Nitrogen (N) is the most abundant element in the atmosphere and is usually the most limiting crop nutrient. Nitrogen cycles through soil in various processes and forms.
Some processes are necessary to convert N into forms which plants can
use. Some processes can lead to N losses such as leaching or volatilization.
Nitrogen is added to soil naturally from N fixation by soil bacteria and legumes and through atmospheric deposition in rainfall. Additional N is typically supplied to the crop by fertilizers, manure, or other organic materials.
Soil nitrate-N
is an excellent indicator of N-cycling in soils, whether carryover nitrogen was
used by the previous crop and whether additional nitrogen is needed.
By
the end of this article, you will be able to discuss Nitrogen content of soils
and explain factors affecting its availability to plants.
Nitrogen Content of Soils
Improved agricultural methods and better crop varieties are demanding more and more of this element. Soils alone seldom can meet the increased demand because they were never well supplied with nitrogen or because they have lost much of their original supply during 50 to 100 years of cultivation.
Nitrogen is of special importance because plants need it in rather large amounts, it is fairly expensive to supply, and it is easily lost from the soil. A major factor in successful farming is the farmer's ability to manage nitrogen efficiently.
Losses of Nitrogen from Soil-Plant System
Apart from nitrate (NO3 – N) and ammonium (NH4+ -N) which are plant utilizable forms of nitrogen in the soil, other forms of inorganic N such as nitrogen dioxide (NO2), nitric oxide (NO) ammonia (NH3), nitrous oxide N 20 and nitrogen gas (N2) do exist having been transformed from organic fractions.
However, these
other forms usually escape to the atmosphere in gaseous forms completely away
from the soil-plant system. They are regarded as nitrogen losses and are usually
the products of denitrification and volatilization. Other losses are through
crop removal or by harvest, erosional losses and liquid leaching losses.
Certain
portions of nitrogen (NH4 – N and simple amino compounds) are held on
negatively charged sites in the soil in the interlayer spaces of 2:1 clay
minerals and are also unavailable to crops but cannot be regarded as losses
since they can still be released for plant use under certain soil conditions.
Inherent Factors Affecting Soil Nitrogen
1) Inherent factors such as soil drainage, soil
texture, and slope steepness impact N-transport and N transformation processes
that limit availability to crops or lead to losses.
2) Inherent factors such as rainfall and
temperature; and site conditions such as moisture, soil aeration (oxygen
levels), and salt content (electrical conductivity/EC) affect rate of N mineralization
from organic matter decomposition, nitrogen cycling, and nitrogen losses
through leaching, runoff, or denitrification.
3) Organic matter decomposes releasing N more
quickly in warm humid climates and slower in cool dry climates. This N release
is also quicker in well aerated soils and much slower on wet saturated soils.
How Leaching Affects Soil Nitrogen
Nitrogen can readily leach out of the root zone in nitrate-N form. The potential for leaching is dependent on soil texture (percentage of sand, silt, and clay) and soil water content.
Water moves more
quickly through large pore spaces in a sandy soil than it does through small
pores in a clayey soil and water holding capacity is much lower in sandy soils,
making them especially vulnerable.
Soils that have poor drainage and are ponded
or saturated with water causes denitrification to occur resulting in loss of N
as a gas which can result in emission of potent greenhouse gases, yield
reduction and increased N fertilizer expense.
Nitrogen Management
Management factors, such as N-rate, N
source, N placement method, timing of application, irrigation management,
residue management, crop type, etc. all can affect how efficiently N is used by
crops and amount of N losses.
Nitrogen management on sandy soils is important because of high potential for leaching losses. Selecting appropriate N rate is the primary management consideration.
However, nitrogen source,
timing N application close to plant uptake, and method of application such as
injecting N to avoid losses are also important. Management measures that
increase organic matter and avoid compaction are also important to stabilize
crop N supply, increase aeration, and to limit N losses due to denitrification
occurring in saturated soils.
Nitrogen Supply to Soils
Nitrogen rates should be based on amount
needed to optimize yield based on agronomic economic and environmental
considerations.
When planning N-fertilizer or manure
application rates appropriate N-credits should be accounted for including; soil
test residual nitrate-N, soil organic matter mineralization, legume credits,
manure or other organic amendments, irrigation water nitrate-N, residue decomposition
and natural N sources. Time N fertilisation to provide adequate amounts of N
when plants are actively growing and using N rapidly.
Losses of applied N from fertilizer can be
reduced by delaying application until the crop has emerged (side dressing).
Split N applications, where some N is applied prior to crop emergence and the
balance after emergence can increase crop N-use efficiency.
Fertilizer source is important to increase N
recovery by crops, avoid N-loss from vitalization and be matched to the type of
placement method to reduce losses and maximise recovery by crops.
