II. NITROGEN ACQUISITION BY CROP PLANTS
A. Nitrogen Availability
Under natural conditions, N enters the soil environment as the result of biological fixation and/or de-
composition of animal or plant residues. Most (90%) of the N in soils is contained in organic matter,
which is relatively stable and not directly available to plants. Although a portion of the N in organic mat-
ter can be made available through mineralization by soil microorganisms, the amount released is variable
depending on management practices and environmental conditions. In addition, the release is normally
too slow to meet the needs of a growing crop, with only 2–3% of the N converted to available forms per
year. As a result, addition of N from chemical fertilizers is usually required to optimize crop growth and
yield.
Nitrogen is unique among the mineral nutrients in that it can be absorbed by plants in two distinct
forms, as either the anion NO 3 or the cation NH 4 . Although numerous N fertilizer formulations are avail-
able that contain varying proportions of NO 3 -N to NH 4 -N, ammoniacal fertilizers are used more exten-
sively because they are lower in cost [8]. However, NO 3 is the predominant form of N absorbed by plants,
regardless of the source of applied N [9,10]. This preference is due to two groups of chemoautotropic soil
bacteria, which rapidly oxidize NH 4 to NO 3 (nitrification) in warm, well-aerated soils that are favorable
to crop growth.
The form of N (NH 4 or NO 3 ) can affect the availability of N to the plant as a result of differences in
mobility of each form in the soil solution. In soil, the positively charged NH 4 ion is bound to negatively
charged soil particles and is relatively immobile. In contrast, the negatively charged NO 3 ion is repelled
by soil particles, which aids in its movement to plant roots. Even though NO 3 is the N form most avail-
able to plants, however, it can be more readily lost from the rooting zone because it is susceptible to leach-
ing and denitrification [11]. Both these economically and environmentally undesirable processes (i.e.,
leaching and denitrification) perpetuate a large amount of the uncertainty associated with N fertilizer
management.
In the United States, N fertilizer recommendations are usually based on the past crop history of the
field and expected yield goal and, to a lesser extent, on formulas calculated to estimate the soil’s capac-
ity for N mineralization [12,13]. Other factors (e.g., fertilizer cost and value of the crop) must also be con-
sidered [14]. While generally sound, problems with fertilizer recommendations can arise if the yield goal
is unrealistic or if growers fail to assess accurately the capacity of the soil to supply the crop with N. As
a result, two main types of test have been developed to measure soil N: tests to determine the soil’s po-
tential to mineralize N from organic matter and direct measurement of residual inorganic N.
Several techniques have been developed to measure mineralization of soil N, which are collectively
known as N availability indices [15,16]. These methods estimate the potential for organic N to be miner-
alized and involve either incubations [16–18] or some type of chemical extraction [19,20]. Some studies
have shown that these tests can provide reasonable estimates of potentially mineralizable N [21,22]. They
have not been widely used for making N fertilizer recommendations, however, because of difficulty in
conducting the measurements and lack of supporting data to help interpret the results.
The other approach to assessing the soil N supply involves measuring the level of inorganic N in the
soil profile and then adjusting the fertilizer N recommendation to account for N that is already present
[15,16,19]. One such test for maize, known as the “late spring nitrate test,” takes some of the uncertainty
associated with N cycling into account by not removing soil samples until after the crop has been estab-
lished (plants are in the early vegetative stage), when the potential for N loss is lessened. Based on soil
analysis and yield response to applied N, a soil NO 3 -N concentration in excess of 20–25 mg kg^1 (ppm)
is considered adequate for maximum yield of maize, whereas lower values indicate the need for additional
fertilizer N [23–26]. Although good at identifying situations in which no fertilizer N is required, the test
does not work as well when the degree of responsiveness to fertilizer N application must be predicted or
when a high percentage of the N is available as NH 4 [27]. In addition, this technique cannot be used if all
the N is applied preplant or if the N is knifed in as anhydrous ammonium. As a result, tests based on plant
characters have also been developed as a way of assessing the soil N supply.
An advantage of plant measurements is that they integrate the effects of soil N availability and plant
N uptake, regardless of the N source or application method. Additionally, because they are based on the
plant, rather than the soil, plant measurements are more likely to reflect the direct impact of N availabil-
ity on growth and yield. Tissue testing of plants to compare N concentrations with critical levels is a well-
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