Home' Grower : December 2009 Contents Organics
The South Australian Grower -- December 2009/January 2010
Nutrients added to the soil by well-managed plants are some of the most important to the crop,
beneficial organisms and the structure and fertility of the soil, resulting in better crop yields.
Weeds not always enemy
THE most economical and effective
way to increase soil carbon is to grow
Plants get between 95 per cent and 98pc of
their minerals from the air and water. If we
look at the chemical composition of an aver-
age plant, carbon, hydrogen and oxygen
account for over 95pc of the minerals. The
remaining 5pc or less comes from the soil.
These minerals are combined using the
energy of the sun via photosynthesis to pro-
duce the carbon-based compounds that
plants need to grow and reproduce.
Research has shown that plant roots put
many tons of carbon molecules and bio-
available minerals into the soil per hectare
every year and are a very important part of
the process of forming topsoils and good
This means that well-managed plants can
put more bio-available nutrients into the soil
than they remove from it. Also, the nutrients
they put into the soil are some of the most
important to the crop, beneficial organisms
and the structure and fertility of the soil.
If weeds are managed properly, and their
residues are allowed to return to the soil,
their nutrient removal from the soil is zero.
In fact, as they are adding between 30pc
and 60pc of the organic compounds they
create through photosynthesis into the soil,
they are increasing soil fertility.
Techniques where weeds are cut down,
pulled or grazed so that their residues will
return to the soil will feed the crop. Cutting
and grazing plants will result in significant
percentages of roots being shed off so that the
weed or cover crop plants can re-establish an
equilibrium between their leaf and root areas.
These cast off roots not only add carbon and
feed the soil microorganisms, they release
nutrients to the crop and significantly lower
nutrient and water competition. This addition
of nutrients encourages the crop roots to
grow deeper in the soil
below the weed roots,
resulting in larger crop
root systems and bet-
ter access to water and
With these tech-
niques, the efficiency
of the farm surface
area capturing sun-
light and using pho-
tosynthesis to make
the carbon-based molecules that eventually
result in the fertile soils that feed our plants
It is the nutrients that are lost off farm --
either through selling the crop, soil leaching or
erosion -- that need to be replaced every year.
Good fertilisation should always ensure
that soil has the optimum level of all the
necessary minerals. If the minerals that we
remove from our soil when we sell our crop
are not replaced, the soil is essentially being
'mined' and run down.
One of the reasons why good organic
farmers notice that weeds do not become a
problem in their systems is because they
ensure they have excellent soil nutrition and
health by using weed management tech-
niques that build up the soil. The process
becomes one of effective weed management
rather than weed eradication.
Organic farmers also use microorganisms
to convert soil carbon into stable forms.
These stable forms of soil carbon, such as
humus and glomalin, are manufactured by
microorganisms. They convert the carbon
compounds that are readily oxidised into
carbon dioxide into stable polymers that can
last thousands of years in the soil.
Some of the current conventional farming
techniques result in the soil carbon deposit-
ed by plant roots being oxidised and con-
verted back into in carbon dioxide. This is
the reason why soil
organic matter levels
continue to decline in
these farming systems.
The other significant
depositories of carbon
are the soil organisms.
Research shows that
they form a consider-
able percentage of soil
carbon. It is essential
to manage the soil to
maintain high levels of soil organisms.
Also, it is essential that farming techniques
stimulate the species of soil microorganisms
that create stable carbons, rather than stim-
ulating the species that consume carbon and
convert it into carbon dioxide.
The process of making compost uses
microbes to build humus and other stable
carbons. The microorganisms that create
compost continue working in the soil after
compost applications, converting the car-
bon gifted by plants roots into stable
Regular applications of compost or com-
post teas will inoculate the soil with beneficial
organisms that build humus and other long
lasting carbon polymers. Over time, these
species will dominate over the species that
chew up carbon into carbon dioxide.
Regular applications of composts or com-
post tea also increase the number and diver-
sity of species living in the soil biomass. This
ensures that a significant proportion of soil
carbon is stored in living species that will
make minerals available to plants and pro-
tect their health.
Details: OFA 07 4098 7610, ofa.org.au
Plant roots add carbon
Managed weeds can have benefits
Fertilise to replace lost nutrients
AT A GLANCE
In part two of his study of soil carbon, Organic Federation of
Australia chairman ANDRE LEU examines techniques for
maximising stable carbon levels
Regular applications of compost
or compost teas will inoculate the
soil with beneficial organisms that
build humus and other long lasting
NO RISK OF WEEDS
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COMPARE THE SOIL BENEFITS WITH YOUR
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Carbon is essential for holding nutrients in the soil.
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