Anhydrous ammonia is usually the least
expensive N source, but this material must be handled safely, and must be
injected/knifed in with ideal soil moisture conditions.
Urea and urea containing materials should be
injected to reduce loss from ammonia volatilization. Surface applied urea N
fertilizers, should not be applied during warm humid conditions, or on wet
residues because of high potential for N losses from vitalization.
Manure or organic amendments can be an
effective N-fertilizer. However, care must be taken to apply manure uniformly
at a known rate, and account for mineralization rate. Placement of N-fertilizer
can be accomplished by several methods.
Methods of N Application
Typical methods include:
1) Side
dress applications after crop emergence.
2) Knifed application placing a band of fertilizer
below the soil surface.
3) Broadcast
applications that uniformly distribute N.
4) Through sprinkler irrigation systems.
Each placement method has its advantages,
and must be matched to type of fertilizer or manure that will be applied.
Irrigation scheduling is important. The goal is to supply enough water to
optimise yield while avoiding excess irrigation which can increase costs and
leach N below the root zone.
Keys to Managing N in Most Efficient
Manner includes these Strategies:
1) Apply recommended rate based on realistic
yield
2) Time N application just before peak crop
demand
3) Select an ammonium containing fertilizer
which provides greater N recovery by crops
4) Inject N if possible to avoid ammonia or
volatisation losses
5) Use N-inhibitors when N is applied
outside of growing season
6) Credit all sources of N
7) Irrigate wisely
8) Monitor crop nitrogen needs by scouting
9) Regular soil testing for nitrate
(including deep samples), and soil salt content (EC)
Symptom
of N Deficiency in Crops
Yellow coloration in a “V” shaped pattern is
symptomatic of nitrogen deficiency. This pattern progresses from leaf end to
leaf collar and from lower to upper leaves. Lower leaves often die when
nitrogen deficiency is severe.
Biological Nitrogen Fixation
Biological nitrogen fixation is the conversion of dinitrogen (N2) in soil air into combined forms useable by the plants and effected by microorganisms.
Some groups of microbes live freely in
the soil where they are able to convert N2 into body tissue nitrogen form and
when they die and decompose, the combined nitrogen is released for plant use. These
groups of free living N-fixers are known as Non-symbiotic nitrogen fixing
microorganisms.
In the tropics Azotobacter and Bjeirinckia species are known to be free-living microbes that fix N2 under aerobic well aerated soils of pH 6.0. Clostridium spp. Are free-living bacteria that fix N2 under anaerobic conditions in soil.
Otherliving soil micro-organisms that fix
nitrogen are blue -green algae and Azospiriullum in the rhizosphere of certain
plants.
The range of fixed N by these group of
non-symbiotic microbes is between 2-25 kg/ha/year. In symbiotic fixation,
bacterial and actinomycetes effect the formation of root nodules (abnormal root
growth) in both legume and non-legume plants and then inhabit those nodules
where they fix nitrogen.
The host plant supplies the bacteria with
carbohydrate as source of energy, and the bacteria supplies the plant with
fixed nitrogen compounds.in effect, both the plant and the microorganism have a
mutually beneficial association known as symbiosis.
There are various species of Rhizobium
bacteria that inhabit different plant species such as R. trifolic which inhabit
clovers, R. japonicum for Glycine max (soyabeans) and R. Phaseoli which
associates with Phaseolus vulgaris (dry beans). Appropriate Rhizobia cultures
are used to inoculate the soil for a particular crop to nodulate for the
fixation of nitrogen.
Nitrogen Cycle
Besides nitrogen (N2) gas within soil pore
space, nitrogen is found in both organic and inorganic forms in soil. Organic
forms occur in soil organic matter which consists of three primary parts
including small (fresh) plant residues and small living soil organisms,
decomposing (active) organic matter, and stable organic matter.
Predominate inorganic forms of N in soils
are ammonium (NH4) and nitrate (NO3), which are both useable by plants. The
nitrogen cycle (Fig. 3) illustrates reactions that various inorganic and
organic N compounds undergo in soil. The nitrogen cycle typically begins with
nitrogen in its simplest stable form, dinitrogen (N2) in air, and follows it
through the processes of fixation, mineralisation, nitrification, leaching,
plant assimilation, ammonia volatilisation, denitrification, and
immobilisation.
Conclusion on Nitrogen Content of Soils
You have learnt that Nitrogen (N) is the most
abundant element in the atmosphere and is usually the most limiting crop
nutrient. Nitrogen cycles through soil in various processes and forms. Some
processes can lead to N losses such as leaching or volatilisation.
Nitrogen is added to soil naturally from N
fixation by soil bacteria and legumes and through atmospheric deposition in
rainfall.
An abundant supply of the essential nitrogen
compounds is required in each plant cell for a good rate of reproduction,
growth, and respiration. Even the green leaf pigment chlorophyll, which enables
plants to use the
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