WW-03770-GO Reviewed 1994
Mike O'Leary, George Rehm and Michael Schmitt
Copyright © 2002 Regents of the University of Minnesota. All rights reserved.
Environmental and economic issues combined have increased the need to
better understand the role and fate of nitrogen (N) in crop production
systems. Nitrogen is the nutrient most often deficient for crop production
in Minnesota and its use can result in substantial economic return for
farmers. However, when N inputs to the soil system exceed crop needs, there
is a possibility that excessive amounts of nitrate (NO
) may enter either ground or surface water.
Managing N inputs to achieve a balance between profitable crop production
and environmentally tolerable levels of NO
in water supplies should be every grower's goal. The behavior of N in the
soil system is complex, yet an understanding of these basic processes is
essential for a more efficient N management program.
Nitrogen exists in the soil system in many forms and changes (transforms)
very easily from one form to another. The route that N follows in and out
of the soil system is collectively called the "nitrogen cycle"
and is biologically influenced. Biological processes, in turn, are
influenced by prevailing climatic conditions along with the physical and
chemical properties of a particular soil. Both climate and soils vary
greatly across Minnesota and affect the N transformations for the different
Figure 1. The Nitrogen Cycle.
Nitrogen can be supplied for plant growth from several sources:
is the major reservoir for N in the N cycle (air is 79% N
gas). Although unavailable to most plants, large amounts of N
can be used by leguminous plants via
. In this biological process, nodule-forming
bacteria inhabit the roots of leguminous plants and through a symbiotic
relationship convert atmospheric N
to a form the plant can use. The amount of N
fixed by legumes into usable N can be substantial, with a potential for
several hundred lbN/acre/year to be fixed in an alfalfa crop. Any portion
of a legume crop, that is left after harvest, including roots and nodules,
supplies N to the soil system. When the plant material is decomposed, N is
released. Several non-symbiotic organisms exist that fix N, but N additions
from these organisms are quite low (1 - 5 lb/acre/year). In addition small
amounts of N are added to soil from
. The amount of N supplied from precipitation averages 5 - 10 lb/acre/year
Commercial N fertilizers
are also derived from the atmospheric N pool. The major step is to combine
with hydrogen (H
) to form ammonia (NH
). Anhydrous ammonia is then used as a starting point in the manufacture of
other nitrogen fertilizers. Anhydrous ammonia or other N products derived
can then supplement other N sources for crop nutrition.
Nitrogen can also become available for plant use from
organic N sources
which must be converted to inorganic forms before they are available to
plants. Nitrogen is available to plants as either ammonium (NH
) or nitrate (NO
). Animal manures and other organic wastes can be important sources of N
for plant growth. The amount of N supplied by manure will vary with the
type of livestock, handling, rate applied, and method of application. Since
the N form and content of manures varies widely, an analysis of manure is
recommended to improve N management.
from non-leguminous plants also contain N, but in relatively small amounts
compared with legumes. Nitrogen exists in crop residues in complex organic
forms and the residue must decay (a process that can take several years)
before N is made available for plant use.
Soil organic matter
is also a major source of N used by crops. Organic matter is composed
primarily of rather stable material called humus that has collected over a
long period of time. Easily decomposed portions of organic material
disappear relatively quickly, leaving behind residues more resistant to
decay. Soils contain approximately 2,000 pounds N in organic forms for each
percent of organic matter. Decomposition of this portion of organic matter
proceeds at a rather slow rate and releases about 20 lbN/acre/year for each
percent of organic matter. A credit for the amount of N released by organic
matter is built into current University of Minnesota N recommendations.
Nitrogen, present or added to the soil, is subject to several changes
(transformations) that dictate the availability of N to plants and
influence the potential movement of NO
to water supplies.
Organic N that is present in soil organic matter, crop residues, and manure
is converted to inorganic N through the process of
. In this process, bacteria digest organic material and release ammonium
) nitrogen. Formation of NH
increases as microbial activity increases. Bacterial growth is directly
related to soil temperature and water content. The ammonium supplied from
fertilizers is the same as the ammonium supplied from organic matter.
Ammonium-N has properties that are of practical importance for N
management. Plants can absorb NH
-N. Ammonium also has a positive charge and, therefore, is attracted or
held by negatively charged soil and soil organic matter. This means that NH
does not move downward in soils. Nitrogen in the ammonium form that is not
taken up by plants is subject to other changes in the soil system.
is the conversion of NH
-N to NO
-N. Nitrification is a biological process and proceeds rapidly in warm,
moist, well-aerated soils. Nitrification slows at soil temperatures below
50 degrees Fthus, the general recommendation is that ammoniacal (NH
forming) fertilizers should not be fall- applied until soils are below 50
degrees F. Nitrate is a negatively charged ion and is not attracted to soil
particles or soil organic matter like NH
. Nitrate-N is water soluble and can move below the crop rooting zone under
is a process by which bacteria convert NO
to N gases that are lost to the atmosphere. Denitrifying bacteria use NO
instead of oxygen in the metabolic processes. Denitrification takes place
where there is waterlogged soil and where there is ample organic matter to
provide energy for bacteria. For these reasons, denitrification is
generally limited to topsoil. Denitrification can proceed rapidly when
soils are warm and become saturated for 2 or 3 days.
