5
Nitrogen in the soil
Background
Nitrogen is a chemical element and is written in the chemical form as N. Plants
and animals require nitrogen to build proteins that are important in the quality
and quantity of plant material produced, and in the health of animals feeding on
healthy plants. In chemical terms, proteins are composed of carbohydrates plus
nitrogen (and usually sulphur). Legume plants are able to fix atmospheric nitrogen
in conjunction with colonies of rhizobia bacteria located in root nodules, and in
time this nitrogen becomes plant available. Azotobacter algae are also able to fix
atmospheric nitrogen that becomes plant available over time. Other sources of plant
available nitrogen are decaying plant and animal matter, and nitrogen compounds
produced by thunderstorms. Nitrogenous fertilisers may also be added to the soil or
sprayed directly onto plants as foliar fertilisers. There is a large difference between
various plants in their nitrogen requirements for optimum growth, and plant
species present can be an indication of the nitrogen status of the soil.
Nitrogen in the soil
For plants to use nitrogen, it must be available to them in a soluble form, and this
is usually as nitrate (NO3), or as ammonium (NH4). These forms of nitrogen can
be produced in or near the soil by the action of soil macro- and micro-organisms
as they break down (decay) organic matter, or can be supplied directly by adding
fertilisers such as urea, various ammonium or nitrate salts such as ammonium
nitrate, potassium nitrate, calcium ammonium nitrate or as anhydrous ammonia.
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26 Soil Essentials
Plants need significant quantities of nitrogen for rapid growth, especially in
spring as the weather warms up, or if soil is showing signs of waterlogging. It
should be mentioned that nitrogen is not used alone by plants, but with a whole
range of other plant nutrients in the correct balance. These are made available over
time, often as a result of complex chemical reactions, as larger molecules are
broken down to simpler molecules that the plant can use.
Nitrogen is removed from the soil by harvesting crops and pastures, and by
grazing animals. Dung and urine from animals does return significant quantities
of nitrogen, but unless paddocks are managed well the returned nitrogen is
unevenly spread, tending to concentrate in stock camps, under trees and near
gateways.
Nitrogen is also removed by leaching through the soil profile, and may leach
below the plant root zone, thus making it unavailable. This is especially true with
light sandy soils in high rainfall areas, or irrigated pastures or crops, but is not
such a problem in heavier clay soils in low rainfall areas or in dryland farming.
Nitrogen may also be lost from the soil in gaseous form, returning to the
atmosphere.
Nitrogen is an important component in the soil pH status, and unused
ammonium nitrogen compounds when converted to the nitrate form can rapidly
acidify soils, for example, if there is an imbalance between legumes (that add
nitrogen) and grasses (that use nitrogen).
Nitrogen in plants
Nitrogen plays an essential role in the production of chlorophyll in plants, so
adequate quantities are vital for healthy plant growth. It is also an integral part of
plant protein, usually in conjunction with sulphur, and is found in other plant
compounds such as amino acids. Chemically, proteins are built by adding nitrogen
to carbohydrates such as sugars and starches that have been made in the plant by
photosynthesis.
Legumes in conjunction with rhizobia bacteria are able to ‘capture’ nitrogen
from the atmosphere and fix it in nodules attached to the plant roots. As these
nodules break down, plant available nitrogen is released to the soil that can be used
by other species of plants such as grasses, and can also be accessed directly by the
nodule forming legume.
The increased production of chlorophyll in plants due to nitrogen leads to
more photosynthesis and hence higher production of sugars and starches within
the plant. This increases vegetative growth, and with plants in the grass family will
lead to increased tillering and seed yield with higher seed protein levels.
If available nitrogen is at low levels, the plant will tend to use what nitrogen
there is to produce vegetative growth at the expense of seed production. Nitrogen
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Nitrogen in the soil 27
can, however, be translocated in the plant from senescing leaves to seed production
as old leaves die and fall off, which will partly make up for the low nitrogen
availability in the soil.
The form in which nitrogen is available to plants can influence plant growth.
Organic nitrogen that is available over time (slow release) will provide nitrogen
over the various growth phases of a plant’s development. This can effectively
increase plant yield as there are no stages of the growth of the plant that are limited
by insufficient nitrogen at a point in time. If highly soluble chemical fertilisers are
used, the nitrogen tends to be available for a relatively short time only, and is at
risk of leaching if applied in excess to the plant’s immediate needs.
Multiple applications of chemical-based fertilisers can achieve the same thing
as slow release organic fertilisers, but may not be economically viable and also runs
the risk of damaging existing soil microbiology.
The nitrogen cycle
Soil and plant nitrogen levels are dynamic, and vary over time according to the
inputs and outputs from the natural environment and from changes brought about
by farming practices. Figure 10 illustrates the basics of this dynamic cycle.
Free nitrogen in air
Fertilizer manufacture
Volatilisation
rog er
nit rtiliz
en
e.g. from urea,
NH3 and urine
Fe
Nitrogen from
Animal life air fixed by
rhizobium in
Grass Legume nodules
Animal excreta
Dead leaves Dead leaves
and roots and roots
Decaying organic litter
Soil organic matter
(or humus)
Absorbed by Ammonium Denitrification
grass roots nitrogen
Nitrate nitrogen
Losses by leaching
Figure 10 The nitrogen cycle. (Price 2006)
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