How much nitrogen is there in the Earth's atmosphere?

Answer 1

The atmosphere is nearly 80% nitrogen.

Answer 2

About 30 percent of the oxygen you breath contains nitrogen.

Answer 3

1
Nitrogen (N) is one of the most abundant elements on earth, and after
carbon (C), hydrogen (H), and oxygen (O), the element living
creatures need most. The atmosphere over each square foot of the
earth’s surface—which is 78% dinitrogen (N2) gas—contains approximately 6,000
pounds of nitrogen. However, the majority of the earth’s nitrogen (98%) is in rock,
sediment, and soils. The amount of nitrogen in rocks is about 50 times more than
that in the atmosphere, and the amount in the atmosphere is approximately 5,000
times more than that found in soils (Stevenson, 1982).
Biological fixation of nitrogen and
atmospheric deposition are the primary means by
which nitrogen is added to soil. (Fixation is the
conversion of dinitrogen gas—which is chemically
unreactive—to nitrogen combined with other
elements, such as oxygen or hydrogen, which can
readily undergo chemical reactions.) The
atmosphere contributes approximately 11.4
pounds of nitrogen per acre to soils annually
(Stevenson, 1982). Biological nitrogen fixation
accounts for 8.2 of the 11.4 pounds of nitrogen per
acre per year. Biological nitrogen fixation occurs
symbiotically (dinitrogen-fixing bacteria, such as
Rhizobium, in conjunction with legumes) and
non-symbiotically (free living organisms such as
photosynthetic bacteria, blue-green algae, and free-living Azotobacter species). The
balance, 3.2 pounds of nitrogen per acre per year, consists of various sources of
ammonium (NH4
+), nitrate (NO3
-) and nitrite (NO2
-) deposited in precipitation. The
amount of nitrogen added each year from atmospheric deposition varies
considerably with climate and proximity to industrial sources of atmospheric
nitrogen, but generally it is too small to significantly affect crop production.
In addition to nitrogen occurring as atmospheric dinitrogen gas in soil pore
spaces, nitrogen occurs in both organic and inorganic forms in the soil.
nOrganic nitrogen
Several organic compounds (compounds containing carbon), grouped into
humic, fulvic, and amino acids, amino sugars, and other proteins, compose the
organic fraction of nitrogen in soil. Soil organic matter exists as decomposing plant
and animal residues, relatively stable products of decomposition-resistant
compounds, and humus. Nitrogen has accumulated in these various organic
fractions during soil development.
Organic matter formation and stability is largely related to long-term moisture
and temperature trends. With higher average temperatures, soil organic matter
decreases. As moisture increases, soil organic matter increases. Higher
temperatures lead to more rapid and complete organic matter decomposition to
soluble products which can leach from soil. Increasing moisture causes more plant
growth, resulting in more organic residue. Trends of moisture, temperature, and
organic matter in soils in the Midwest and Great Plains are shown in Figure 1-1.
1
Nitrogen
Richard B. Ferguson
UNL Professor of Agronomy
Forms of
Nitrogen in Soil
Chapter 1 n Nitrogen
2
FIGURE 1-1
Soil organic matter in
relation to temperature and
moisture. Soil organic
matter decreases with higher
average temperatures. It
increases as moisture
increases.
Through thousands of years of development, soils in the Midwest have accumulated
significant quantities of organic matter; yet organic matter levels have declined by
cultivating virgin soils, thereby increasing organic matter oxidation and decreasing
soil organic matter nitrogen through crop uptake (Figure 1-2). Soils that once
contained 4% to 5% organic matter may contain only 1% to 2% after 50 years of
cultivation. However, soils under cultivation in the Midwest have, for the most part,
reached a new equilibrium of organic matter levels with widespread commercial
fertilizer use. Reduced tillage techniques in combination with legume rotations and
judicial fertilizer use may increase organic matter levels with time.
MONTANA
WYOMING
COLORADO
NEW
MEXICO
TEXAS
OKLAHOMA
KANSAS
NEBRASKA
SOUTH
DAKOTA
NORTH
DAKOTA
MINNESOTA
WISCONSIN
IOWA
ILLINOIS
MS
MISSOURI
ARKANSAS
LA
Average temperature
increases, soil organic
matter decreases
Average rainfall increases,
soil organic matter increases
.10
.14
.18
.22
0 10 20 30 40
Years of Cropping
Nitrogen in Soil (percent)
Hays, KS
Sheridan, WY
Colby, KS
Garden City, KS (a)
Garden City, KS (b)
FIGURE 1-2
The influence of long-term
cropping on organic
nitrogen in soils in the
Midwest (adapted from W.J.
