Table of Contents
Effect of Nitrogen on
Fate of Nitrogen in the
of Nitrogen to Uphold Water Quality
Jack V. Baird, Extension Soil Science Specialist
North Carolina Cooperative Extension Service
August 1990 (DED)
Last Web Update:
December 1997 (DBL)
Whatever its source,
nitrogen (N) is essential for achieving optimum yields of
grain, forage, and other crops. The same is true of phosphorus (P)
and other nutrients. Applying too much nitrogen or phosphorus to cropland,
however, can have adverse effects on the environment. Achieving optimum
yields without applying excessive nutrients should therefore be a goal
of all farmers. Excess nitrogen and phosphorus in surface waters and nitrogen
in groundwater cause eutrophication (excess algae growth) in surface waters
and health problems in humans and livestock as a result of high intake
of nitrogen in its nitrate form.
Eutrophication is the slow, natural nutrient
enrichment of streams and lakes and is responsible for the "aging"
of ponds, lakes, and reservoirs. Excessive amounts of nutrients, especially
nitrogen and phosphorus, speed up the eutrophication process. As algae
grow and then decompose they deplete the dissolved oxygen in the water.
This condition usually results in fish kills, offensive odors, unsightliness,
and reduced attractiveness of the water for recreation and other public
uses. These poor conditions have been observed in eastern North Carolina
in the Neuse, Chowan, and Pamlico river systems. However, this condition
occurs only when excessive nutrients are present; a certain amount of
nitrogen and phosphorus is essential for any life to exist in water.
Excessive nitrate (NO3)
in drinking water can cause human and animal health problems, particularly
for small babies. The United States Public Health Service has established
a specific standard of 10 milligrams of nitrate nitrogen
per liter as the maximum concentration safe for human consumption. Problems
in adults that drink water with excessive nitrate are essentially nonexistent
and are rare in infants. The principal sources of nitrate and nitrite
(NO2) for adults are vegetables and cured meats, which supply
more than 95 percent of the total nitrate in typical diets.
Less than 1 percent is from drinking water if it comes from
a low-nitrate source, as is usually the case.
Nitrate toxicity does occur in livestock,
and the nitrate concentrations that produce toxicity are much higher than
those for humans. Nitrate poisoning in livestock depends more on nitrate
in feed than in water. Nitrate-contaminated water is usually a problem
only when it adds to high nitrate concentrations already present in some
The long-term fate of land-applied nitrogen
is the same whether it comes from field-applied fertilizer, legume residues,
animal, industrial, or municipal wastes, or other sources. The possible
outcomes of this applied nitrogen are listed on the following pages.
Remaining in the Soil
The total nitrogen in cultivated soils remains
relatively constant over a period of years. Most of the nitrogen is present
in organic matter, which varies among soils and among cropping systems
on the same soil but is relatively constant for a particular soil on which
a given crop rotation is used. Thus, regardless of how much nitrogen from
fertilizers, legumes, or animal waste is used on a particular soil, nitrogen
does not normally accumulate in the soil. Nitrogen may carry over from
one crop to another, but the nitrogen content of a selected cultivated
soil does not increase greatly over a period of years. Thus, most of the
nitrogen is lost from the soil in one way or another. Regardless of whether
nitrogen is in the organic or inorganic form when applied to crops on
North Carolina soils, it undergoes transformation to yield nitrate as
an end product.
of Nitrogen in Harvested Crop
The amount of nitrogen harvested by crop
plants is less than most people assume; for example, recovery of only
50 percent of the applied nitrogen is a good average. However,
the recovery rate varies for different crops and soils. Research in North
Carolina has shown that 90 percent or more of the nitrogen
applied to sod crops (such as bluegrass or coastal bermudagrass) is commonly
recovered. Flue-cured tobacco recovers 70 to 80 percent in
seasons of average rainfall. The recovery of nitrogen applied to corn
for grain largely depends on the amount applied and the yield obtained.
Approximately 0.8 pound of nitrogen is harvested with each bushel of corn.
Farmers who apply 150 pounds of nitrogen per acre and harvest
100 bushels per acre recover approximately 53 percent
of the applied nitrogen. However, if farmers harvested only 50 bushels
per acre from the same nitrogen application, they would have recovered
only 27 percent of the applied nitrogen. The percentage of
recovery would be increased considerably if the corn were harvested for
The above examples suggest that in most seasons
an average of 30 to 50 percent of applied nitrogen may not be used by
the crops. This nitrogen may be lost by leaching or runoff and represents
a potential source of pollution.
Nitrogen Lost to the Air as Gas
North Carolina State University soil scientists
have documented that some of the nitrogen that moves below the plant root
zone is lost to the atmosphere through a process called denitrification.
This process is the breakdown of nitrate to simple nitrogen (N2)
and oxygen (02) gases that return to the atmosphere.
Loss of nitrogen as a gas by this process is not extensive in well-aerated,
cultivated soils. Nitrogen applications to high-water-table soils of the
lower coastal plain that are poorly drained and high in organic matter
are the least likely to contribute to contamination of groundwater by
nitrate. The organic matter in the shallow groundwater provides energy
for microorganisms that promote denitrification and thus much of the nitrogen
is lost in the gaseous form rather than as nitrate.
