wheat (Triticum aestivum L.) was established, vernalized, and grown to maturity in the growth chamber. At first internode elongation, the chamber was closed and sealed for the duration of the experiment. Air supplied to the chamber during this period was bubbled through 1 N H2SO4 to remove ambient NH3 and through a reagent specific to NO and NO2 to remove these gases. Air samples were continuously drawn from the chamber and bubbled through 0.2 N H2SO4 to trap evolved NH3. These samples were collected weekly and analyzed using steam distillation. A second portion of the air sample was washed through the NO‐NO2 specific reagent and analyzed colorimetrically. Only trace amounts of NO and NO2 could be found at any time during the experiment leading to the conclusion that volatilization of these gases does not contribute significantly to the loss of N from the plant. Ammonia volatilized from the system at the rate of 0.34 to 0.89 ✕ 10−1 mg NH3‐N/m2/day prior to flowering. After flowering the rate of NH3‐N evolution increased to 1.03 to 1.32 ✕ 10−1 mg/m2/day. This dramatic increase in the rate of NH3‐N evolution at flowering coincides with the plant growth stage that researchers have begun to observe deficits in total N accumulations in the above ground portion of the plants. This data supports the hypothesis forwarded here, that volatilization of NH3 from plant tissue can partially account for the deficits in total N accumulation observed in plant tissue following flowering.