Hase et al., 1957; Nitrogen
and carbon changes in Great
Plains soils as influenced by
cropping and soil
treatments; Technical
bulletin 1167, USDA,
Washington D.C.).
Part I n Fertility Principles
3
nThe nitrogen cycle
Ammonium and nitrate are the predominate inorganic forms of nitrogen in
soils. Ammonium exists as exchangeable and nonexchangeable forms. Nitrite and
nitrous oxide (N2O) are present in soil in lesser quantities. Plants normally use
nitrogen in only the ammonium and nitrate forms. Nitrite is actually toxic to plants.
The nitrogen cycle (Figure 1-3) shows reactions that various inorganic nitrogen
compounds undergo in soil. The nitrogen cycle begins with nitrogen in its simplest
stable form, dinitrogen (N2), and follows it through the processes of fixation,
mineralization, nitrification, leaching, plant assimilation, ammonia volatilization,
denitrification, and immobilization.
Plant & Animal Wastes
Atmospheric Fixation
Industrial Fixation
Biological Fixation
Ammonia
Nitrous oxide
Nitric oxide
Dinitrogen
Protein
Atmospheric Nitrogen
Urea
Ammonium
Nitrite Nitrate
Soil Organic Matter
Hydrolysis Immoblization
Plant
Assimilation
Ammonia
Volatilization
Denitrification
Aminization
Ammonification
Nitrification
Leaching
Fixation
Clay Minerals
FIGURE 1-3
The nitrogen cycle.
Fixation
As described earlier, fixation is the process of converting dinitrogen gas to
chemically reactive forms—where nitrogen combines with other elements such as
oxygen, hydrogen and carbon. Energy is required to convert dinitrogen to ammonia
or other forms of fixed nitrogen. Lightening fixes nitrogen into various oxides that
rain and snow deposit, typically less than 10 pounds of total nitrogen per acre per
year. Bacteria can convert nitrogen to organic forms through fixation. Fixation can
occur either in free-living organisms or symbiotically in association with legumes.
Nitrogen is also fixed industrially through several processes using fossil fuel as an
energy source.
Chapter 1 n Nitrogen
4
Mineralization
Once nitrogen is fixed, it is subject to several chemical reactions which can
convert it to different organic or inorganic forms. Mineralization occurs in soil as
microorganisms convert organic nitrogen to inorganic forms. The first step of
mineralization is called aminization, in which microorganisms (primarily
heterotrophs) break down complex proteins to simpler amino acids, amides, and
amines. Heterotrophic microorganisms require preformed organic compounds as
sources of carbon and energy. Autotrophic microorganisms can derive energy from
the oxidation of inorganic elements or compounds such as iron (Fe), sulfur (S),
ammonium, nitrite, or from radiant energy; they derive their carbon from carbon
dioxide (CO2). For example, urea is an amide added directly to soil either in animal
urine or as commercial fertilizer.
Aminization: Proteins ® R*-NH2 + CO2
(*R designates a carbon chain of indefinite length.)
Ammonification is the second step of mineralization in which amino (NH2)
groups are converted to ammonium. Again, microorganisms (primarily autotrophic)
accomplish this action.
Ammonification: R-NH2 + H2O ® NH3 + R-OH
Nitrification
Microbial activity is also responsible for the two steps of nitrification.
Nitrosomonas (obligate autotrophic bacteria) convert ammonium to nitrite.
Nitrification inhibitors, such as nitrapyrin (N-Serveâ) or dicyandiamide (DCD)
interfere with the function of these bacteria, blocking ammonium conversion to
leachable nitrate. The second step of nitrification occurs through Nitrobacter
species, which convert nitrite to nitrate. This step rapidly follows ammonium
conversion to nitrite, and consequently nitrite concentrations are normally low in
soils.
2NH4
+ + 3O2 ® 2NO2
- + 2H2O + 4H+
2NO2
- + O2 ® 2NO3
-
Mineralization and nitrification are influenced by environmental factors that
affect biological activity such as temperature, moisture, aeration and pH.
Nitrification, for example, occurs very slowly at cold temperatures and ceases once
the temperature declines below freezing (Figure 1-4). The rate increases with
increasing temperature until the point at which bacterial viability is reduced, (around
95o F to 100o F) and then nitrification begins to decline with increasing temperature.
