Nitrogen in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 26

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