Biological Nitrogen Fixation
D.H. Hubbell and Gerald Kidder
1. This document is SL-16, one of a series of fact sheets of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida.
First printed: July 1978. Revised February 1992. Reviewed April 1998, September 2003.
Please visit the
EDIS Web site at http://edis.ifas.ufl.edu.
2. D.H. Hubbell, professor emeritus (deceased) and Gerald Kidder, professor emeritus, Soil and Water Science Department, Florida Cooperative Extension
Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611-0290.
services only to individuals and institutions that
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For information on obtaining other extension publications, contact your county Cooperative Extension
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
The Importance of Nitrogen
Molecular nitrogen (N
) is the major
component (approximately 80%) of the earth's
The element nitrogen is an essential part
of many of the chemical compounds, such as proteins
and nucleic acids, which are the basis of all life
cannot be used directly by
biological systems to build the chemicals required for
growth and reproduction.
Before its incorporation
into a living system, N
must first be combined with
the element hydrogen.
This process of reduction of
, commonly referred to as "nitrogen fixation"
(N-fixation) may be accomplished chemically or
Nitrogen (N) is the nutrient element most
frequently found limiting to the growth of green
This results from the continual loss of
nitrogen from the reserve of combined or fixed
nitrogen, which is present in soil and available for use
It is continually depleted by such processes
as microbial denitrification, soil erosion, leaching,
chemical volatilization, and perhaps most important,
removal of nitrogen-containing crop residues from
The nitrogen reserve of agricultural soils
must therefore be replenished periodically in order to
maintain an adequate (non-growth limiting) level for
This replacement of soil nitrogen is
generally accomplished by the addition of chemically
fixed nitrogen in the form of commercial inorganic
fertilizers or by the activity of biological nitrogen
fixation (BNF) systems.
The significance of BNF as
the major mechanism of recycling of nitrogen from
the unavailable atmospheric form to available forms
in the biosphere cannot be overemphasized.
The Mechanism of Nitrogen Fixation