Moisture is necessary for microbial function in both the mineralization and
nitrification processes. Excessive moisture limits oxygen availability, reducing
mineralization and nitrification rates, which, perhaps, lead to anaerobic conditions in
the soil. Rates of mineralization and nitrification proceed most rapidly at pH levels
near 7, and decline as soils become either excessively acid or alkaline.
EQUATIONS 1-1 AND 1-2
Mineralization.
EQUATIONS 1-3 AND 1-4
Nitrification.
Part I n Fertility Principles
5
Denitrification
Denitrification—the conversion of nitrate to various gaseous forms of
nitrogen which can be lost to the atmosphere (nitric oxide, nitrous oxide,
dinitrogen)—occurs under oxygen-limiting conditions when anaerobic bacteria use
nitrate in respiration in the presence of a carbon source such as organic matter.
Low areas of fields that are subject to ponded water for sustained periods
during the irrigation season often exhibit nitrogen deficiencies related to
denitrification losses.
NO3
- ® NO2
- ® NO ® N2O ® N2
Denitrification losses from saturated soil will vary with temperature and the
amount of carbon (organic matter) available. Table 1-1 illustrates the effect that
time and temperature can have on potential nitrogen losses from denitrification.
3 6 9 12
Time (weeks)
Nitrification (% complete)
20
40
60
80
100 75 F
52 F
47 F
42 F
37 F
o
o
o
o
o
FIGURE 1-4
Reductions in nitrification
based on temperature.
EQUATION 1-5
Denitrification.
TABLE 1-1
Denitrification rates from
saturated soil*.
Time Temperature N loss
days degrees F percent
5 55 - 60 10
10 55 - 60 25
3 75 - 80 60
*Denitrifcation loss will be less with soils having less than 1%
organic matter.
Chapter 1 n Nitrogen
6
Ammonia Volatilization
Ammonia (NH3) loss to the atmosphere is called ammonia volatilization.
Technically, ammonia volatilization is different from gaseous loss of applied
anhydrous ammonia, which is not retained in the soil. Instead, ammonia
volatilization occurs when ammonium in the soil, because of pH, is converted to
ammonia, which can be lost as a gas. Ammonia volatilization is normally only a
problem in Nebraska with fertilizers containing urea, such as urea or urea
ammonium nitrate (UAN) solution. Urea in soil is decomposed, or hydrolyzed,
enzymatically by the enzyme urease to ammonium.
Ammonia loss can be significant where the producer surface-applies fertilizers
containing urea without incorporation, particularly if significant amounts of residue
are present and conditions are warm and moist. The amount of total nitrogen loss
from fertilizers containing urea due to ammonia volatilization can vary
considerably, from no loss to 50% or more of the applied nitrogen. Typical losses
from urea broadcast to a silt loam soil in the spring, without rain for at least a week
following application, may be in the range of 10% to 20% of the applied nitrogen.
The potential for ammonia volatilization is influenced by soil moisture, temperature,
soil pH, soil buffering capacity, urease activity, residue cover, precipitation, wind
and other factors. Warm, moist soil with heavy residue and urea broadcast to the
surface are ideal conditions for ammonia loss. Precipitation or irrigation of ½ inch
or more is adequate to move urea far enough into the soil to minimize volatilization
loss potential.
EQUATION 1-6
Ammonia volatilization.
FIGURE 1-5
Urea granules on corn
residue.
CO(NH urease 2)2 + H+ + 2H2O ® 2NH4
+ + HCO3
-
+ H+ « CO2­ + H2O + NH4 « NH3­ + H+
Part I n Fertility Principles
7
Leaching
In order for leaching to occur, nitrogen must be in a water soluble, mobile
form and abundant enough to transport nitrogen through the soil. Although urea and
nitrite are mobile, neither exists in significant concentrations in soil. Nitrate is the
form of nitrogen most susceptible to leaching loss. Nitrate leached below the root
zone (four to six feet) of most agronomic crops will eventually leach downward
until it reaches a saturated zone, either an aquifer or aquitard. Nitrate leached below
four to six feet is generally unrecoverable by most crops except deep rooted
species such as alfalfa. The rate of nitrate movement downward depends on a
variety of factors, including soil texture, precipitation and irrigation amounts, crop
uptake of water and nitrate, and so on. Nitrate leaching from relatively sandy soils
overlying coarse-textured vadose zones and shallow aquifers (such as in the Central
Platte Valley) can leave the root zone and enter the aquifer in a matter of months,
while nitrate leaching from upland, silt loam soils overlying aquifers 100 feet or
more below the surface can take 25 to 30 years to reach the aquifer.
