after flowering. However, these types might also be more susceptible to N deficiency early in the grow-
ing season and less likely to recover from such setbacks. Further complicating the ability to characterize
a cultivar’s response to fertilizer N is the fact that these two strategies can interact to determine the final
N use. In other words, the amount of N required for maximum yield may or may not be related to when
the N is accumulated by the plant.
Because of its economic value and high requirement for fertilizer N, much of the recent effort in iden-
tifying genotypic variation for N use has been directed toward maize. Although differences in N use
among maize genotypes have been reported for inbreds [223] and open-pollinated populations [121],
there is considerable controversy regarding whether these differences can be used to improve N fertilizer
management of hybrids. Whereas some studies have reported large differences among maize hybrids in
their response to fertilizer N [127,224], others have shown no or limited differences [117,225]. Similarly,
a separate large-scale study did not observe hybrid N rate interactions, although it was noted that hy-
brids in an individual location responded differently to the level of soil-applied N [226]. In an attempt to
reconcile these differences, hybrids were divided into breeding groups based on their inbred parents, and
differences in the N rate required for maximum yield were observed between, but not within, the groups
[126]. Collectively, these studies suggest that maize hybrids do respond differently to the level of fertil-
izer-applied N but that the growing environment and the hybrid’s genetic makeup can markedly influence
the response.
C. Form of Nitrogen
As mentioned earlier, N can be utilized by plants as either NO 3 or NH 4 , although under production condi-
tions the greater amount is thought to be absorbed as NO 3. Thus, enhancing the supply of soil N as NH 4 is
one way to improve N fertilizer management (i.e., by minimizing the potential for N losses). Increasing
the supply of NH 4 in soils could also enhance plant performance: a survey of the literature shows numer-
ous examples of improved vegetative growth and N accumulation when growing cereal plants are provided
with mixtures of NO 3 and NH 4 compared with NO 3 alone [64,84,227–229]. Similar results have been re-
ported for tomato [230], soybean [231], and sunflower [232]. These data imply that seedlings of many plant
species cannot acquire sufficient N for maximum vegetative growth when N is supplied solely as NO 3.
Although much of the earlier work involved vegetative growth, later studies show that cereal crops
supplied with both NO 3 and NH 4 (mixed-N nutrition) also produce higher yields than those supplied with
only NO 3 [233–238]. For a variety of maize hybrids grown in field hydroponics, an equal mixture of NO 3
and NH 4 increased the yield by 11–14% [217,235,239] compared with plants grown with only NO 3. Even
greater mixed-N–induced yield increases (average of 21–43%) have been reported for hydroponically or
pot-grown spring wheat [236,238,240,241].
Although these responses were obtained with hydroponics or pot culture, where a finer degree of
control over the NO 3 /NH 4 ratio is possible, there is evidence that enhancing the supply of NH 4 to cereal
396 BELOWFigure 3 Representation of the two major ways in which maize cultivars can vary in their use of N. Culti-
vars can differ in the amount of fertilizer N required for maximum yield (high or low N response types; left) or
in their timing of N acquisition (high or low postflowering uptake types; right).