Studies have shown that nitrate levels are
not alarming even in moderately to well-drained soils with low organic
matter where little denitrification occurs (Figure
The lack of nitrate below 13 feet
may also be explained by the presence of an almost impermeable horizon
(soil layer) between the 9- and 12-foot depths.
These confining beds or layers are very common in the upper and middle
coastal plain. Water reaching these layers flows laterally to a lower
elevation where it frequently enters a stream through a seep. Furthermore,
in many places throughout the coastal plain (and other regions of the
state), much of the nitrate flowing laterally to an outlet is either used
by plants in these wet natural areas or is lost through denitrification.
The same processes have been observed in Georgia and Maryland. Thus, nature
has a very effective way of removing much of the nitrate before it can
A limited number of samples collected in
piedmont soils have shown low nitrate concentration levels within the
soil profiles (Figures 2 and
Groundwater NO3 concentration of a Goldsboro soil
near Kinston, North Carolina, cropped to corn.
Groundwater NO3 concentration in an old cultivated
Vance soil profile at the Oxford, North Carolina, Research
Groundwater NO3 concentration in an old cultivated
Appling soil profile at the Upper Piedmont Research Station,
Reidsville, North Carolina.
Nitrogen Removed from the Soil in Surface and Subsurface Drainage
Nitrogen from fertilizers may enter streams
through surface or subsurface drainage (leaching). Considerable loss of
nitrogen may occur if heavy rains immediately follow a surface application
of fertilizer on a moist soil surface, particularly if there is considerable
slope. However, fertilizer nitrogen in surface runoff will be low if the
fertilizer is mixed with the soil. The loss of organic nitrogen (contained
in crop residues, animal waste, or soil material) could be considerable
if intense rainfall results in substantial soil and debris movement.
Because it has a high solubility, nitrate
nitrogen normally moves readily into the soil with the initial rainfall.
Thus, if fertilizer nitrogen is a source of pollution, it is usually from
leaching or subsurface drainage.
Leaching has been studied within and slightly
below the root zone; however, the study of nitrate movement from the point
of application to the groundwater has been very limited. In some sandy
North Carolina soils, considerable nitrate has been observed 2 feet
below the point of application after only 3 inches of percolated
water. On similar soils in Georgia, considerable nitrate leached below
3 feet in 50 weeks with 42 inches
of rainfall. Although evidence is not available to state precisely how
much fertilizer nitrogen gets into the water via leaching, the foregoing
information can be used to make some estimates. Because nitrogen does
not accumulate in the soil and 30 to 50 percent of the applied fertilizer
is not harvested with the crop, this nitrogen must be escaping into the
air or water.
In North Carolina, evapotranspiration (water
that evaporates from leaves and soil surfaces) exceeds rainfall only during
the summer months (April to August). The average annual rainfall in North
Carolina is approximately 49 inches and the average evapotranspiration
is approximately 35 inches. The excess 14 inches of water
either runs off or percolates through the soil; in either case it ultimately
enters streams, groundwater, or both. This water can become contaminated
with excessive soluble nutrients from various sources.
In 1985 and 1986, a total of 245,751
tons of fertilizer nitrogen were used in North Carolina. If all
this nitrogen were put directly into the 14 inches of drainage
water over the state's total area, the average nitrogen concentration
would be only 4.6 parts per million. In previous sections
it was shown that on the average at least 50 percent of the
applied nitrogen is used by the crop. Soil scientists propose a method
that shows an average nitrogen concentration in waters resulting from
the use of fertilizer (Table 1).
From these data it appears that fertilizer nitrogen does not contribute
greatly to the nitrogen content of streams.
1. Potential Average Nitrogen Concentration*
|Applied N Used By
||Potential Avg. N
Concentration In Drainage Water (ppm)
In drainage waters based on
fertilizer usage in N.C. in 1985 and 1986 assuming that all nitrogen
not used in the crop gets into the water.
Because nitrate in groundwater and surface
water is a potential health hazard and contributes to current eutrophication
problems, fertilizer nitrogen must be used prudently on crops. Listed
below are some techniques for guarding against the possibility of unused
nitrate contaminating surface water and groundwater supplies.
- Apply two-thirds to three-fourths of the
planned fertilizer nitrogen just before the crop enters a period of
rapid growth. Proper timing ensures maximum daily nitrogen uptake and
minimizes the likelihood of unused nitrogen leaching below the plant
- Apply a reasonable amount of nitrogen
to your crop. When grain and forage yields are low, less nitrogen will
be removed with the grain, silage, or hay crop or by grazing. Because
a soil test is not a reliable means of predicting nitrogen response,
consider analyzing plant samples collected early in a period of rapid
growth. The need for additional nitrogen can be determined and applied
before the crop matures.
- If your crop will follow peanuts, soybeans,
or forage legumes (clover or alfalfa) of average or greater yield, reduce
the amount of nitrogen you apply. Soybeans and peanuts may provide 20
to 40 pounds of carryover nitrogen per acre. A "strong"
alfalfa stand may provide 80 to 100 pounds
of nitrogen per acre for the next crop.
- Be sure to analyze animal, municipal,
and industrial wastes for nitrogen content when applied to cropland.
Guard against "dumping," as this practice may contaminate
water with excess nitrate.
- Throughout the sandy soil surfaces of
the coastal plain, do not apply nitrogen in the fall for springplanted
crops. Piedmont fields may receive some nitrogen (up to one-half
of crop needs) for springplanted crops.