Figure 1-6 shows the nitrate levels in the vadose zone of a 35-year continuous
irrigated corn field and a native grass pasture. In this example from Seward County,
the native grass pasture contains 307 pounds of nitrate-nitrogen per acre to a depth
of 80 feet, while the continuous corn field contains 1,224 pounds of nitrate-nitrogen
per acre to a depth of 100 feet.
FIGURE 1-6
The effect of nitrogen
fertilization and irrigation on
vadose zone nitrate levels
(Upper Big Blue Natural
Resources District - Mid-
Nebraska Water Quality
Demonstration Project).
0 5 10 15 20
Nitrate-N (ppm)
-120
-100
-80
-60
-40
-20
0
Depth (ft)
Pasture
Corn
Chapter 1 n Nitrogen
8
Immobilization
Immobilization, or the temporary tying up of inorganic nitrogen by soil
microorganisms decomposing plant residues, is a recycling process. Immobilized
nitrogen will be unavailable to plants for a time, but will eventually become
available again as residue decomposition proceeds and populations of
microorganisms decline (Figure 1-7). Fertilizer nitrogen immobilization can be
reduced by placing fertilizers below crop residues, instead of incorporating fertilizer
into the soil with residue. The producer can accomplish this most directly by knifing
in anhydrous ammonia or solutions.
Time
Increase
Activity of
Decay Organisms &
Nitrate Level
in Soil
Residue with high C:N
ratio added to soil here
Residue with low C:N
ratio remain
CO2 evolution
Nitrate Depression Period
FIGURE 1-7
Levels of nitrogen available
to plants based on microbial
decomposition.
TABLE 1-2
Typical carbon-to-nitrogen
ratios for selected organic
materials.
The duration of the nitrate depression period during immobilization is
dependent on environmental factors such as moisture and temperature and the
carbon-to-nitrogen (C:N) ratio of the residue. Soil organic matter contains an
average of approximately 50% carbon and 5% nitrogen. This ratio (10:1) is
relatively constant for organic matter. The C:N ratio of plant residue ranges from
10:1 for young leguminous plant tissue to as high as 200:1 for straw of some grains.
Plant tissues low in nitrogen generally are more resistant to decomposition and
require a longer time before the nitrogen is available to plants.
Source C:N ratio
Organic matter in
undisturbed top soil
10:1
Alfalfa 13:1
Cattle manure 20:1
Corn stalks 60:1
Wheat straw 80:1
Coal and shale oil 124:1
Oak 200:1
Part I n Fertility Principles
9
FIGURE 1-8
Field example of
immobilized urea (chlorotic
area where urea was
incorporated with crop
residue) next to NH3-injected
field (darker green area).
When a high C:N ratio plant residue is incorporated into the soil, microbial
decomposition of the residue starts. Microorganism populations increase rapidly,
evidenced by increased release of CO2 leaving the soil through respiration. The
microorganisms take nitrogen from the soil for synthesis. Consequently, for a period
of time the concentration of inorganic nitrogen in the soil declines and may be
deficient for plant growth. As residue decomposes, the C:N ratio becomes narrower.
At a ratio of approximately 17:1, nitrogen becomes available for plant use.
Decomposition continues until the ratio is approximately 10:1 to 15:1.
Plant Assimilation
Plants use nitrogen in primarily the nitrate or ammonium forms. If any
preference exists, it is usually for ammonium early and nitrate late in the growing
season. Research has shown that growth is optimized with a mixture of both
ammonia and nitrate, with ammonium used preferentially for synthesis of amino
acids and proteins. Some plants can also directly use urea (Harper, 1984), although
in most cases urea-nitrogen will hydrolyze to ammonium-nitrogen prior to uptake.
In order to take up nitrate-nitrogen, plants require that nitrogen move with water
toward the root—a process called mass flow. Consequently, nitrate-nitrogen that has
moved below the root zone has potential to move up into the root zone, as surface
horizons of soil dry out and crops use water deeper in the profile. Conversely, plants
may exhibit symptoms of nitrogen deficiency even though the soil contains
adequate amounts of nitrogen, if moisture and consequently mass flow of nitrogen,
is limited.
Chapter 1 n Nitrogen
10
FIGURES 1-9 AND 1-10
Nitrogen fertilizer application
timings: preplant (top) and at
planting (bottom).
