Nitrogen

Nitrogen is a very important and needed for plant growth. It is

found in healthy soils, and give plants the energy to grow, and
produce fruit or vegetables. Nitrogen is actually considered the
most important component for supporting plant growth.

Nitrogen is part of the chlorophyll molecule, which gives plants

their green color and is involved in creating food for the plant
through photosynthesis. Lack of nitrogen shows up as general
yellowing (chlorosis) of the plant. Because nitrogen can move
around in the plant, older growth often yellows more than the
new growth.

Nitrogen accounts for 79 percent of the air we breathe. The

surface 6 inches of a fertile prairie soil may contain 2 to 3 tons of
nitrogen per acre. The air above this same acre will contain about
35,000 tons of inert nitrogen gas (N2). Most of the nitrogen found
in soil originated as N2 gas and nearly all the nitrogen in the
atmosphere is N2 gas. This inert nitrogen cannot be used by the
plant until it is changed to ammonium (NH4 +) or nitrate (NO3– )
forms.
Three important methods for changing nitrogen gas (N2) to
ammonium (NH4 +) are:

1-Free-living N2-fixing bacteria

2-N2-fixing bacteria in nodules on the roots of leguminous
plants.
3-Nitrogen fertilizer production factories.

Another important method of N2 conversion is through

lightning. When lightning flashes, the nitrogen gas in super-
heated air is converted to nitrate (NO3 – ) and nitrite (NO2 – ).
Lightning may account for 1 to 50 pounds of plant-available
nitrogen per acre per year.

Although nitrogen enters the soil in several chemical forms, it

eventually converts to the inorganic nitrate (NO3 – ) ion. Figure
1 shows that NO3 – can be used by plants, be converted back to
nitrogen gas or be leached downward with soil water.

Commercial fertilizers, plant residues, animal manures and

sewage are the most common sources of nitrogen addition to
soils. Rates of application vary widely. Single application rates
may be as high as 150 pounds of nitrogen equivalent per acre
for crops such as coastal bermudagrass. However, such high
application rates should be limited to soils with a low potential
for erosion and runoff.

Nitrogen in organic materials (plant residues, animal manures,

sewage, soil organic matter) is present as part of proteins, amino
acids and other plant and microbial materials. It becomes
available to plants only after the compound is decomposed by
soil microorganisms. This is called “mineralization” (Fig. 2). The
first step of mineralization is “ammonification.” The ammonium
(NH4 +) derived from ammonification is then converted to
nitrate-nitrogen (NO3 – -N) by “nitrifying” bacteria in the soil
through the process called “nitrification.”
Removal of Nitrogen from Soil
Nitrogen is removed from soils by two major processes:
1-Plant uptake
2-Gaseous loss

Plant uptake refers to nitrogen absorption by roots. Cotton,

corn, tomatoes and turf grasses require 60 to 300 pounds of
nitrogen per acre to produce good growth and profitable yields
or desired aesthetics.
Gaseous loss of nitrogen takes place by denitrification or
ammonia volatilization. Denitrification is a process through
which nitrate nitrogen (NO3- -N) is converted to gaseous nitrogen
oxide (N2O) or elemental nitrogen (N2). This involved the action
of anaerobic bacteria (those which do not require free oxygen)
and commonly occurs in wet or water-logged soils.
Preventing Nitrogen Loss
The best way to prevent losses of nitrogen from agricultural
lands is through good soil and water management practices. The
first step in reducing potential nitrogen losses is to have the soil
tested. A properly obtained soil sample will provide an estimate
of nitrate-nitrogen (NO3 – –N) present in the soil, and can be used
as a guide for applying the appropriate amount of nitrogen
fertilizer for the crop being grown.

Nitrogen: found in chlorophyll, nucleic acids and amino acids;

component of protein and enzymes.
Nitrogen: is a component of chlorophyll, so when nitrogen is
insufficient, leaves will take on a yellow (chlorotic) appearance
down the middle of the leaf. New plant growth will be reduced
as well, and may appear red or red-brown. Because of its
essential role in amino acids and proteins, deficient plants and
grains will have low protein content. Nitrogen excess results in
extremely dark green leaves, and promotes vegetative plant
growth. This growth, particularly of grains, may exceed the
plant's ability to hold itself upright, and increased lodging is
observed. Nitrogen is mobile both in the soil and in the plant,
which affects its application and management, as discussed
later.
Fields with adequate (left) and inadequate (right) nitrogen.
(Original image location)
Translocation(mobile-immobile)
Nutrients that are mobile in the plant will move to new growth
areas, so the deficiency symptoms will first show up in older
leaves.
Nutrients that are not mobile in the plant will not move to new
growth areas, so deficiency symptoms will first show up in the
new growth.
NO3- nitrogen is very mobile in the soil, and will leach easily.
Excessive or improper application increases the risk of water
contamination.
The role of nitrogen in crops
Nitrogen is an important component of many essential
structural, genetic and metabolic compounds in plant cells. It is
also an elementary constituent of numerous important organic
compounds including amino acids, proteins, nucleic acids,
enzymes, and the chlorophyll molecule.

Of all the essential nutrients, nitrogen is the one that is most

often limiting for crop growth. Nitrogen is the nutrient which
normally produces the greatest yield response in crop plants,
promoting rapid vegetative growth and giving the plant a healthy
green color.

Roots take up nitrogen in its inorganic forms, nitrate (NO3-) and

ammonium (NH4+) ions. Once inside the plant, NO3 is reduced to
the NH2 form and is assimilated to form the organic compounds.

Adding nitrogen is not recommended for legume crops such as

soybean, since they manufacture their own nitrogen supply.
Nitrogen-fixing soil organisms (rhizobium) associated with the
roots of legumes capture atmospheric nitrogen and make it
available to the plant.