Home' Grower : January 2010 Contents The South Australian Grower -- February 2010 1 3
Synthetic nitrogen fertilisers are one of the major causes behind the decline of soil carbon because they stimulate a range
of bacteria that feed on nitrogen and carbon to form amino acids for their growth and reproduction.
Think twice before fertilising
THE continuous application
of carbon as composts,
manures, mulches and
through plant growth will not
increase soil carbon levels if farm-
ing practices destroy soil carbon.
This article will discuss some of
the practices that result in a
decline in carbon and offer alter-
natives that prevent this loss.
Synthetic nitrogen fertilisers
are one of the major causes
behind the decline of soil car-
bon. This is because they stimu-
late a range of bacteria that feed
on nitrogen and carbon to form
amino acids for their growth and
reproduction. These bacteria
have carbon to nitrogen ratios of
In other words, every tonne of
nitrogen applied results in the
bacteria consuming between 20
to 30t of soil carbon. This
equates to between 73.4-110.1t
of carbon dioxide that will be
released into the atmosphere.
Scientists from the University
of Illinois analysed the results of
a 50-year agricultural trial and
found that synthetic nitrogen
fertiliser resulted in all the car-
bon residues from the crop dis-
appearing as well as an average
loss of about 10,000 kilograms
of soil carbon a hectare.
This is about 33,000kg of car-
bon dioxide/ha on top of the
many thousands of kilograms of
crop residue that is converted in
to CO2 every year.
The researcher found that the
higher the application of synthet-
ic nitrogen fertiliser, the greater
the amount of soil carbon lost as
CO2. This is one of the major
reasons why conventional agri-
cultural systems have a decline in
soil carbon while organic systems
increase soil carbon.
Freshly deposited carbon com-
pounds tend to readily oxidise
into CO2 unless they are con-
verted into more stable forms.
Stable forms of carbon take time
to form. In many cases, it
requires years to rebuild the
bank of stable carbon back to
Ensuring that a carbon source
is included with nitrogen fer-
tilisers protects the soil carbon
bank, as the microbes will use
the added carbon, rather than
degrading the stable soil carbon.
Composts, animal manures,
green manures and legumes are
good examples of carbon-based
Where possible, nitrogen should
be obtained through rhizobium
bacteria in legumes and free living
nitrogen fixing microorganisms.
These microorganisms work at a
stable rate, fixing the nitrogen in
the soil into plant-available forms.
They can utilise the steady stream
of newly-deposited carbon from
plant roots to create amino acids,
rather than destroying humus and
other stable carbon polymers.
The use of synthetic nitrogen
fertilisers changes the soil biota
to favour microorganisms that
consume carbon, rather than the
species that build humus and
other stable forms of carbon.
By stimulating high levels of
species that consume soil car-
bon, the carbon never gets to
increase and usually continues
to slowly decline.
The use of composts with
microorganisms that build stable
carbons will lead to carbon levels
increasing if the farm avoids
practices that destroy soil car-
Research shows that the use of
biocides: herbicides, pesticides
and fungicides which cause a
decline in beneficial microorgan-
isms. As early as 1962, Rachel
Carson quoted research about
the detrimental effect of biocides
on soil microorganisms in her
ground-breaking book 'Silent
Spring'. Since then, there have
been regular studies confirming
the damage agricultural chemi-
cal are causing to our soil biota.
Recently, the work of one of the
world's leading microbiologists,
Dr Elaine Ingham, has shown
that these chemicals cause a sig-
nificant decline in the beneficial
microorganisms that build
humus, suppress diseases and
make nutrients available to plants.
Many herbicides and fungicides
have been shown to kill off bene-
ficial soil fungi. These types of
fungi have been shown to sup-
press diseases, increase nutrient
uptake -- particularly phosphorus
-- and form glomalin.
Glomalin is a stable carbon poly-
mer that forms long strings that
work like reinforcing rods in the
soil. Research is showing that they
form a significant role in building
a good soil structure that is resist-
ant to erosion and compaction.
The structure facilitates good aer-
ation and water infiltration.
Avoiding the use of toxic chem-
icals is an important part of the
process of developing healthy
soils that are teeming with the
beneficial species that will build
the stable forms of carbon.
Details: OFA 07 4098 7610, ofa.org.au
Organic Federation of Australia chairman
ANDRE LEU continues his examination of the
importance of soil carbon, and this month
focuses on limiting carbon-reducing
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