Nitrogen is a nutrient easily lost from soil through several pathways, as
already discussed. Consequently, plants use nitrogen most efficiently if the producer
applies it as close as possible to the time of crop uptake. Ideally, this might include
multiple applications of nitrogen during a growing season. Center pivot irrigation
systems equipped for fertigation and high clearance applicators are two methods to
accomplish multiple nitrogen applications. The use of center pivot irrigation
systems for fertigation also facilitates the use of a chlorophyll meter to detect
nitrogen deficiency and apply nitrogen according to crop demand. Sidedress
nitrogen application also allows for more efficient fertilizer use since the producer
applies nitrogen close to the period of maximum nitrogen uptake for corn and
sorghum. Nitrogen application prior to or at planting is still more efficient than fall
application for row crops such as corn and grain sorghum. Fall application may still
be a viable option on some soils for row crops. In that case, the producer should
only apply anhydrous ammonia in the fall (because it initially is not leachable), if
soils are fine-textured and when the soil temperature is 50o F, on average, for a week
or longer. With either fall or spring preplant application, nitrification inhibitors,
such as N-Serve® or DCD, help reduce the potential for leaching or denitrification
losses of nitrogen. For nitrogen application to winter wheat, late winter or early
spring topdress application allows the producer to assess moisture status and crop
condition before deciding on the appropriate nitrogen rate.
Nitrogen
Fertilizer
Management
Part I n Fertility Principles
11
Crops use nitrogen more efficiently when it is placed beneath the soil surface.
Broadcasting nitrogen on the soil surface increases the likelihood that some
nitrogen will be lost due to ammonia volatilization or runoff. This is one reason why
anhydrous ammonia, which must be injected, sometimes appears to be a better
nitrogen source than urea or UAN solution, which can be applied on the soil surface.
In general, as long as nitrogen fertilizers are correctly applied, all are agronomically
equal. If the farmer must apply nitrogen fertilizers to the soil surface, he can
increase efficiency by banding, which concentrates the fertilizer and reduces soil/
fertilizer contact. Sprinkler irrigation water application is another efficient method,
as long as application rates are not excessive.
The primary nitrogen fertilizers available in Nebraska are anhydrous ammonia
(82% nitrogen), urea (44% to 46% nitrogen), UAN solution (28% to 32% nitrogen),
ammonium nitrate (33% to 35% nitrogen), and ammonium sulfate (21% nitrogen).
Other fertilizers can contain significant amounts of nitrogen, but they are used
primarily as sources of nutrients other than nitrogen. All of the above are effective
fertilizers when properly applied. Anhydrous ammonia is historically the least
expensive nitrogen fertilizer, but it requires injection into the soil, which is a more
expensive application method than broadcasting or surface banding. Tillage,
irrigation and rainfall soon after application reduces the potential for significant
ammonia loss from urea fertilizers. A recent management option for urea fertilizers
is the urease inhibitor Agrotain®. This material contains the active ingredient N-(nbutyl)
thiophosphoric triamide (NBPT), which inhibits the function of the urease
enzyme (responsible for breaking urea down into ammonium and potentially
FIGURES 1-11 AND 1-12
Nitrogen fertilizer application
methods: sidedress (top) and
broadcast (bottom).
Chapter 1 n Nitrogen
12
ammonia ) for up to several weeks, depending on temperature. This delay in
decomposition of urea can increase the chances of rain or tillage moving the urea
into the soil where it is protected from volatile loss. Using a urease inhibitor reduces
the risk of applying urea fertilizers on the surface in minimum-tillage, high residue
conditions which otherwise have considerable potential for ammonia loss. Urease
inhibitors, like nitrification inhibitors, will not guarantee a yield increase every year,
but they can protect against yield reductions in years when climatic conditions are
conducive to nitrogen loss.
Efficient nitrogen fertilizer use requires that the producer gives proper credit
for sources of nitrogen other than the fertilizer before selecting the appropriate
nitrogen rate. Significant sources of nitrogen include soil residual nitrate
(determined by deep soil sampling), manure and organic materials (determined by
analyzing a sample of the material), legumes (determined according to the previous
crop), and irrigation water (determined by irrigation water sampling). Actual
nitrogen credits from these sources can vary widely, but in many cases the nitrogen
fertilizer rate can be reduced significantly after accounting for these credits. More
information on nitrogen accounting is available in the resources listed at the end of
this chapter.
