Digital Signal Processing Reference
In-Depth Information
atmospheric nitrogen (N
2
), ammonia (NH
3
), ammonium ion (NH
4
+
), nitrite ion (NO
2
-
), and
nitrate ion (NO
3
-
). Each nitrogen form has characteristics that relate to plant utilization and
possible impacts on water resources.
Nitrogen Availability to Plants
In their need for nitrogen, non-leguminous plants, such as lawn and turf grasses, corn and most fruit
and vegetable crops, must rely on either bacteria that live in the soil to "fix" the nitrogen (N
2
) into a
usable form or nitrogen from decomposing organic matter, or fertilizers. The forms of nitrogen that
most plants can use are ammonium ion (NH
4
+
) and nitrate ion (NO
3
-
), as shown in
Figure (2-4)
.
Of
these, the ammonium and nitrate ions are the most common forms taken in through plant roots.
Ammonium is converted to the nitrite and nitrate forms rather quickly by nitrifying bacteria, such as
Nitrosomonas .sp
and
Nitrobacter .sp
, which add oxygen to the ammonium ion and convert it to
nitrate. However, the legumes, for example, alfalfa, clover, soybeans and peanuts, have nodules on
their roots that contain bacteria. The plants benefit by having the bacteria that fix atmospheric nitrogen
into a usable form for the plant, while the bacteria benefit from the energy obtained in the chemical
conversion. It is to be taken into consideration that the ammonia and nitrite forms of nitrogen are
highly toxic to humans.
Figure (2-4): The nitrogen cycle in soil
Nitrogen Loss from Availability to Plants
Nitrogen can become unavailable to plants primarily in three ways. First, most nitrogen is lost through
denitrification, which is a problem in wet or compact soils. Since these soils contain little oxygen,
denitrifying bacteria remove the oxygen from nitrite (NO
2
-
) and nitrate (NO
3
-
) ions for their own use,
releasing N
2
and/or N
2
O back to the atmosphere. The second means of nitrogen loss is by nitrate
leaching, which is a particular concern with the nitrate ion (NO
3
-
). Leaching occurs when the water-
soluble nitrate ion moves through the soil as water percolates downward beyond the reach of plant
roots. Surface volatilization (conversion to the gaseous phase) is the third method of nitrogen loss.
This loss occurs when ammonia (NH
3
), usually in the form of urea, volatilizes and is lost to the
atmosphere. Surface volatilization is usually a problem in areas with high temperatures, and with soils
that have a high pH value. Soils that have been compacted by field operations and other human
activities also are a problem because it may not be possible to properly mix the urea with the
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