Geoscience Reference
In-Depth Information
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 (NO 3 ), or as ammonium (NH 4 ). 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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