Whenever possible, the farmer should practice rotating crops such as corn
and grain sorghum with crops that intensively use nitrogen such as soybeans, alfalfa
and clover. Aside from reducing fertilizer nitrogen requirements, crop rotations
provide other proven benefits in terms of reduced insect and weed infestation levels
and disease pressure. The nitrogen credit to corn following soybeans is not only
because of the additional nitrogen in the soil from the soybeans, but also because
the low C:N ratio of soybean residue immobilizes less soil nitrogen, and mineralizes
nitrogen from residue sooner the following season; this allows more soil nitrogen to
be available to the subsequent crop. Legumes such as alfalfa or clover that are tilled
in prior to planting corn do increase the level of available nitrogen in the soil as the
legume residue mineralizes. Legumes are efficient scavengers of soil nitrate, and
can substantially reduce soil nitrate levels following corn. Figure 1-13 illustrates the
effect of an annual corn-soybean rotation on the amount of nitrate in the vadose
zone. This example, from vadose zone soil cores taken in 1992 from the Long-Term
Tillage Study at the University of Nebraska South Central Research and Extension
Center Farm, shows the effect of implementing a corn-soybean rotation in 1984.
Nitrogen
Accounting and
Credits
Crop Rotations
Part I n Fertility Principles
13
FIGURE 1-13
The effect of an annual
corn/soybean rotation on the
amount of nitrate found in
the vadose zone (A.
Katupitiya, 1995; Long-term
tillage effects on nitrate
accumulation and movement
and denitrification in the root
and intermediate vadose
zones; Ph.D. dissertation,
University of Nebraska).
Monitoring Crops
for Nitrogen
Deficiency
0 5 10 15
Nitrate-N (ppm)
-70
-60
-50
-40
-30
-20
-10
0
Depth (ft)
Corn/corn
Corn/soybean
Nitrogen deficiency in plants is fairly easy to diagnose because of the unique
symptoms expressed in plants: initial yellowing of lower leaves with the leaf tip and
margin affected first. However, by the time such deficiency symptoms become
evident, yield reduction may have occurred, depending on the stage of growth. The
chlorophyll meter is a relatively new tool for nitrogen management which can detect
developing nitrogen deficiencies before they are visible, and before they can
significantly reduce yield. Other recent methods for detecting nitrogen stress are the
lower stalk nitrate test (which indicates after the season if the nitrogen supply to the
crop has been adequate or limiting), and remotely sensed imaging (which can detect
developing nitrogen stress similar to the chlorophyll meter). The producer may be
able to use a chlorophyll meter or remote sensing to detect nitrogen stress in time to
correct a deficiency during a growing season, and then apply necessary nitrogen
through high clearance applicators or via fertigation through center pivot irrigation
systems.
Chapter 1 n Nitrogen
14
Resources
Summary
FIGURES 1-14 AND 1-15
Nitrogen deficiency
symptoms in corn.
Nitrogen is usually the nutrient most limiting to cereal crop production in
Nebraska. It is subject to a variety of transformations in the soil. Some of these
transformations are necessary to convert nitrogen into forms which plants can use.
Other transformation or transport processes limit the availability of nitrogen to
plants by converting nitrogen into forms which plants cannot use, or moving
nitrogen away from the root zone.
Management factors, such as choice of nitrogen source, nitrogen placement
method, irrigation management, tillage and residue management all can affect how
efficiently crops use nitrogen.
1. Brady, N.C. 1974. The Nature and Property of Soils. 8th Ed. Macmillan
Publishing Company, Inc. New York, N.Y.
2. Ferguson, R.B., C.A. Shapiro, and G.W. Hergert. 1994. Fertilizer Nitrogen Best
Management Practices. NebGuide G94-1178A. University of Nebraska,
Cooperative Extension, Lincoln, NE.
3. Harper, J.F. 1984. Uptake of Organic Nitrogen Forms by Roots and Leaves. In
R.D. Hauck (ed.) Nitrogen in Crop Production. American Society of Agronomy,
Madison, WI.
4. Kissel, D.E. Management of Urea Fertilizers. North Central Regional Extension
Publication No. 326. University of Nebraska, Cooperative Extension, Lincoln,
NE.
5. Peterson, T.A., T.M. Blackmer, D.D. Francis, and J.S. Schepers. 1993. Using a
Chlorophyll Meter to Improve N Management. NebGuide G93-1171A.
University of Nebraska, Cooperative Extension, Lincoln, NE.
6. Stevenson, F.J. Origin and Distribution of N in Soil. In F.J. Stevenson (ed.)
Nitrogen in Agricultural Soils. 1982. American Society of Agronomy, Madison,
WI.