Em4885 irrigation management practices to protect ground water

3 CHAPTER
Nitrogen as a Potential Pollutant
Nitrogen is the most critical of the essential elements for plant growth. The addition of nitrogen
fertilizer to enhance crop growth is basic to modern irrigated agriculture. The ultimate source of this
nitrogen is the inert gas N which makes up about 78% of the atmosphere. However, except for
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legumes, plants cannot use nitrogen in this form. Most plants use nitrogen in either the ammonium
+ - - (NH ) form or the nitrate (NO3 ) form. Both occur in solution. However, the nitrate (NO3 ) form is
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+ much more leachable than the ammonium (NH ) form.
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Nitrogen is added to the soil in a number of ways:
- 1. Lightning may cause the formation of the nitrate (NO ) form which falls to earth
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with rainfall.
2. Legumes such as alfalfa, soybeans, and clover can convert atmospheric N 2 into usable
form through “rhizobia” bacteria. Rhizobia exist in a symbiotic (mutually beneficial)
relationship with the crop. They form and inhabit nodules (abnormal growths) on the
root systems of legumes. This process is termed “symbiotic fixation.”
3. “Non-symbiotic fixation” occurs through some types of blue-green algae and what are
known as “free-living” bacteria. These require no other plants for existence. The amount
of nitrogen fixed by non-symbiotic fixation is relatively small, with estimates in the range
of ten pounds/acre annually.
4. By far, the most important sources of plant-available nitrogen are through addition of
commercial fertilizer and manure.
The disposition of nitrogen that enters the soil is a complex process with many possible avenues
as indicated in Figure 3-1.
AMMONIUM FERTILIZERS
RAINFALL
CROPPED OR GRAZED
+ _
NH + NO
4 AMMONIUM VOLATILIZATION 3
+ _
(NH 4 3 NO in alkaline soils)
MINERALIZATION
LEACHING
(Decomposition of organic
TO IMMOBILIZATION
matter to 4 NH )
+
DENITRIFICATION
(Bacterial change of
-
NO 3 to 2
N )
IMMOBILIZATION
(Used by microbes)
NITRIFICATION
(Bacterial oxidation
to NO ) 3
CLAY COLLOIDS
-
(Some NH 4 fixation)
+
HUMUS
(Organic colloids)
LEACHING
FIGURE 3-1. The nitrogen cycle, showing the different ways that nitrogen enters and leaves the soil.
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N OF AIR
2
FERTILIZER
NITROGEN FIXATION
(N used by bacteria)
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