Agriculture Reference
In-Depth Information
of 0-10 cm is normally sufficient to evaluate the nutrient status and to make liming and fertilizer
recommendations.
The inorganic combined nitrogen in most soils is in the form of ammonium
(
NH
4
+
and nitrate
)
+
−
. Hence,
the soil test values of nitrogen (
NO
3
−
and
NH
4
+
) should be calibrated against the straw yield or grain
yield of crops. However, in the author's opinion, as yet there does not seem to be any well-accepted
method for testing soils for available N. This is a reflection, in part, on the fact that 97-99% of
the N in the soil is present in very complex organic compounds that are not available to plants
(Dahnke and Vasey, 1973). These authors further reported that developing a test for available soil
N is further complicated by the fact that the mineralization process of N in the soil is very complex
and influenced by several factors such as temperature, moisture, aeration, type of organic matter,
pH, and other factors. In addition, the soil test values of
NO
3
−
and
NH
4
+
are not stable during crop
growth because they change very rapidly due to nitrification and losses by volatilization, leaching,
and denitrification. In addition, the
NO
3
−
ion is very soluble and generally does not form insoluble
compounds with any of the soil constituents. Another complicating factor in the study of NO
3
-N is
that it can be rapidly immobilized by soil microorganisms if a suitable source of energy is present,
only to reappear after an indefinite period of time as the microorganisms reduce the C/N ratio of the
energy source (Dahnke and Vasey, 1973).
Meisinger (1984) concluded, in his review of methods to predict soil N availability to crops,
that since scientists have been unsuccessful in their attempts, over the past 75 years, to develop a
quick test to predict N mineralization in the field, it is unlikely that, such a test will be developed
in the near future. Meisinger et al. (2008) also reported that, in contrast to P, N that is not used by
a crop cannot reliably be banked in the soil from year to year and both the spatial and temporal
variabilities in soil N provide limitations to the potential accuracy of fertilizer N recommendations.
Similarly, Anthony et al. (2012) also reported that the challenges of site-specific N management are
different from those involving P, primarily because of the nature of the transient nature of soil N.
Hence, soil test calibration for rapid movable or loosing form of N (
NO
3
−
and
NH
4
+
) has doubt about
its applicability under field conditions.
Soil test calibration values are recommended for immobile nutrients such as P and K. However,
data regarding NO
3
-N (which is a major form of N in the oxidized soils) interpretation for crop
production are available in the literature (Table 3.2). The NO
3
-N values in Table 3.2 can serve only
as reference points. Hence, these values of the NO
3
-N test should be used with caution for different
crop species and soil types in crop production processes. In the state of Iowa, the initial interpreta-
tion of the NO
3
test for corn was that if the test result is above a critical value of 25 mg kg
−1
, no N is
recommended. However, if the test result is below 25 mg kg
−1
, 9 kg N ha
−1
is recommended for each
1 mg kg
−1
below the critical level (Blackmer et al., 1993). This interpretation has been modified a
little bit in 1997 (Blackmer et al., 1997). The critical level dropped by 3-5 mg kg
−1
when the spring
rainfall was more than 20% above normal.
NO
3
−
Nitrogen is also taken up by crop plants in the form of both NO
3
-N and
NH
(
).
N
4
TABLE 3.2
Interpretation of
NO
3
−
Soil Test Values for Making Nitrogen
Recommendations for Crop Plants
NO
3
Values (mg kg
−1
)
Interpretation
Low
<10
10-30
Medium
High
>30
Source:
Adapted from Keeney, D. R. and D. W. Nelson. 1982.
Methods of Soil Analysis.
Part 2,
2nd edition, eds., A. L. Page et al., pp. 643-687. Madison, Wisconsin: ASA and SSSA.
