Em4885 irrigation management practices to protect ground water

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CHAPTER 3
The nitrogen cycle, shown schematically in Figure 3-1, is the name given to the movement of
nitrogen in its different forms from the atmospheric gas N 2 into the soil in some form or another and
then back to the atmosphere. Some of the more important processes that occur are:
1. Fixation - addition of nitrogen to the soil through the symbiotic action of rhizobia
bacteria on the root systems of legumes or by other microorganisms (non-symbiotic)
in the soil and water.
2. Mineralization - the breakdown of soil organic matter by the activity of microbes.
+ Mineralization converts organic nitrogen to the ammonium (NH ) form, which
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is available to the plant.
+ - 3. Nitrification - the conversion of the ammonium (NH ) form to the nitrate (NO3 ) form.
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- Nitrification is the result of activity by soil bacteria. Nitrate (NO ) nitrogen is readily
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- available to plants. However, nitrate (NO ) nitrogen is also readily leached since it stays
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in solution and does not adhere to soil particles. Nitrification is a relatively quick
process. Nitrogen added to the soil via commercial fertilizer in the ammonium form can
be transformed to the nitrate form within one to two weeks if conditions are favorable.
4. Immobilization - the conversion of inorganic nitrogen in organic matter which occurs
when carbon is added to the soil. Plant residues are the chief source of this carbon.
However, as the decomposition of plant residues continues, nitrogen is again released
through mineralization as explained previously.
- 5. Denitrification - conversion of nitrate (NO ) nitrogen into the atmospheric gas N2 by soil
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bacteria in wet, poorly aerated conditions, such as would be found in water-logged, heavy
soils. This process can also occur relatively rapidly but requires decomposing organic
matter as a carbon source.
6. Volatilization - the movement of nitrogen in the form of ammonia gas to the atmosphere.
+ Volatilization occurs when ammonium (NH ) forms of nitrogen are applied to the soil
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surface and not properly worked into the soil. Volatilization increases with high
temperatures and calcareous soils. Lack of rain following the application and high
amounts of crop residue also increase the process.
- 7. Leaching - the movement of the nitrate (NO ) form below the crop’s root zone. Both
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+ - - the ammonium (NH ) and nitrate (NO3 ) forms will leach. However, the nitrate (NO3 )
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form is highly soluble and thus, is more readily leachable. Leaching occurs with deep
percolation, the movement of soil water below the root zone. Unfortunately, many times,
leachate moves to an aquifer and contaminates ground water.
An example of a path in the nitrogen cycle that includes the use of commercial fertilizer is:
+ 1. Conversion of atmospheric N into the ammonium (NH ) form by a commercial
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fertilizer manufacturer.
+ 2. Addition of the ammonium (NH ) fertilizer to the soil by a grower.
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