A temporary reduction in the amount of plant-available N can occur from
(tie up) of soil N. Bacteria that decompose high carbon-low N residues,
such as corn stalks or small grain straw, need more N to digest the
material than is present in the residue. Immobilization occurs when nitrate
and/or ammonium present in the soil is used by the growing microbes to
build proteins. The actively growing bacteria that immobilize some soil N
also break down soil organic matter to release available N during the
growing season. There is often a net gain of N during the growing season
because the additional N in the residue will be the net gain after
Nitrogen is lost from the soil system in several ways:
In contrast to the biological transformations previously described, loss of
is a physical event. Leaching is the loss of soluble NO
as it moves with soil water, generally excess water, below the root zone.
Nitrate that moves below the root zone has potential to enter either
groundwater or surface water through tile drainage systems.
Coarse-textured soils have a lower water-holding capacity and, therefore, a
higher potential to lose nitrate from leaching when compared with
fine-textured soils. Some sandy soils, for instance, may retain only
inch of water per foot of soil while some silt loam or clay loam soils may
retain up to 2 inches of water per foot. Nitrate can be leached from any
soil if rainfall or irrigation moves water through the root zone.
can be a major loss mechanism of NO
when soils are saturated with water for 2 or 3 days. Nitrogen in the NH
form is not subject to this loss. Management alternatives are available if
denitrification losses are a potential problem.
Significant losses from some surface-applied N sources can occur through
the process of
. In this process, N is lost as the ammonia (NH
) gas. Nitrogen can be lost in this way from manure and fertilizer products
containing urea. Ammonia is an intermediate form of N during the process in
which urea is transformed to NH
. Incorporation of these N sources will virtually eliminate volatilization
losses. Loss of N from volatilization is greater when soil pH is higher
than 7.3, the air temperature is high, the soil surface is moist, and there
is a lot of residue on the soil.
Substantial amounts of N are lost from the soil system through
. A 150 bu/acre corn crop, for example, removes approximately 135 pounds of
N with the grain. Crop removal accounts for a majority of the N that leaves
the soil system.
Nitrogen can be lost from agricultural lands through soil
erosion and runoff
. Losses through these events do not normally account for a large portion
of the soil N budget, but should be considered for surface water quality
issues. Incorporation or injection of manure and fertilizer can help to
protect against N loss through erosion or runoff. Where soils are highly
erodible, conservation tillage can reduce soil erosion and runoff,
resulting in less surface loss of N.
In considering the many transformations and reactions of N in soils, there
are some major points to keep in mind. Although N can be added to soil in
either organic or inorganic forms, plants take up only inorganic N (that
). One form is not more important than the other and all sources of N can
be converted to nitrate. Commercial N fertilizers, legumes, manures, and
crop residues are all initial sources of NO
and once in the plant or in the water supply it is impossible to identify
the initial source.
Nitrate is always present in the soil solution and will move with the soil
water. Inhibiting the conversion of NH
can result in less N loss and more plant uptake; however, it is not
possible to totally prevent nitrification. There is no way to totally
prevent the movement of some NO
to water supplies, but sound management practices can keep losses within
This publication discusses several factors that are key to understanding N
behavior in a soil system. Numerous sources of N exist and must be
considered when evaluating the N budget for any field or region. Nitrogen's
mobility factor in the soil must be considered when developing N programs
and evaluating environmental effects. Nitrogen loss from the soil system is
greatly affected by soil type and climate. Sandy soils may lose N through
leaching while on heavy, poorly drained soils it may be lost through
denitrification. Because Minnesota has such diverse soils and climate,
interpretation of the N cycle should be site specific.
The following publications which discuss several aspects of N management in
more detail can be requested through the county offices of the Minnesota
Extension Service or from the Extension Store, 20 Coffey Hall, 1420
Eckles Ave., University of Minnesota, St. Paul, MN 55108-6069.
FO-2392, Managing Nitrogen for Corn Production on Irrigated Sandy Soils
FO-3425, Potato Fertilization on Irrigated Soils
FO-0636, Fertilizer Urea
FO-3073, Using Anhydrous Ammonia in Minnesota
Mike O'Leary, former assistant extension specialist, soil science, is now
with the Minnesota Department of Agriculture.
George Rehm and Michael Schmitt are extension soil scientists, soil fertility.
This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.