Mg, respectively). In contrast, the fourth 67-kg increment increased yield by only 0.1 Mg, which would
not be considered economically (or environmentally) sound. Therefore, although it is obvious that fertil-
izer N was needed to maximize yield, the optimal rate required is much less clear.
In addition to demonstrating the need for fertilizer N, the foregoing data show that a reasonable yield
(8.4 Mg ha^1 ) was produced without adding any N to the soil (Table 1). The crop accumulated 124 kg
ha^1 from the soil, of which 83 kg was removed with the grain. For heavily fertilized plants (267 kg ha^1
N), these values increased to 262 kg of plant N and 180 kg of grain N. When the level of fertilizer N lim-
ited grain yield (at N rates of 134 kg ha^1 or less), more N was removed with the grain than was provided
by the fertilizer. Conversely, N levels in excess of those needed for maximum yields (202 kg ha^1 and
up) resulted in the removal of less N than had been applied. This situation greatly increases the potential
for accumulation of residual N (usually as NO 3 ) in the soil. Based on this example, and other published
reports [216,217], it is suggested that the N level that just maximizes grain yield also results in the best
balance between fertilizer N added and the amount removed with the grain.
Similarly to grain yield, the largest increase in total plant N accumulation, and the greatest plant re-
covery, occurred with the first increment of N. For example, the first 67 kg of fertilizer N increased plant
N accumulation by 46 kg, representing a plant recovery of 68% (calculated from data of Table 1). Of this
46 kg of plant N, 36 kg was removed with the grain for an N removal recovery of just over 50%. In con-
trast, plant N accumulation was increased by just 21 kg (32% recovery) for the fourth N increment, of
which only 10 kg (15% recovery) was removed with the grain. These data demonstrate the inherent inef-
ficiency with which fertilizer N is recovered by the maize plant, emphasizing the potential for environ-
mental damage at excessive rates.
B. Genotypic Variation
Different cultivars grown at the same location can exhibit different response patterns to N fertilization,
and such variation has been observed for wheat [218,219], rice [220], sorghum [221], and maize
[126,127]. However, as might be expected, this variation is highly affected by the environment and grow-
ing conditions and is most apparent under controlled conditions (e.g., in hydroponics) [135,222]. Interest
in identifying genetic differences in responsiveness to N fertilizer is intensifying, as producers and agri-
cultural consultants see genotypic variation as one way to fine-tune N fertilizer management. There is also
a desire to develop or identify genotypes that perform well under a low N supply or, conversely, to find
genotypes that will respond to high fertility conditions.
From a botanical standpoint, plants can vary in their use of N in two major ways: in how much N the
plant uses to produce maximum yield or in when (i.e., at what stage during the growing season) the plant
acquires its N. An example of this type of variation is depicted for maize in Figure 3. In this example, the
low N response type produced its maximum yield at 120 kg N ha^1 compared with an N requirement of
200 kg ha^1 for the high N type (Figure 3, left). Although high N types are usually capable of producing
the highest yields, the low N types may outyield the high N types at low levels of soil N. Cultivars can
also differ in their timing of N acquisition: some accumulate the majority of their N before flowering,
whereas others may have a substantial requirement for N accumulation after flowering (Figure 3, right).
A cultivar’s N acquisition pattern can affect N accumulation and productivity because plants acquir-
ing most of their N by flowering should be less subject to fluctuations in the N supply during grain fill.
These types may be more consistent from year to year because adverse growing conditions usually occur
NITROGEN METABOLISM AND CROP PRODUCTIVITY 395
TABLE 1 General Effect of N Fertilizer Rate on Grain Yield and N Recovery of Maize
Grown on Highly Fertile Silt Loam Soil in Illinoisa
Nitrogen rate Grain yield N in crop N removed with grain
(kg ha^1 ) (Mg ha^1 ) (kg ha^1 ) (kg ha^1 )
0 8.4 124 83
67 10.5 170 119
134 12.2 212 150
202 13.1 241 170
267 13.2 262 180
aValues are averaged over two hybrids grown at the University of Illinois research farm in 1990.